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Cannot access a disposed object. A common cause of this error is disposing a context that was resolved from dependency injection [duplicate]

From my understanding one of the main things that async and await do is to make code easy to write and read - but is using them equal to spawning background threads to perform long duration logic?
I'm currently trying out the most basic example. I've added some comments inline. Can you clarify it for me?
// I don't understand why this method must be marked as `async`.
private async void button1_Click(object sender, EventArgs e)
{
Task<int> access = DoSomethingAsync();
// task independent stuff here
// this line is reached after the 5 seconds sleep from
// DoSomethingAsync() method. Shouldn't it be reached immediately?
int a = 1;
// from my understanding the waiting should be done here.
int x = await access;
}
async Task<int> DoSomethingAsync()
{
// is this executed on a background thread?
System.Threading.Thread.Sleep(5000);
return 1;
}

When using async and await the compiler generates a state machine in the background.
Here's an example on which I hope I can explain some of the high-level details that are going on:
public async Task MyMethodAsync()
{
Task<int> longRunningTask = LongRunningOperationAsync();
// independent work which doesn't need the result of LongRunningOperationAsync can be done here
//and now we call await on the task
int result = await longRunningTask;
//use the result
Console.WriteLine(result);
}
public async Task<int> LongRunningOperationAsync() // assume we return an int from this long running operation
{
await Task.Delay(1000); // 1 second delay
return 1;
}
OK, so what happens here:
Task<int> longRunningTask = LongRunningOperationAsync(); starts executing LongRunningOperation
Independent work is done on let's assume the Main Thread (Thread ID = 1) then await longRunningTask is reached.
Now, if the longRunningTask hasn't finished and it is still running, MyMethodAsync() will return to its calling method, thus the main thread doesn't get blocked. When the longRunningTask is done then a thread from the ThreadPool (can be any thread) will return to MyMethodAsync() in its previous context and continue execution (in this case printing the result to the console).
A second case would be that the longRunningTask has already finished its execution and the result is available. When reaching the await longRunningTask we already have the result so the code will continue executing on the very same thread. (in this case printing result to console). Of course this is not the case for the above example, where there's a Task.Delay(1000) involved.

From my understanding one of the main things that async and await do is to make code easy to write and read.
They're to make asynchronous code easy to write and read, yes.
Is it the same thing as spawning background threads to perform long duration logic?
Not at all.
// I don't understand why this method must be marked as 'async'.
The async keyword enables the await keyword. So any method using await must be marked async.
// This line is reached after the 5 seconds sleep from DoSomethingAsync() method. Shouldn't it be reached immediately?
No, because async methods are not run on another thread by default.
// Is this executed on a background thread?
No.
You may find my async/await intro helpful. The official MSDN docs are also unusually good (particularly the TAP section), and the async team put out an excellent FAQ.

Explanation
Here is a quick example of async/await at a high level. There are a lot more details to consider beyond this.
Note: Task.Delay(1000) simulates doing work for 1 second. I think it's best to think of this as waiting for a response from an external resource. Since our code is waiting for a response, the system can set the running task off to the side and come back to it once it's finished. Meanwhile, it can do some other work on that thread.
In the example below, the first block is doing exactly that. It starts all the tasks immediately (the Task.Delay lines) and sets them off to the side. The code will pause on the await a line until the 1 second delay is done before going to the next line. Since b, c, d, and e all started executing at almost the exact same time as a (due to lack of the await), they should finish at roughly the same time in this case.
In the example below, the second block is starting a task and waiting for it to finish (that is what await does) before starting the subsequent tasks. Each iteration of this takes 1 second. The await is pausing the program and waiting for the result before continuing. This is the main difference between the first and second blocks.
Example
Console.WriteLine(DateTime.Now);
// This block takes 1 second to run because all
// 5 tasks are running simultaneously
{
var a = Task.Delay(1000);
var b = Task.Delay(1000);
var c = Task.Delay(1000);
var d = Task.Delay(1000);
var e = Task.Delay(1000);
await a;
await b;
await c;
await d;
await e;
}
Console.WriteLine(DateTime.Now);
// This block takes 5 seconds to run because each "await"
// pauses the code until the task finishes
{
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
}
Console.WriteLine(DateTime.Now);
OUTPUT:
5/24/2017 2:22:50 PM
5/24/2017 2:22:51 PM (First block took 1 second)
5/24/2017 2:22:56 PM (Second block took 5 seconds)
Extra info regarding SynchronizationContext
Note: This is where things get a little foggy for me, so if I'm wrong on anything, please correct me and I will update the answer. It's important to have a basic understanding of how this works but you can get by without being an expert on it as long as you never use ConfigureAwait(false), although you will likely lose out on some opportunity for optimization, I assume.
There is one aspect of this which makes the async/await concept somewhat trickier to grasp. That's the fact that in this example, this is all happening on the same thread (or at least what appears to be the same thread in regards to its SynchronizationContext). By default, await will restore the synchronization context of the original thread that it was running on. For example, in ASP.NET you have an HttpContext which is tied to a thread when a request comes in. This context contains things specific to the original Http request such as the original Request object which has things like language, IP address, headers, etc. If you switch threads halfway through processing something, you could potentially end up trying to pull information out of this object on a different HttpContext which could be disastrous. If you know you won't be using the context for anything, you can choose to "not care" about it. This basically allows your code to run on a separate thread without bringing the context around with it.
How do you achieve this? By default, the await a; code actually is making an assumption that you DO want to capture and restore the context:
await a; //Same as the line below
await a.ConfigureAwait(true);
If you want to allow the main code to continue on a new thread without the original context, you simply use false instead of true so it knows it doesn't need to restore the context.
await a.ConfigureAwait(false);
After the program is done being paused, it will continue potentially on an entirely different thread with a different context. This is where the performance improvement would come from -- it could continue on on any available thread without having to restore the original context it started with.
Is this stuff confusing? Hell yeah! Can you figure it out? Probably! Once you have a grasp of the concepts, then move on to Stephen Cleary's explanations which tend to be geared more toward someone with a technical understanding of async/await already.

Further to the other answers, have a look at await (C# Reference)
and more specifically at the example included, it explains your situation a bit
The following Windows Forms example illustrates the use of await in an
async method, WaitAsynchronouslyAsync. Contrast the behavior of that
method with the behavior of WaitSynchronously. Without an await
operator applied to a task, WaitSynchronously runs synchronously
despite the use of the async modifier in its definition and a call to
Thread.Sleep in its body.
private async void button1_Click(object sender, EventArgs e)
{
// Call the method that runs asynchronously.
string result = await WaitAsynchronouslyAsync();
// Call the method that runs synchronously.
//string result = await WaitSynchronously ();
// Display the result.
textBox1.Text += result;
}
// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
await Task.Delay(10000);
return "Finished";
}
// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
// Add a using directive for System.Threading.
Thread.Sleep(10000);
return "Finished";
}

For fastest learning..
Understand method execution flow(with a diagram): 3 mins
Question introspection (learning sake): 1 min
Quickly get through syntax sugar: 5 mins
Share the confusion of a developer : 5 mins
Problem: Quickly change a real-world implementation of normal code to
Async code: 2 mins
Where to Next?
Understand method execution flow(with a diagram): 3 mins
In this image, just focus on #6 (nothing more)
At #6 step, execution ran out of work and stopped. To continue it needs a result from getStringTask(kind of a function). Therefore, it uses an await operator to suspend its progress and give control back(yield) to the caller(of this method we are in). The actual call to getStringTask was made earlier in #2. At #2 a promise was made to return a string result. But when will it return the result? Should we(#1:AccessTheWebAsync) make a 2nd call again? Who gets the result, #2(calling statement) or #6(awaiting statement)?
The external caller of AccessTheWebAsync() also is waiting now. So caller waiting for AccessTheWebAsync, and AccessTheWebAsync is waiting for GetStringAsync at the moment. Interesting thing is AccessTheWebAsync did some work(#4) before waiting perhaps to save time from waiting. The same freedom to multitask is also available for the external caller(and all callers in the chain) and this is the biggest plus of this 'async' thingy! You feel like it is synchronous..or normal but it is not.
#2 and #6 is split so we have the advantage of #4(work while waiting). But we can also do it without splitting. So #2 will be: string urlContents = await client.GetStringAsync("...");. Here we see no advantage but somewhere in the chain one function will be splitting while rest of them call it without splitting. It depends which function/class in the chain you use. This change in behavior from function to function is the most confusing part about this topic.
Remember, the method was already returned(#2), it cannot return again(no second time). So how will the caller know? It is all about Tasks! Task was returned. Task status was waited for (not method, not value). Value will be set in Task. Task status will be set to complete. Caller just monitors Task(#6). So 6# is the answer to where/who gets the result. Further reads for later here.
Question introspection for learning sake: 1 min
Let us adjust the question a bit:
How and When to use async and await Tasks?
Because learning Task automatically covers the other two(and answers your question).
The whole idea is pretty simple. A method can return any data type(double, int, object, etc.) but here we just deny that and force a 'Task' object return! But we still need the returned data(except void), right? That will be set in a standard property inside 'Task' object eg: 'Result' property.
Quickly get through syntax sugar: 5 mins
Original non-async method
internal static int Method(int arg0, int arg1)
{
int result = arg0 + arg1;
IO(); // Do some long running IO.
return result;
}
a brand new Task-ified method to call the above method
internal static Task<int> MethodTask(int arg0, int arg1)
{
Task<int> task = new Task<int>(() => Method(arg0, arg1));
task.Start(); // Hot task (started task) should always be returned.
return task;
}
Did we mention await or async? No. Call the above method and you get a task which you can monitor. You already know what the task returns(or contains).. an integer.
Calling a Task is slightly tricky and that is when the keywords starts to appear. If there was a method calling the original method(non-async) then we need to edit it as given below. Let us call MethodTask()
internal static async Task<int> MethodAsync(int arg0, int arg1)
{
int result = await HelperMethods.MethodTask(arg0, arg1);
return result;
}
Same code above added as image below:
We are 'awaiting' task to be finished. Hence the await(mandatory syntax)
Since we use await, we must use async(mandatory syntax)
MethodAsync with Async as the prefix (coding standard)
await is easy to understand but the remaining two (async,Async) may not be :). Well, it should make a lot more sense to the compiler though.Further reads for later here
So there are 2 parts.
Create 'Task' (only one task and it will be an additional method)
Create syntactic sugar to call the task with await+async(this involves changing existing code if you are converting a non-async method)
Remember, we had an external caller to AccessTheWebAsync() and that caller is not spared either... i.e it needs the same await+async too. And the chain continues(hence this is a breaking change which could affect many classes). It can also be considered a non-breaking change because the original method is still there to be called. Change it's access (or delete and move it inside a task) if you want to impose a breaking change and then the classes will be forced to use Task-method. Anyways, in an async call there will always be a Task at one end and only one.
All okay, but one developer was surprised to see Task
missing...
Share the confusion of a developer: 5 mins
A developer has made a mistake of not implementing Task but it still works! Try to understand the question and just the accepted answer provided here. Hope you have read and fully understood. The summary is that we may not see/implement 'Task' but it is implemented somewhere in a parent/associated class. Likewise in our example calling an already built MethodAsync() is way easier than implementing that method with a Task (MethodTask()) ourself. Most developers find it difficult to get their head around Tasks while converting a code to Asynchronous one.
Tip: Try to find an existing Async implementation (like MethodAsync or ToListAsync) to outsource the difficulty. So we only need to deal with Async and await (which is easy and pretty similar to normal code)
Problem: Quickly change a real-world implementation of normal code to
Async operation: 2 mins
Code line shown below in Data Layer started to break(many places). Because we updated some of our code from .Net framework 4.2.* to .Net core. We had to fix this in 1 hour all over the application!
var myContract = query.Where(c => c.ContractID == _contractID).First();
easypeasy!
We installed EntityFramework nuget package because it has QueryableExtensions. Or in other words it does the Async implementation(task), so we could survive with simple Async and await in code.
namespace = Microsoft.EntityFrameworkCore
calling code line got changed like this
var myContract = await query.Where(c => c.ContractID == _contractID).FirstAsync();
Method signature changed from
Contract GetContract(int contractnumber)
to
async Task<Contract> GetContractAsync(int contractnumber)
calling method also got affected: GetContract(123456); was called as GetContractAsync(123456).Result;
Wait! what is that Result? Good catch! GetContractAsync only returns a Task not the value we wanted(Contract). Once the result of an operation is available, it is stored and is returned immediately on subsequent calls to the Result property.
We can also do a time-out implementation with a similar 'Wait()'
TimeSpan ts = TimeSpan.FromMilliseconds(150);
if (! t.Wait(ts))
Console.WriteLine("The timeout interval elapsed.");
We changed it everywhere in 30 minutes!
But the architect told us not to use EntityFramework library just for this! oops! drama! Then we made a custom Task implementation(yuk!). Which you know how. Still easy! ..still yuk..
Where to Next?
There is a wonderful quick video we could watch about Converting Synchronous Calls to Asynchronous in ASP.Net Core, perhaps that is likely the direction one would go after reading this. Or have I explained enough? ;)

Showing the above explanations in action in a simple console program:
class Program
{
static void Main(string[] args)
{
TestAsyncAwaitMethods();
Console.WriteLine("Press any key to exit...");
Console.ReadLine();
}
public async static void TestAsyncAwaitMethods()
{
await LongRunningMethod();
}
public static async Task<int> LongRunningMethod()
{
Console.WriteLine("Starting Long Running method...");
await Task.Delay(5000);
Console.WriteLine("End Long Running method...");
return 1;
}
}
And the output is:
Starting Long Running method...
Press any key to exit...
End Long Running method...
Thus,
Main starts the long running method via TestAsyncAwaitMethods. That immediately returns without halting the current thread and we immediately see 'Press any key to exit' message
All this while, the LongRunningMethod is running in the background. Once its completed, another thread from Threadpool picks up this context and displays the final message
Thus, not thread is blocked.

I think you've picked a bad example with System.Threading.Thread.Sleep
Point of an async Task is to let it execute in background without locking the main thread, such as doing a DownloadFileAsync
System.Threading.Thread.Sleep isn't something that is "being done", it just sleeps, and therefore your next line is reached after 5 seconds ...
Read this article, I think it is a great explanation of async and await concept: http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx

Async & Await Simple Explanation
Simple Analogy
A person may wait for their morning train. This is all they are doing as this is their primary task that they are currently performing. (synchronous programming (what you normally do!))
Another person may await their morning train whilst they smoke a cigarette and then drink their coffee. (Asynchronous programming)
What is asynchronous programming?
Asynchronous programming is where a programmer will choose to run some of his code on a separate thread from the main thread of execution and then notify the main thread on it's completion.
What does the async keyword actually do?
Prefixing the async keyword to a method name like
async void DoSomething(){ . . .
allows the programmer to use the await keyword when calling asynchronous tasks. That's all it does.
Why is this important?
In a lot of software systems the main thread is reserved for operations specifically relating to the User Interface. If I am running a very complex recursive algorithm that takes 5 seconds to complete on my computer, but I am running this on the Main Thread (UI thread) When the user tries to click on anything on my application, it will appear to be frozen as my main thread has queued and is currently processing far too many operations. As a result the main thread cannot process the mouse click to run the method from the button click.
When do you use Async and Await?
Use the asynchronous keywords ideally when you are doing anything that doesn't involve the user interface.
So lets say you're writing a program that allows the user to sketch on their mobile phone but every 5 seconds it is going to be checking the weather on the internet.
We should be awaiting the call the polling calls every 5 seconds to the network to get the weather as the user of the application needs to keep interacting with the mobile touch screen to draw pretty pictures.
How do you use Async and Await
Following on from the example above, here is some pseudo code of how to write it:
//ASYNCHRONOUS
//this is called using the await keyword every 5 seconds from a polling timer or something.
async Task CheckWeather()
{
var weather = await GetWeather();
//do something with the weather now you have it
}
async Task<WeatherResult> GetWeather()
{
var weatherJson = await CallToNetworkAddressToGetWeather();
return deserializeJson<weatherJson>(weatherJson);
}
//SYNCHRONOUS
//This method is called whenever the screen is pressed
void ScreenPressed()
{
DrawSketchOnScreen();
}
Additional Notes - Update
I forgot to mention in my original notes that in C# you can only await methods that are wrapped in Tasks. for example you may await this method:
// awaiting this will return a string.
// calling this without await (synchronously) will result in a Task<string> object.
async Task<string> FetchHelloWorld() {..
You cannot await methods that are not tasks like this:
async string FetchHelloWorld() {..
Feel free to review the source code for the Task class here.

Here is a quick console program to make it clear to those who follow. The TaskToDo method is your long running method that you want to make async. Making it run async is done by the TestAsync method. The test loops method just runs through the TaskToDo tasks and runs them async. You can see that in the results because they don't complete in the same order from run to run - they are reporting to the console UI thread when they complete. Simplistic, but I think the simplistic examples bring out the core of the pattern better than more involved examples:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace TestingAsync
{
class Program
{
static void Main(string[] args)
{
TestLoops();
Console.Read();
}
private static async void TestLoops()
{
for (int i = 0; i < 100; i++)
{
await TestAsync(i);
}
}
private static Task TestAsync(int i)
{
return Task.Run(() => TaskToDo(i));
}
private async static void TaskToDo(int i)
{
await Task.Delay(10);
Console.WriteLine(i);
}
}
}

All the answers here use Task.Delay() or some other built in async function. But here is my example that use none of those async functions:
// Starts counting to a large number and then immediately displays message "I'm counting...".
// Then it waits for task to finish and displays "finished, press any key".
static void asyncTest ()
{
Console.WriteLine("Started asyncTest()");
Task<long> task = asyncTest_count();
Console.WriteLine("Started counting, please wait...");
task.Wait(); // if you comment this line you will see that message "Finished counting" will be displayed before we actually finished counting.
//Console.WriteLine("Finished counting to " + task.Result.ToString()); // using task.Result seems to also call task.Wait().
Console.WriteLine("Finished counting.");
Console.WriteLine("Press any key to exit program.");
Console.ReadLine();
}
static async Task<long> asyncTest_count()
{
long k = 0;
Console.WriteLine("Started asyncTest_count()");
await Task.Run(() =>
{
long countTo = 100000000;
int prevPercentDone = -1;
for (long i = 0; i <= countTo; i++)
{
int percentDone = (int)(100 * (i / (double)countTo));
if (percentDone != prevPercentDone)
{
prevPercentDone = percentDone;
Console.Write(percentDone.ToString() + "% ");
}
k = i;
}
});
Console.WriteLine("");
Console.WriteLine("Finished asyncTest_count()");
return k;
}

This answer aims to provide some info specific to ASP.NET.
By utilizing async/await in the MVC controller, it is possible to increase thread pool utilization and achieve much better throughput, as explained in the below article,
http://www.asp.net/mvc/tutorials/mvc-4/using-asynchronous-methods-in-aspnet-mvc-4
In web applications that see a large number of concurrent requests at
start-up or have a bursty load (where concurrency increases suddenly),
making these web service calls asynchronous will increase the
responsiveness of your application. An asynchronous request takes the
same amount of time to process as a synchronous request. For example,
if a request makes a web service call that requires two seconds to
complete, the request takes two seconds whether it is performed
synchronously or asynchronously. However, during an asynchronous call,
a thread is not blocked from responding to other requests while it
waits for the first request to complete. Therefore, asynchronous
requests prevent request queuing and thread pool growth when there are
many concurrent requests that invoke long-running operations.

Async / Await
Actually, Async / Await is a pair of keywords that are just syntactic sugar for creating a callback of an asynchronous task.
Take by example this operation:
public static void DoSomeWork()
{
var task = Task.Run(() =>
{
// [RUNS ON WORKER THREAD]
// IS NOT bubbling up due to the different threads
throw new Exception();
Thread.Sleep(2000);
return "Hello";
});
// This is the callback
task.ContinueWith((t) => {
// -> Exception is swallowed silently
Console.WriteLine("Completed");
// [RUNS ON WORKER THREAD]
});
}
The code above has several disadvantages. Errors are not passed on and it's hard to read.
But Async and Await come in to help us out:
public async static void DoSomeWork()
{
var result = await Task.Run(() =>
{
// [RUNS ON WORKER THREAD]
// IS bubbling up
throw new Exception();
Thread.Sleep(2000);
return "Hello";
});
// every thing below is a callback
// (including the calling methods)
Console.WriteLine("Completed");
}
Await calls have to be in Async methods. This has some advantages:
Returns the result of the Task
creates automatically a callback
checks for errors and lets them bubble up in callstack (only up to none-await calls in callstack)
waits for the result
frees up the main thread
runs the callback on the main thread
uses a worker thread from the threadpool for the task
makes the code easy to read
and a lot more
NOTE: Async and Await are used with asynchronous calls not to make these. You have to use Task Libary for this, like Task.Run() .
Here is a comparison between await and none await solutions
This is the none async solution:
public static long DoTask()
{
stopWatch.Reset();
stopWatch.Start();
// [RUNS ON MAIN THREAD]
var task = Task.Run(() => {
Thread.Sleep(2000);
// [RUNS ON WORKER THREAD]
});
// goes directly further
// WITHOUT waiting until the task is finished
// [RUNS ON MAIN THREAD]
stopWatch.Stop();
// 50 milliseconds
return stopWatch.ElapsedMilliseconds;
}
This is the async method:
public async static Task<long> DoAwaitTask()
{
stopWatch.Reset();
stopWatch.Start();
// [RUNS ON MAIN THREAD]
await Task.Run(() => {
Thread.Sleep(2000);
// [RUNS ON WORKER THREAD]
});
// Waits until task is finished
// [RUNS ON MAIN THREAD]
stopWatch.Stop();
// 2050 milliseconds
return stopWatch.ElapsedMilliseconds;
}
You can actually call an async method without the await keyword but this means that any Exception here is swallowed in release mode:
public static Stopwatch stopWatch { get; } = new Stopwatch();
static void Main(string[] args)
{
Console.WriteLine("DoAwaitTask: " + DoAwaitTask().Result + " ms");
// 2050 (2000 more because of the await)
Console.WriteLine("DoTask: " + DoTask() + " ms");
// 50
Console.ReadKey();
}
Async and Await are not meant for parallel computing. They are used to not block your main thread. When it's about asp.net or Windows applications, blocking your main thread due to a network call is a bad thing. If you do this, your app will get unresponsive or even crash.
Check out MS docs for more examples.

To be honest I still think the best explanation is the one about future and promises on the Wikipedia: http://en.wikipedia.org/wiki/Futures_and_promises
The basic idea is that you have a separate pool of threads that execute tasks asynchronously. When using it. The object does however make the promise that it will execute the operation at some time and give you the result when you request it. This means that it will block when you request the result and hasn't finished, but execute in the thread pool otherwise.
From there you can optimize things: some operations can be implemented async and you can optimize things like file IO and network communication by batching together subsequent requests and/or reordering them. I'm not sure if this is already in the task framework of Microsoft - but if it isn't that would be one of the first things I would add.
You can actually implement the future pattern sort-of with yields in C# 4.0. If you want to know how it works exactly, I can recommend this link that does a decent job: http://code.google.com/p/fracture/source/browse/trunk/Squared/TaskLib/ . However, if you start toying with it yourself, you will notice that you really need language support if you want to do all the cool things -- which is exactly what Microsoft did.

See this fiddle https://dotnetfiddle.net/VhZdLU (and improve it if possible) for running a simple console application which shows usages of Task, Task.WaitAll(), async and await operators in the same program.
This fiddle should clear your execution cycle concept.
Here is the sample code
using System;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
var a = MyMethodAsync(); //Task started for Execution and immediately goes to Line 19 of the code. Cursor will come back as soon as await operator is met
Console.WriteLine("Cursor Moved to Next Line Without Waiting for MyMethodAsync() completion");
Console.WriteLine("Now Waiting for Task to be Finished");
Task.WaitAll(a); //Now Waiting
Console.WriteLine("Exiting CommandLine");
}
public static async Task MyMethodAsync()
{
Task<int> longRunningTask = LongRunningOperation();
// independent work which doesn't need the result of LongRunningOperationAsync can be done here
Console.WriteLine("Independent Works of now executes in MyMethodAsync()");
//and now we call await on the task
int result = await longRunningTask;
//use the result
Console.WriteLine("Result of LongRunningOperation() is " + result);
}
public static async Task<int> LongRunningOperation() // assume we return an int from this long running operation
{
Console.WriteLine("LongRunningOperation() Started");
await Task.Delay(2000); // 2 second delay
Console.WriteLine("LongRunningOperation() Finished after 2 Seconds");
return 1;
}
}
Trace coming from Output Window:

I'd like to give my two cents to this, I'm sorry if any other answer contains what I will explain, I read most of it and haven't find it, but I could have missed something.
I saw a lot of missconceptions and a lot of good explanations, just want to explain async in terms of how it differs from parallel programming, that I believe will make things easier to understand.
When you need to do long computations, processor intensive work, you should opt to use parallel programming, if it's possible, to optimize cores usage. This opens some threads and process things simultaneosly.
Say you have an array of numbers and want to make some expensive long calculation with every and each one of than. Parallel is your friend.
Asyncronous programming is used in a different use case.
It's used to free your thread when you are waiting for something that do not depend on your processor, like IO for example (writing and reading to/from disk), your thread does nothing when you do IO, same thing when you are awaiting for some result from an expensive query to return from DB.
Async methods free your thread when it's waiting for something long to return results. This thread can be used by other parts of your application (in a web app it process other requests, for example) or can return to OS for other use.
When your result is done, the same thread (or another one) is given back to your application to resume processing.
Async programming is not mandatory (but a good practice) in a multithreaded environment like .net, in a web app other threads will respond to new requests, but if you are in a singlethreaded framework like nodejs it's mandatory, because you can't block your only thread, or you won't be able to anwser any other request.
To summarize, long processor intensive calculations will benefit more from parallel programming and long waiting periods that do not depend on your processor, like IO or DB query or a call to some API will benefit more from async programming.
That's why Entity Framework, for example, has an async api to save, list, find, etc...
Remember that async/await is not the same as wait or waitAll, the contexts are different. Async/await release the thread and are asyncronous programming. wait / waitAll blocks all threads (they are not released) to force syncronization in parallel context... different stuff...
Hope this is usefull for someone...

On a higher level:
1) Async keyword enables the await and that's all it does. Async keyword does not run the method in a separate thread. The beginning f async method runs synchronously until it hits await on a time-consuming task.
2) You can await on a method that returns Task or Task of type T. You cannot await on async void method.
3) The moment main thread encounters await on time-consuming task or when the actual work is started, the main thread returns to the caller of the current method.
4) If the main thread sees await on a task that is still executing, it doesn't wait for it and returns to the caller of the current method. In this way, the application remains responsive.
5) Await on processing task, will now execute on a separate thread from the thread pool.
6) When this await task is completed, all the code below it will be executed by the separate thread
Below is the sample code. Execute it and check the thread id
using System;
using System.Threading;
using System.Threading.Tasks;
namespace AsyncAwaitDemo
{
class Program
{
public static async void AsynchronousOperation()
{
Console.WriteLine("Inside AsynchronousOperation Before AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//Task<int> _task = AsyncMethod();
int count = await AsyncMethod();
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod Before Await, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//int count = await _task;
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await Before DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
DependentMethod(count);
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await After DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
}
public static async Task<int> AsyncMethod()
{
Console.WriteLine("Inside AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
int count = 0;
await Task.Run(() =>
{
Console.WriteLine("Executing a long running task which takes 10 seconds to complete, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(20000);
count = 10;
});
Console.WriteLine("Completed AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
return count;
}
public static void DependentMethod(int count)
{
Console.WriteLine("Inside DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId + ". Total count is " + count);
}
static void Main(string[] args)
{
Console.WriteLine("Started Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
AsynchronousOperation();
Console.WriteLine("Completed Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Console.ReadKey();
}
}
}

The way I understand it is also, there should be a third term added to the mix: Task.
Async is just a qualifier you put on your method to say it's an asynchronous method.
Task is the return of the async function. It executes asynchronously.
You await a Task. When code execution reaches this line, control jumps out back to caller of your surrounding original function.
If instead, you assign the return of an async function (ie Task) to a variable, when code execution reaches this line, it just continues past that line in the surrounding function while the Task executes asynchronously.

public static void Main(string[] args)
{
string result = DownloadContentAsync().Result;
Console.ReadKey();
}
// You use the async keyword to mark a method for asynchronous operations.
// The "async" modifier simply starts synchronously the current thread.
// What it does is enable the method to be split into multiple pieces.
// The boundaries of these pieces are marked with the await keyword.
public static async Task<string> DownloadContentAsync()// By convention, the method name ends with "Async
{
using (HttpClient client = new HttpClient())
{
// When you use the await keyword, the compiler generates the code that checks if the asynchronous operation is finished.
// If it is already finished, the method continues to run synchronously.
// If not completed, the state machine will connect a continuation method that must be executed WHEN the Task is completed.
// Http request example.
// (In this example I can set the milliseconds after "sleep=")
String result = await client.GetStringAsync("http://httpstat.us/200?sleep=1000");
Console.WriteLine(result);
// After completing the result response, the state machine will continue to synchronously execute the other processes.
return result;
}
}

The best example is here,enjoy:

is using them equal to spawning background threads to perform long
duration logic?
This article MDSN:Asynchronous Programming with async and await (C#) explains it explicitly:
The async and await keywords don't cause additional threads to be
created. Async methods don't require multithreading because an async
method doesn't run on its own thread. The method runs on the current
synchronization context and uses time on the thread only when the
method is active.

Below is code which reads excel file by opening dialog and then uses async and wait to run asynchronous the code which reads one by one line from excel and binds to grid
namespace EmailBillingRates
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
lblProcessing.Text = "";
}
private async void btnReadExcel_Click(object sender, EventArgs e)
{
string filename = OpenFileDialog();
Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename);
Microsoft.Office.Interop.Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
Microsoft.Office.Interop.Excel.Range xlRange = xlWorksheet.UsedRange;
try
{
Task<int> longRunningTask = BindGrid(xlRange);
int result = await longRunningTask;
}
catch (Exception ex)
{
MessageBox.Show(ex.Message.ToString());
}
finally
{
//cleanup
// GC.Collect();
//GC.WaitForPendingFinalizers();
//rule of thumb for releasing com objects:
// never use two dots, all COM objects must be referenced and released individually
// ex: [somthing].[something].[something] is bad
//release com objects to fully kill excel process from running in the background
Marshal.ReleaseComObject(xlRange);
Marshal.ReleaseComObject(xlWorksheet);
//close and release
xlWorkbook.Close();
Marshal.ReleaseComObject(xlWorkbook);
//quit and release
xlApp.Quit();
Marshal.ReleaseComObject(xlApp);
}
}
private void btnSendEmail_Click(object sender, EventArgs e)
{
}
private string OpenFileDialog()
{
string filename = "";
OpenFileDialog fdlg = new OpenFileDialog();
fdlg.Title = "Excel File Dialog";
fdlg.InitialDirectory = #"c:\";
fdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*";
fdlg.FilterIndex = 2;
fdlg.RestoreDirectory = true;
if (fdlg.ShowDialog() == DialogResult.OK)
{
filename = fdlg.FileName;
}
return filename;
}
private async Task<int> BindGrid(Microsoft.Office.Interop.Excel.Range xlRange)
{
lblProcessing.Text = "Processing File.. Please wait";
int rowCount = xlRange.Rows.Count;
int colCount = xlRange.Columns.Count;
// dt.Column = colCount;
dataGridView1.ColumnCount = colCount;
dataGridView1.RowCount = rowCount;
for (int i = 1; i <= rowCount; i++)
{
for (int j = 1; j <= colCount; j++)
{
//write the value to the Grid
if (xlRange.Cells[i, j] != null && xlRange.Cells[i, j].Value2 != null)
{
await Task.Delay(1);
dataGridView1.Rows[i - 1].Cells[j - 1].Value = xlRange.Cells[i, j].Value2.ToString();
}
}
}
lblProcessing.Text = "";
return 0;
}
}
internal class async
{
}
}

Answering your second question - WHEN to use async - here's a fairly easy approach we use:
Long-running I/O bound task that runs longer than 50ms - use async.
Long-running CPU-bound task - use parallel execution, threads etc.
Explanation: when you're doing I/O work - sending a network request, reading data from disk etc - the actual work is done by "external" silicon (network card, disk controller etc). Once the work is done - the I/O device driver will "ping" the OS back, and the OS will execute your continuation code, callback/etc. Until then the CPU is free to do it's own work (and as a bonus you might also free up a threadpool thread which is a very nice bonus for web app scalability)
P.S. The 50ms threshold is MS's recommendation. Otherwise the overhead added by async (creating the state machine, execution context etc) eats up all the benefits. Can't find the original MS article now, but it's mentioned here too https://www.red-gate.com/simple-talk/dotnet/net-framework/the-overhead-of-asyncawait-in-net-4-5/

The answers here are useful as a general guidance about await/async. They also contain some detail about how await/async is wired. I would like to share some practical experience with you that you should know before using this design pattern.
The term "await" is literal, so whatever thread you call it on will wait for the result of the method before continuing. On the foreground thread, this is a disaster. The foreground thread carries the burden of constructing your app, including views, view models, initial animations, and whatever else you have boot-strapped with those elements. So when you await the foreground thread, you stop the app. The user waits and waits when nothing appears to happen. This provides a negative user experience.
You can certainly await a background thread using a variety of means:
Device.BeginInvokeOnMainThread(async () => { await AnyAwaitableMethod(); });
// Notice that we do not await the following call,
// as that would tie it to the foreground thread.
try
{
Task.Run(async () => { await AnyAwaitableMethod(); });
}
catch
{}
The complete code for these remarks is at https://github.com/marcusts/xamarin-forms-annoyances. See the solution called AwaitAsyncAntipattern.sln.
The GitHub site also provides links to a more detailed discussion on this topic.

The async is used with a function to makes it into an asynchronous function. The await keyword is used to invoke an asynchronous function synchronously. The await keyword holds the JS engine execution until promise is resolved.
We should use async & await only when we want the result immediately. Maybe the result returned from the function is getting used in the next line.
Follow this blog, It is very well written in simple word

Maybe my insight is relevant. async tells the compiler to treat a function specially, the function is suspendable/resumable, it saves state in some way. await suspends a function, but is also a way to enforce discipline, is restrictive; you need to specify what you are waiting for, you can't just suspend without cause, which is what makes the code more readable and perhaps also more efficient. This opens up another question. Why not await multiple things, why just one at a time? I believe this is because such a pattern established itself and programmers are following the principle of least astonishment. There exists the possibility of ambiguity: are you satisfied with just one of conditions being fulfilled, or do you want all to be fulfilled, perhaps just some of them?

Does the following code sequence make sense in C#

I haven't used async/await very often and I'm not fully comfortable with them. I am trying to make an operation that is synchronous to be run asynchronously. Can you tell me if the following code snippet makes sense:
public static async Task<string> FileVersionAsync(string localFilePath)
{
FileVersionInfo versionInfo;
try
{
versionInfo = await Task.FromResult(FileVersionInfo.GetVersionInfo(localFilePath));
}
catch (SecurityException)
{
return "File Version Checker does not have permission to read file version";
}
catch (FileNotFoundException)
{
return "Unable to find configured file";
}
if (versionInfo.FileVersion == null)
return "N/A";
return versionInfo.FileVersion;
}

Adding Task, await and async do not make something asynchronous; they merely provide the plumbing to work with asynchronous code when it happens. In your example: it never happens asynchronously, so all you are doing is adding plumbing overhead for no benefit.
The compiler will generate a ton of extra code that will turn out to never be hit, because when it gets to the await it'll discover that the task is already complete, and will simply continue on the existing thread.
To actually be asynchronous, you need ... something that is async. This could be external IO, or could be something threading related - but note that simply jumping to another thread doesn't buy you anything: it just adds a context switch.
If there was a FileVersionInfo.GetVersionInfoAsync method, what you are doing might be worthwhile.

No it does not make sense.
The only reason to make your function async is if somewhere inside it awaits other functions that are async. In fact your compiler warns you if you forget to await somewhere.
The async-await syntax was invented as a replacement for other task functions like Task.ContinueWith, Task.FromResult, Task.FromException etc.
In this interview Eric Lippert compared async-await with a cook who has to prepare breakfast Search somewhere in the middle for async-await.
If a cook has to prepare breakfase he starts to boil water. But instead of waiting for the water to cook, he starts slicing bread, and do other things. Only after he has nothing to do anymore he starts waiting idly for the water to boil after which he makes the tea.
Similarly: if a program has to wait for an external process to perform a request, like a database query, write data to a file, get information from the internet etc. async-await makes sure that your thread doesn't wait idly. Instead your thread goes up its call stack to see if one of the callers can continue working without the result from the other process.
You'll see this in the following code:
public async Task<string> ReadTextFile()
{
StreamReader txtReader = File.OpenText(...);
// read the complete text file; don't wait until finished yet
Task<String> taskTxt = txtReader.ReadToEndAsync();
// as we didn't use await, we can continue processing:
DoSomething();
// now we need the result from the read action: await for it
// the result of await Task<TResult> is the TResult:
string importedTxt = await taskTxt;
return importedTxt;
}
Some guidelines for async-await:
Only use async-await if you call at least one other async function
instead of void return Task, instead of TResult return Task<TResult>
Only exception: the async event handler: this async function returns void
The return of await Task is void; the return of await Task<TResult> is TResult
If you don't need the result of an async function right now, and if you can do something meaningful while the task is processing: start the task by calling the async function, but don't await for it. Only await when you need the result
.
public async void Button_Clicked(object sender, EventArgs e)
{
// indicate you will do something time consuming:
this.ProgressBar1.Visible = true;
await DoSomethingTimeconsumingAsync(...);
// finished:
this.progressBar1.Visible = false;
}
This will make sure that whenever your thread has to wait something, it can do other things, and thus your GUI remains responsive.

No. There appears to be absolutely no need to make this method async, because Task.FromResult is ready immediately.

Await client async [duplicate]

From my understanding one of the main things that async and await do is to make code easy to write and read - but is using them equal to spawning background threads to perform long duration logic?
I'm currently trying out the most basic example. I've added some comments inline. Can you clarify it for me?
// I don't understand why this method must be marked as `async`.
private async void button1_Click(object sender, EventArgs e)
{
Task<int> access = DoSomethingAsync();
// task independent stuff here
// this line is reached after the 5 seconds sleep from
// DoSomethingAsync() method. Shouldn't it be reached immediately?
int a = 1;
// from my understanding the waiting should be done here.
int x = await access;
}
async Task<int> DoSomethingAsync()
{
// is this executed on a background thread?
System.Threading.Thread.Sleep(5000);
return 1;
}

When using async and await the compiler generates a state machine in the background.
Here's an example on which I hope I can explain some of the high-level details that are going on:
public async Task MyMethodAsync()
{
Task<int> longRunningTask = LongRunningOperationAsync();
// independent work which doesn't need the result of LongRunningOperationAsync can be done here
//and now we call await on the task
int result = await longRunningTask;
//use the result
Console.WriteLine(result);
}
public async Task<int> LongRunningOperationAsync() // assume we return an int from this long running operation
{
await Task.Delay(1000); // 1 second delay
return 1;
}
OK, so what happens here:
Task<int> longRunningTask = LongRunningOperationAsync(); starts executing LongRunningOperation
Independent work is done on let's assume the Main Thread (Thread ID = 1) then await longRunningTask is reached.
Now, if the longRunningTask hasn't finished and it is still running, MyMethodAsync() will return to its calling method, thus the main thread doesn't get blocked. When the longRunningTask is done then a thread from the ThreadPool (can be any thread) will return to MyMethodAsync() in its previous context and continue execution (in this case printing the result to the console).
A second case would be that the longRunningTask has already finished its execution and the result is available. When reaching the await longRunningTask we already have the result so the code will continue executing on the very same thread. (in this case printing result to console). Of course this is not the case for the above example, where there's a Task.Delay(1000) involved.

From my understanding one of the main things that async and await do is to make code easy to write and read.
They're to make asynchronous code easy to write and read, yes.
Is it the same thing as spawning background threads to perform long duration logic?
Not at all.
// I don't understand why this method must be marked as 'async'.
The async keyword enables the await keyword. So any method using await must be marked async.
// This line is reached after the 5 seconds sleep from DoSomethingAsync() method. Shouldn't it be reached immediately?
No, because async methods are not run on another thread by default.
// Is this executed on a background thread?
No.
You may find my async/await intro helpful. The official MSDN docs are also unusually good (particularly the TAP section), and the async team put out an excellent FAQ.

Explanation
Here is a quick example of async/await at a high level. There are a lot more details to consider beyond this.
Note: Task.Delay(1000) simulates doing work for 1 second. I think it's best to think of this as waiting for a response from an external resource. Since our code is waiting for a response, the system can set the running task off to the side and come back to it once it's finished. Meanwhile, it can do some other work on that thread.
In the example below, the first block is doing exactly that. It starts all the tasks immediately (the Task.Delay lines) and sets them off to the side. The code will pause on the await a line until the 1 second delay is done before going to the next line. Since b, c, d, and e all started executing at almost the exact same time as a (due to lack of the await), they should finish at roughly the same time in this case.
In the example below, the second block is starting a task and waiting for it to finish (that is what await does) before starting the subsequent tasks. Each iteration of this takes 1 second. The await is pausing the program and waiting for the result before continuing. This is the main difference between the first and second blocks.
Example
Console.WriteLine(DateTime.Now);
// This block takes 1 second to run because all
// 5 tasks are running simultaneously
{
var a = Task.Delay(1000);
var b = Task.Delay(1000);
var c = Task.Delay(1000);
var d = Task.Delay(1000);
var e = Task.Delay(1000);
await a;
await b;
await c;
await d;
await e;
}
Console.WriteLine(DateTime.Now);
// This block takes 5 seconds to run because each "await"
// pauses the code until the task finishes
{
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
await Task.Delay(1000);
}
Console.WriteLine(DateTime.Now);
OUTPUT:
5/24/2017 2:22:50 PM
5/24/2017 2:22:51 PM (First block took 1 second)
5/24/2017 2:22:56 PM (Second block took 5 seconds)
Extra info regarding SynchronizationContext
Note: This is where things get a little foggy for me, so if I'm wrong on anything, please correct me and I will update the answer. It's important to have a basic understanding of how this works but you can get by without being an expert on it as long as you never use ConfigureAwait(false), although you will likely lose out on some opportunity for optimization, I assume.
There is one aspect of this which makes the async/await concept somewhat trickier to grasp. That's the fact that in this example, this is all happening on the same thread (or at least what appears to be the same thread in regards to its SynchronizationContext). By default, await will restore the synchronization context of the original thread that it was running on. For example, in ASP.NET you have an HttpContext which is tied to a thread when a request comes in. This context contains things specific to the original Http request such as the original Request object which has things like language, IP address, headers, etc. If you switch threads halfway through processing something, you could potentially end up trying to pull information out of this object on a different HttpContext which could be disastrous. If you know you won't be using the context for anything, you can choose to "not care" about it. This basically allows your code to run on a separate thread without bringing the context around with it.
How do you achieve this? By default, the await a; code actually is making an assumption that you DO want to capture and restore the context:
await a; //Same as the line below
await a.ConfigureAwait(true);
If you want to allow the main code to continue on a new thread without the original context, you simply use false instead of true so it knows it doesn't need to restore the context.
await a.ConfigureAwait(false);
After the program is done being paused, it will continue potentially on an entirely different thread with a different context. This is where the performance improvement would come from -- it could continue on on any available thread without having to restore the original context it started with.
Is this stuff confusing? Hell yeah! Can you figure it out? Probably! Once you have a grasp of the concepts, then move on to Stephen Cleary's explanations which tend to be geared more toward someone with a technical understanding of async/await already.

Further to the other answers, have a look at await (C# Reference)
and more specifically at the example included, it explains your situation a bit
The following Windows Forms example illustrates the use of await in an
async method, WaitAsynchronouslyAsync. Contrast the behavior of that
method with the behavior of WaitSynchronously. Without an await
operator applied to a task, WaitSynchronously runs synchronously
despite the use of the async modifier in its definition and a call to
Thread.Sleep in its body.
private async void button1_Click(object sender, EventArgs e)
{
// Call the method that runs asynchronously.
string result = await WaitAsynchronouslyAsync();
// Call the method that runs synchronously.
//string result = await WaitSynchronously ();
// Display the result.
textBox1.Text += result;
}
// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
await Task.Delay(10000);
return "Finished";
}
// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
// Add a using directive for System.Threading.
Thread.Sleep(10000);
return "Finished";
}

For fastest learning..
Understand method execution flow(with a diagram): 3 mins
Question introspection (learning sake): 1 min
Quickly get through syntax sugar: 5 mins
Share the confusion of a developer : 5 mins
Problem: Quickly change a real-world implementation of normal code to
Async code: 2 mins
Where to Next?
Understand method execution flow(with a diagram): 3 mins
In this image, just focus on #6 (nothing more)
At #6 step, execution ran out of work and stopped. To continue it needs a result from getStringTask(kind of a function). Therefore, it uses an await operator to suspend its progress and give control back(yield) to the caller(of this method we are in). The actual call to getStringTask was made earlier in #2. At #2 a promise was made to return a string result. But when will it return the result? Should we(#1:AccessTheWebAsync) make a 2nd call again? Who gets the result, #2(calling statement) or #6(awaiting statement)?
The external caller of AccessTheWebAsync() also is waiting now. So caller waiting for AccessTheWebAsync, and AccessTheWebAsync is waiting for GetStringAsync at the moment. Interesting thing is AccessTheWebAsync did some work(#4) before waiting perhaps to save time from waiting. The same freedom to multitask is also available for the external caller(and all callers in the chain) and this is the biggest plus of this 'async' thingy! You feel like it is synchronous..or normal but it is not.
#2 and #6 is split so we have the advantage of #4(work while waiting). But we can also do it without splitting. So #2 will be: string urlContents = await client.GetStringAsync("...");. Here we see no advantage but somewhere in the chain one function will be splitting while rest of them call it without splitting. It depends which function/class in the chain you use. This change in behavior from function to function is the most confusing part about this topic.
Remember, the method was already returned(#2), it cannot return again(no second time). So how will the caller know? It is all about Tasks! Task was returned. Task status was waited for (not method, not value). Value will be set in Task. Task status will be set to complete. Caller just monitors Task(#6). So 6# is the answer to where/who gets the result. Further reads for later here.
Question introspection for learning sake: 1 min
Let us adjust the question a bit:
How and When to use async and await Tasks?
Because learning Task automatically covers the other two(and answers your question).
The whole idea is pretty simple. A method can return any data type(double, int, object, etc.) but here we just deny that and force a 'Task' object return! But we still need the returned data(except void), right? That will be set in a standard property inside 'Task' object eg: 'Result' property.
Quickly get through syntax sugar: 5 mins
Original non-async method
internal static int Method(int arg0, int arg1)
{
int result = arg0 + arg1;
IO(); // Do some long running IO.
return result;
}
a brand new Task-ified method to call the above method
internal static Task<int> MethodTask(int arg0, int arg1)
{
Task<int> task = new Task<int>(() => Method(arg0, arg1));
task.Start(); // Hot task (started task) should always be returned.
return task;
}
Did we mention await or async? No. Call the above method and you get a task which you can monitor. You already know what the task returns(or contains).. an integer.
Calling a Task is slightly tricky and that is when the keywords starts to appear. If there was a method calling the original method(non-async) then we need to edit it as given below. Let us call MethodTask()
internal static async Task<int> MethodAsync(int arg0, int arg1)
{
int result = await HelperMethods.MethodTask(arg0, arg1);
return result;
}
Same code above added as image below:
We are 'awaiting' task to be finished. Hence the await(mandatory syntax)
Since we use await, we must use async(mandatory syntax)
MethodAsync with Async as the prefix (coding standard)
await is easy to understand but the remaining two (async,Async) may not be :). Well, it should make a lot more sense to the compiler though.Further reads for later here
So there are 2 parts.
Create 'Task' (only one task and it will be an additional method)
Create syntactic sugar to call the task with await+async(this involves changing existing code if you are converting a non-async method)
Remember, we had an external caller to AccessTheWebAsync() and that caller is not spared either... i.e it needs the same await+async too. And the chain continues(hence this is a breaking change which could affect many classes). It can also be considered a non-breaking change because the original method is still there to be called. Change it's access (or delete and move it inside a task) if you want to impose a breaking change and then the classes will be forced to use Task-method. Anyways, in an async call there will always be a Task at one end and only one.
All okay, but one developer was surprised to see Task
missing...
Share the confusion of a developer: 5 mins
A developer has made a mistake of not implementing Task but it still works! Try to understand the question and just the accepted answer provided here. Hope you have read and fully understood. The summary is that we may not see/implement 'Task' but it is implemented somewhere in a parent/associated class. Likewise in our example calling an already built MethodAsync() is way easier than implementing that method with a Task (MethodTask()) ourself. Most developers find it difficult to get their head around Tasks while converting a code to Asynchronous one.
Tip: Try to find an existing Async implementation (like MethodAsync or ToListAsync) to outsource the difficulty. So we only need to deal with Async and await (which is easy and pretty similar to normal code)
Problem: Quickly change a real-world implementation of normal code to
Async operation: 2 mins
Code line shown below in Data Layer started to break(many places). Because we updated some of our code from .Net framework 4.2.* to .Net core. We had to fix this in 1 hour all over the application!
var myContract = query.Where(c => c.ContractID == _contractID).First();
easypeasy!
We installed EntityFramework nuget package because it has QueryableExtensions. Or in other words it does the Async implementation(task), so we could survive with simple Async and await in code.
namespace = Microsoft.EntityFrameworkCore
calling code line got changed like this
var myContract = await query.Where(c => c.ContractID == _contractID).FirstAsync();
Method signature changed from
Contract GetContract(int contractnumber)
to
async Task<Contract> GetContractAsync(int contractnumber)
calling method also got affected: GetContract(123456); was called as GetContractAsync(123456).Result;
Wait! what is that Result? Good catch! GetContractAsync only returns a Task not the value we wanted(Contract). Once the result of an operation is available, it is stored and is returned immediately on subsequent calls to the Result property.
We can also do a time-out implementation with a similar 'Wait()'
TimeSpan ts = TimeSpan.FromMilliseconds(150);
if (! t.Wait(ts))
Console.WriteLine("The timeout interval elapsed.");
We changed it everywhere in 30 minutes!
But the architect told us not to use EntityFramework library just for this! oops! drama! Then we made a custom Task implementation(yuk!). Which you know how. Still easy! ..still yuk..
Where to Next?
There is a wonderful quick video we could watch about Converting Synchronous Calls to Asynchronous in ASP.Net Core, perhaps that is likely the direction one would go after reading this. Or have I explained enough? ;)

Showing the above explanations in action in a simple console program:
class Program
{
static void Main(string[] args)
{
TestAsyncAwaitMethods();
Console.WriteLine("Press any key to exit...");
Console.ReadLine();
}
public async static void TestAsyncAwaitMethods()
{
await LongRunningMethod();
}
public static async Task<int> LongRunningMethod()
{
Console.WriteLine("Starting Long Running method...");
await Task.Delay(5000);
Console.WriteLine("End Long Running method...");
return 1;
}
}
And the output is:
Starting Long Running method...
Press any key to exit...
End Long Running method...
Thus,
Main starts the long running method via TestAsyncAwaitMethods. That immediately returns without halting the current thread and we immediately see 'Press any key to exit' message
All this while, the LongRunningMethod is running in the background. Once its completed, another thread from Threadpool picks up this context and displays the final message
Thus, not thread is blocked.

I think you've picked a bad example with System.Threading.Thread.Sleep
Point of an async Task is to let it execute in background without locking the main thread, such as doing a DownloadFileAsync
System.Threading.Thread.Sleep isn't something that is "being done", it just sleeps, and therefore your next line is reached after 5 seconds ...
Read this article, I think it is a great explanation of async and await concept: http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx

Async & Await Simple Explanation
Simple Analogy
A person may wait for their morning train. This is all they are doing as this is their primary task that they are currently performing. (synchronous programming (what you normally do!))
Another person may await their morning train whilst they smoke a cigarette and then drink their coffee. (Asynchronous programming)
What is asynchronous programming?
Asynchronous programming is where a programmer will choose to run some of his code on a separate thread from the main thread of execution and then notify the main thread on it's completion.
What does the async keyword actually do?
Prefixing the async keyword to a method name like
async void DoSomething(){ . . .
allows the programmer to use the await keyword when calling asynchronous tasks. That's all it does.
Why is this important?
In a lot of software systems the main thread is reserved for operations specifically relating to the User Interface. If I am running a very complex recursive algorithm that takes 5 seconds to complete on my computer, but I am running this on the Main Thread (UI thread) When the user tries to click on anything on my application, it will appear to be frozen as my main thread has queued and is currently processing far too many operations. As a result the main thread cannot process the mouse click to run the method from the button click.
When do you use Async and Await?
Use the asynchronous keywords ideally when you are doing anything that doesn't involve the user interface.
So lets say you're writing a program that allows the user to sketch on their mobile phone but every 5 seconds it is going to be checking the weather on the internet.
We should be awaiting the call the polling calls every 5 seconds to the network to get the weather as the user of the application needs to keep interacting with the mobile touch screen to draw pretty pictures.
How do you use Async and Await
Following on from the example above, here is some pseudo code of how to write it:
//ASYNCHRONOUS
//this is called using the await keyword every 5 seconds from a polling timer or something.
async Task CheckWeather()
{
var weather = await GetWeather();
//do something with the weather now you have it
}
async Task<WeatherResult> GetWeather()
{
var weatherJson = await CallToNetworkAddressToGetWeather();
return deserializeJson<weatherJson>(weatherJson);
}
//SYNCHRONOUS
//This method is called whenever the screen is pressed
void ScreenPressed()
{
DrawSketchOnScreen();
}
Additional Notes - Update
I forgot to mention in my original notes that in C# you can only await methods that are wrapped in Tasks. for example you may await this method:
// awaiting this will return a string.
// calling this without await (synchronously) will result in a Task<string> object.
async Task<string> FetchHelloWorld() {..
You cannot await methods that are not tasks like this:
async string FetchHelloWorld() {..
Feel free to review the source code for the Task class here.

Here is a quick console program to make it clear to those who follow. The TaskToDo method is your long running method that you want to make async. Making it run async is done by the TestAsync method. The test loops method just runs through the TaskToDo tasks and runs them async. You can see that in the results because they don't complete in the same order from run to run - they are reporting to the console UI thread when they complete. Simplistic, but I think the simplistic examples bring out the core of the pattern better than more involved examples:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace TestingAsync
{
class Program
{
static void Main(string[] args)
{
TestLoops();
Console.Read();
}
private static async void TestLoops()
{
for (int i = 0; i < 100; i++)
{
await TestAsync(i);
}
}
private static Task TestAsync(int i)
{
return Task.Run(() => TaskToDo(i));
}
private async static void TaskToDo(int i)
{
await Task.Delay(10);
Console.WriteLine(i);
}
}
}

All the answers here use Task.Delay() or some other built in async function. But here is my example that use none of those async functions:
// Starts counting to a large number and then immediately displays message "I'm counting...".
// Then it waits for task to finish and displays "finished, press any key".
static void asyncTest ()
{
Console.WriteLine("Started asyncTest()");
Task<long> task = asyncTest_count();
Console.WriteLine("Started counting, please wait...");
task.Wait(); // if you comment this line you will see that message "Finished counting" will be displayed before we actually finished counting.
//Console.WriteLine("Finished counting to " + task.Result.ToString()); // using task.Result seems to also call task.Wait().
Console.WriteLine("Finished counting.");
Console.WriteLine("Press any key to exit program.");
Console.ReadLine();
}
static async Task<long> asyncTest_count()
{
long k = 0;
Console.WriteLine("Started asyncTest_count()");
await Task.Run(() =>
{
long countTo = 100000000;
int prevPercentDone = -1;
for (long i = 0; i <= countTo; i++)
{
int percentDone = (int)(100 * (i / (double)countTo));
if (percentDone != prevPercentDone)
{
prevPercentDone = percentDone;
Console.Write(percentDone.ToString() + "% ");
}
k = i;
}
});
Console.WriteLine("");
Console.WriteLine("Finished asyncTest_count()");
return k;
}

This answer aims to provide some info specific to ASP.NET.
By utilizing async/await in the MVC controller, it is possible to increase thread pool utilization and achieve much better throughput, as explained in the below article,
http://www.asp.net/mvc/tutorials/mvc-4/using-asynchronous-methods-in-aspnet-mvc-4
In web applications that see a large number of concurrent requests at
start-up or have a bursty load (where concurrency increases suddenly),
making these web service calls asynchronous will increase the
responsiveness of your application. An asynchronous request takes the
same amount of time to process as a synchronous request. For example,
if a request makes a web service call that requires two seconds to
complete, the request takes two seconds whether it is performed
synchronously or asynchronously. However, during an asynchronous call,
a thread is not blocked from responding to other requests while it
waits for the first request to complete. Therefore, asynchronous
requests prevent request queuing and thread pool growth when there are
many concurrent requests that invoke long-running operations.

Async / Await
Actually, Async / Await is a pair of keywords that are just syntactic sugar for creating a callback of an asynchronous task.
Take by example this operation:
public static void DoSomeWork()
{
var task = Task.Run(() =>
{
// [RUNS ON WORKER THREAD]
// IS NOT bubbling up due to the different threads
throw new Exception();
Thread.Sleep(2000);
return "Hello";
});
// This is the callback
task.ContinueWith((t) => {
// -> Exception is swallowed silently
Console.WriteLine("Completed");
// [RUNS ON WORKER THREAD]
});
}
The code above has several disadvantages. Errors are not passed on and it's hard to read.
But Async and Await come in to help us out:
public async static void DoSomeWork()
{
var result = await Task.Run(() =>
{
// [RUNS ON WORKER THREAD]
// IS bubbling up
throw new Exception();
Thread.Sleep(2000);
return "Hello";
});
// every thing below is a callback
// (including the calling methods)
Console.WriteLine("Completed");
}
Await calls have to be in Async methods. This has some advantages:
Returns the result of the Task
creates automatically a callback
checks for errors and lets them bubble up in callstack (only up to none-await calls in callstack)
waits for the result
frees up the main thread
runs the callback on the main thread
uses a worker thread from the threadpool for the task
makes the code easy to read
and a lot more
NOTE: Async and Await are used with asynchronous calls not to make these. You have to use Task Libary for this, like Task.Run() .
Here is a comparison between await and none await solutions
This is the none async solution:
public static long DoTask()
{
stopWatch.Reset();
stopWatch.Start();
// [RUNS ON MAIN THREAD]
var task = Task.Run(() => {
Thread.Sleep(2000);
// [RUNS ON WORKER THREAD]
});
// goes directly further
// WITHOUT waiting until the task is finished
// [RUNS ON MAIN THREAD]
stopWatch.Stop();
// 50 milliseconds
return stopWatch.ElapsedMilliseconds;
}
This is the async method:
public async static Task<long> DoAwaitTask()
{
stopWatch.Reset();
stopWatch.Start();
// [RUNS ON MAIN THREAD]
await Task.Run(() => {
Thread.Sleep(2000);
// [RUNS ON WORKER THREAD]
});
// Waits until task is finished
// [RUNS ON MAIN THREAD]
stopWatch.Stop();
// 2050 milliseconds
return stopWatch.ElapsedMilliseconds;
}
You can actually call an async method without the await keyword but this means that any Exception here is swallowed in release mode:
public static Stopwatch stopWatch { get; } = new Stopwatch();
static void Main(string[] args)
{
Console.WriteLine("DoAwaitTask: " + DoAwaitTask().Result + " ms");
// 2050 (2000 more because of the await)
Console.WriteLine("DoTask: " + DoTask() + " ms");
// 50
Console.ReadKey();
}
Async and Await are not meant for parallel computing. They are used to not block your main thread. When it's about asp.net or Windows applications, blocking your main thread due to a network call is a bad thing. If you do this, your app will get unresponsive or even crash.
Check out MS docs for more examples.

To be honest I still think the best explanation is the one about future and promises on the Wikipedia: http://en.wikipedia.org/wiki/Futures_and_promises
The basic idea is that you have a separate pool of threads that execute tasks asynchronously. When using it. The object does however make the promise that it will execute the operation at some time and give you the result when you request it. This means that it will block when you request the result and hasn't finished, but execute in the thread pool otherwise.
From there you can optimize things: some operations can be implemented async and you can optimize things like file IO and network communication by batching together subsequent requests and/or reordering them. I'm not sure if this is already in the task framework of Microsoft - but if it isn't that would be one of the first things I would add.
You can actually implement the future pattern sort-of with yields in C# 4.0. If you want to know how it works exactly, I can recommend this link that does a decent job: http://code.google.com/p/fracture/source/browse/trunk/Squared/TaskLib/ . However, if you start toying with it yourself, you will notice that you really need language support if you want to do all the cool things -- which is exactly what Microsoft did.

See this fiddle https://dotnetfiddle.net/VhZdLU (and improve it if possible) for running a simple console application which shows usages of Task, Task.WaitAll(), async and await operators in the same program.
This fiddle should clear your execution cycle concept.
Here is the sample code
using System;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
var a = MyMethodAsync(); //Task started for Execution and immediately goes to Line 19 of the code. Cursor will come back as soon as await operator is met
Console.WriteLine("Cursor Moved to Next Line Without Waiting for MyMethodAsync() completion");
Console.WriteLine("Now Waiting for Task to be Finished");
Task.WaitAll(a); //Now Waiting
Console.WriteLine("Exiting CommandLine");
}
public static async Task MyMethodAsync()
{
Task<int> longRunningTask = LongRunningOperation();
// independent work which doesn't need the result of LongRunningOperationAsync can be done here
Console.WriteLine("Independent Works of now executes in MyMethodAsync()");
//and now we call await on the task
int result = await longRunningTask;
//use the result
Console.WriteLine("Result of LongRunningOperation() is " + result);
}
public static async Task<int> LongRunningOperation() // assume we return an int from this long running operation
{
Console.WriteLine("LongRunningOperation() Started");
await Task.Delay(2000); // 2 second delay
Console.WriteLine("LongRunningOperation() Finished after 2 Seconds");
return 1;
}
}
Trace coming from Output Window:

I'd like to give my two cents to this, I'm sorry if any other answer contains what I will explain, I read most of it and haven't find it, but I could have missed something.
I saw a lot of missconceptions and a lot of good explanations, just want to explain async in terms of how it differs from parallel programming, that I believe will make things easier to understand.
When you need to do long computations, processor intensive work, you should opt to use parallel programming, if it's possible, to optimize cores usage. This opens some threads and process things simultaneosly.
Say you have an array of numbers and want to make some expensive long calculation with every and each one of than. Parallel is your friend.
Asyncronous programming is used in a different use case.
It's used to free your thread when you are waiting for something that do not depend on your processor, like IO for example (writing and reading to/from disk), your thread does nothing when you do IO, same thing when you are awaiting for some result from an expensive query to return from DB.
Async methods free your thread when it's waiting for something long to return results. This thread can be used by other parts of your application (in a web app it process other requests, for example) or can return to OS for other use.
When your result is done, the same thread (or another one) is given back to your application to resume processing.
Async programming is not mandatory (but a good practice) in a multithreaded environment like .net, in a web app other threads will respond to new requests, but if you are in a singlethreaded framework like nodejs it's mandatory, because you can't block your only thread, or you won't be able to anwser any other request.
To summarize, long processor intensive calculations will benefit more from parallel programming and long waiting periods that do not depend on your processor, like IO or DB query or a call to some API will benefit more from async programming.
That's why Entity Framework, for example, has an async api to save, list, find, etc...
Remember that async/await is not the same as wait or waitAll, the contexts are different. Async/await release the thread and are asyncronous programming. wait / waitAll blocks all threads (they are not released) to force syncronization in parallel context... different stuff...
Hope this is usefull for someone...

On a higher level:
1) Async keyword enables the await and that's all it does. Async keyword does not run the method in a separate thread. The beginning f async method runs synchronously until it hits await on a time-consuming task.
2) You can await on a method that returns Task or Task of type T. You cannot await on async void method.
3) The moment main thread encounters await on time-consuming task or when the actual work is started, the main thread returns to the caller of the current method.
4) If the main thread sees await on a task that is still executing, it doesn't wait for it and returns to the caller of the current method. In this way, the application remains responsive.
5) Await on processing task, will now execute on a separate thread from the thread pool.
6) When this await task is completed, all the code below it will be executed by the separate thread
Below is the sample code. Execute it and check the thread id
using System;
using System.Threading;
using System.Threading.Tasks;
namespace AsyncAwaitDemo
{
class Program
{
public static async void AsynchronousOperation()
{
Console.WriteLine("Inside AsynchronousOperation Before AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//Task<int> _task = AsyncMethod();
int count = await AsyncMethod();
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod Before Await, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//int count = await _task;
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await Before DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
DependentMethod(count);
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await After DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
}
public static async Task<int> AsyncMethod()
{
Console.WriteLine("Inside AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
int count = 0;
await Task.Run(() =>
{
Console.WriteLine("Executing a long running task which takes 10 seconds to complete, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(20000);
count = 10;
});
Console.WriteLine("Completed AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
return count;
}
public static void DependentMethod(int count)
{
Console.WriteLine("Inside DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId + ". Total count is " + count);
}
static void Main(string[] args)
{
Console.WriteLine("Started Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
AsynchronousOperation();
Console.WriteLine("Completed Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Console.ReadKey();
}
}
}

The way I understand it is also, there should be a third term added to the mix: Task.
Async is just a qualifier you put on your method to say it's an asynchronous method.
Task is the return of the async function. It executes asynchronously.
You await a Task. When code execution reaches this line, control jumps out back to caller of your surrounding original function.
If instead, you assign the return of an async function (ie Task) to a variable, when code execution reaches this line, it just continues past that line in the surrounding function while the Task executes asynchronously.

public static void Main(string[] args)
{
string result = DownloadContentAsync().Result;
Console.ReadKey();
}
// You use the async keyword to mark a method for asynchronous operations.
// The "async" modifier simply starts synchronously the current thread.
// What it does is enable the method to be split into multiple pieces.
// The boundaries of these pieces are marked with the await keyword.
public static async Task<string> DownloadContentAsync()// By convention, the method name ends with "Async
{
using (HttpClient client = new HttpClient())
{
// When you use the await keyword, the compiler generates the code that checks if the asynchronous operation is finished.
// If it is already finished, the method continues to run synchronously.
// If not completed, the state machine will connect a continuation method that must be executed WHEN the Task is completed.
// Http request example.
// (In this example I can set the milliseconds after "sleep=")
String result = await client.GetStringAsync("http://httpstat.us/200?sleep=1000");
Console.WriteLine(result);
// After completing the result response, the state machine will continue to synchronously execute the other processes.
return result;
}
}

The best example is here,enjoy:

is using them equal to spawning background threads to perform long
duration logic?
This article MDSN:Asynchronous Programming with async and await (C#) explains it explicitly:
The async and await keywords don't cause additional threads to be
created. Async methods don't require multithreading because an async
method doesn't run on its own thread. The method runs on the current
synchronization context and uses time on the thread only when the
method is active.

Below is code which reads excel file by opening dialog and then uses async and wait to run asynchronous the code which reads one by one line from excel and binds to grid
namespace EmailBillingRates
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
lblProcessing.Text = "";
}
private async void btnReadExcel_Click(object sender, EventArgs e)
{
string filename = OpenFileDialog();
Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename);
Microsoft.Office.Interop.Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
Microsoft.Office.Interop.Excel.Range xlRange = xlWorksheet.UsedRange;
try
{
Task<int> longRunningTask = BindGrid(xlRange);
int result = await longRunningTask;
}
catch (Exception ex)
{
MessageBox.Show(ex.Message.ToString());
}
finally
{
//cleanup
// GC.Collect();
//GC.WaitForPendingFinalizers();
//rule of thumb for releasing com objects:
// never use two dots, all COM objects must be referenced and released individually
// ex: [somthing].[something].[something] is bad
//release com objects to fully kill excel process from running in the background
Marshal.ReleaseComObject(xlRange);
Marshal.ReleaseComObject(xlWorksheet);
//close and release
xlWorkbook.Close();
Marshal.ReleaseComObject(xlWorkbook);
//quit and release
xlApp.Quit();
Marshal.ReleaseComObject(xlApp);
}
}
private void btnSendEmail_Click(object sender, EventArgs e)
{
}
private string OpenFileDialog()
{
string filename = "";
OpenFileDialog fdlg = new OpenFileDialog();
fdlg.Title = "Excel File Dialog";
fdlg.InitialDirectory = #"c:\";
fdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*";
fdlg.FilterIndex = 2;
fdlg.RestoreDirectory = true;
if (fdlg.ShowDialog() == DialogResult.OK)
{
filename = fdlg.FileName;
}
return filename;
}
private async Task<int> BindGrid(Microsoft.Office.Interop.Excel.Range xlRange)
{
lblProcessing.Text = "Processing File.. Please wait";
int rowCount = xlRange.Rows.Count;
int colCount = xlRange.Columns.Count;
// dt.Column = colCount;
dataGridView1.ColumnCount = colCount;
dataGridView1.RowCount = rowCount;
for (int i = 1; i <= rowCount; i++)
{
for (int j = 1; j <= colCount; j++)
{
//write the value to the Grid
if (xlRange.Cells[i, j] != null && xlRange.Cells[i, j].Value2 != null)
{
await Task.Delay(1);
dataGridView1.Rows[i - 1].Cells[j - 1].Value = xlRange.Cells[i, j].Value2.ToString();
}
}
}
lblProcessing.Text = "";
return 0;
}
}
internal class async
{
}
}

Answering your second question - WHEN to use async - here's a fairly easy approach we use:
Long-running I/O bound task that runs longer than 50ms - use async.
Long-running CPU-bound task - use parallel execution, threads etc.
Explanation: when you're doing I/O work - sending a network request, reading data from disk etc - the actual work is done by "external" silicon (network card, disk controller etc). Once the work is done - the I/O device driver will "ping" the OS back, and the OS will execute your continuation code, callback/etc. Until then the CPU is free to do it's own work (and as a bonus you might also free up a threadpool thread which is a very nice bonus for web app scalability)
P.S. The 50ms threshold is MS's recommendation. Otherwise the overhead added by async (creating the state machine, execution context etc) eats up all the benefits. Can't find the original MS article now, but it's mentioned here too https://www.red-gate.com/simple-talk/dotnet/net-framework/the-overhead-of-asyncawait-in-net-4-5/

The answers here are useful as a general guidance about await/async. They also contain some detail about how await/async is wired. I would like to share some practical experience with you that you should know before using this design pattern.
The term "await" is literal, so whatever thread you call it on will wait for the result of the method before continuing. On the foreground thread, this is a disaster. The foreground thread carries the burden of constructing your app, including views, view models, initial animations, and whatever else you have boot-strapped with those elements. So when you await the foreground thread, you stop the app. The user waits and waits when nothing appears to happen. This provides a negative user experience.
You can certainly await a background thread using a variety of means:
Device.BeginInvokeOnMainThread(async () => { await AnyAwaitableMethod(); });
// Notice that we do not await the following call,
// as that would tie it to the foreground thread.
try
{
Task.Run(async () => { await AnyAwaitableMethod(); });
}
catch
{}
The complete code for these remarks is at https://github.com/marcusts/xamarin-forms-annoyances. See the solution called AwaitAsyncAntipattern.sln.
The GitHub site also provides links to a more detailed discussion on this topic.

The async is used with a function to makes it into an asynchronous function. The await keyword is used to invoke an asynchronous function synchronously. The await keyword holds the JS engine execution until promise is resolved.
We should use async & await only when we want the result immediately. Maybe the result returned from the function is getting used in the next line.
Follow this blog, It is very well written in simple word

Maybe my insight is relevant. async tells the compiler to treat a function specially, the function is suspendable/resumable, it saves state in some way. await suspends a function, but is also a way to enforce discipline, is restrictive; you need to specify what you are waiting for, you can't just suspend without cause, which is what makes the code more readable and perhaps also more efficient. This opens up another question. Why not await multiple things, why just one at a time? I believe this is because such a pattern established itself and programmers are following the principle of least astonishment. There exists the possibility of ambiguity: are you satisfied with just one of conditions being fulfilled, or do you want all to be fulfilled, perhaps just some of them?

Why is this code running synchronously?

I am trying to understand concurrency by doing it in code. I have a code snippet which I thought was running asynchronously. But when I put the debug writeline statements in, I found that it is running synchronously. Can someone explain what I need to do differently to push ComputeBB() onto another thread using Task.Something?
Clarification I want this code to run ComputeBB in some other thread so that the main thread will keep on running without blocking.
Here is the code:
{
// part of the calling method
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = await Task.Run(() => returnDTM.ComputeBB());
Debug.WriteLine("Just called await ComputBB.");
return returnDTM;
}
private ptsBoundingBox2d ComputeBB()
{
Debug.WriteLine("Starting ComputeBB.");
Stopwatch sw = new Stopwatch(); sw.Start();
var point1 = this.allPoints.FirstOrDefault().Value;
var returnBB = new ptsBoundingBox2d(
point1.x, point1.y, point1.z, point1.x, point1.y, point1.z);
Parallel.ForEach(this.allPoints,
p => returnBB.expandByPoint(p.Value.x, p.Value.y, p.Value.z)
);
sw.Stop();
Debug.WriteLine(String.Format("Compute BB took {0}", sw.Elapsed));
return returnBB;
}
Here is the output in the immediate window:
About to call ComputeBB
Starting ComputeBB.
Compute BB took 00:00:00.1790574
Just called await ComputBB.
Clarification If it were really running asynchronously it would be in this order:
About to call ComputeBB
Just called await ComputBB.
Starting ComputeBB.
Compute BB took 00:00:00.1790574
But it is not.
Elaboration
The calling code has signature like so: private static async Task loadAsBinaryAsync(string fileName) At the next level up, though, I attempt to stop using async. So here is the call stack from top to bottom:
static void Main(string[] args)
{
aTinFile = ptsDTM.CreateFromExistingFile("TestSave.ptsTin");
// more stuff
}
public static ptsDTM CreateFromExistingFile(string fileName)
{
ptsDTM returnTin = new ptsDTM();
Task<ptsDTM> tsk = Task.Run(() => loadAsBinaryAsync(fileName));
returnTin = tsk.Result; // I suspect the problem is here.
return retunTin;
}
private static async Task<ptsDTM> loadAsBinaryAsync(string fileName)
{
// do a lot of processing
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = await Task.Run(() => returnDTM.ComputeBB());
Debug.WriteLine("Just called await ComputBB.");
return returnDTM;
}

I have a code snippet which I thought was running asynchronously. But when I put the debug writeline statements in, I found that it is running synchronously.
await is used to asynchronously wait an operations completion. While doing so, it yields control back to the calling method until it's completion.
what I need to do differently to push ComputeBB() onto another thread
It is already ran on a thread pool thread. If you don't want to asynchronously wait on it in a "fire and forget" fashion, don't await the expression. Note this will have an effect on exception handling. Any exception which occurs inside the provided delegate would be captured inside the given Task, if you don't await, there is a chance they will go about unhandled.
Edit:
Lets look at this piece of code:
public static ptsDTM CreateFromExistingFile(string fileName)
{
ptsDTM returnTin = new ptsDTM();
Task<ptsDTM> tsk = Task.Run(() => loadAsBinaryAsync(fileName));
returnTin = tsk.Result; // I suspect the problem is here.
return retunTin;
}
What you're currently doing is synchronously blocking when you use tsk.Result. Also, for some reason you're calling Task.Run twice, once in each method. That is unnecessary. If you want to return your ptsDTM instance from CreateFromExistingFile, you will have to await it, there is no getting around that. "Fire and Forget" execution doesn't care about the result, at all. It simply wants to start whichever operation it needs, if it fails or succeeds is usually a non-concern. That is clearly not the case here.
You'll need to do something like this:
private PtsDtm LoadAsBinary(string fileName)
{
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = returnDTM.ComputeBB();
Debug.WriteLine("Just called ComputeBB.");
return returnDTM;
}
And then somewhere up higher up the call stack, you don't actually need CreateFromExistingFiles, simply call:
Task.Run(() => LoadAsBinary(fileName));
When needed.
Also, please, read the C# naming conventions, which you're currently not following.

await's whole purpose is in adding the synchronicity back in asynchronous code. This allows you to easily partition the parts that are happenning synchronously and asynchronously. Your example is absurd in that it never takes any advantage whatsoever of this - if you just called the method directly instead of wrapping it in Task.Run and awaiting that, you would have had the exact same result (with less overhead).
Consider this, though:
await
Task.WhenAll
(
loadAsBinaryAsync(fileName1),
loadAsBinaryAsync(fileName2),
loadAsBinaryAsync(fileName3)
);
Again, you have the synchronicity back (await functions as the synchronization barrier), but you've actually performed three independent operations asynchronously with respect to each other.
Now, there's no reason to do something like this in your code, since you're using Parallel.ForEach at the bottom level - you're already using the CPU to the max (with unnecessary overhead, but let's ignore that for now).
So the basic usage of await is actually to handle asynchronous I/O rather than CPU work - apart from simplifying code that relies on some parts of CPU work being synchronised and some not (e.g. you have four threads of execution that simultaneously process different parts of the problem, but at some point have to be reunited to make sense of the individual parts - look at the Barrier class, for example). This includes stuff like "making sure the UI doesn't block while some CPU intensive operation happens in the background" - this makes the CPU work asynchronous with respect to the UI. But at some point, you still want to reintroduce the synchronicity, to make sure you can display the results of the work on the UI.
Consider this winforms code snippet:
async void btnDoStuff_Click(object sender, EventArgs e)
{
lblProgress.Text = "Calculating...";
var result = await DoTheUltraHardStuff();
lblProgress.Text = "Done! The result is " + result;
}
(note that the method is async void, not async Task nor async Task<T>)
What happens is that (on the GUI thread) the label is first assigned the text Calculating..., then the asynchronous DoTheUltraHardStuff method is scheduled, and then, the method returns. Immediately. This allows the GUI thread to do whatever it needs to do. However - as soon as the asynchronous task is complete and the GUI is free to handle the callback, the execution of btnDoStuff_Click will continue with the result already given (or an exception thrown, of course), back on the GUI thread, allowing you to set the label to the new text including the result of the asynchronous operation.
Asynchronicity is not an absolute property - stuff is asynchronous to some other stuff, and synchronous to some other stuff. It only makes sense with respect to some other stuff.
Hopefully, now you can go back to your original code and understand the part you've misunderstood before. The solutions are multiple, of course, but they depend a lot on how and why you're trying to do what you're trying to do. I suspect you don't actually need to use Task.Run or await at all - the Parallel.ForEach already tries to distribute the CPU work over multiple CPU cores, and the only thing you could do is to make sure other code doesn't have to wait for that work to finish - which would make a lot of sense in a GUI application, but I don't see how it would be useful in a console application with the singular purpose of calculating that single thing.
So yes, you can actually use await for fire-and-forget code - but only as part of code that doesn't prevent the code you want to continue from executing. For example, you could have code like this:
Task<string> result = SomeHardWorkAsync();
Debug.WriteLine("After calling SomeHardWorkAsync");
DoSomeOtherWorkInTheMeantime();
Debug.WriteLine("Done other work.");
Debug.WriteLine("Got result: " + (await result));
This allows SomeHardWorkAsync to execute asynchronously with respect to DoSomeOtherWorkInTheMeantime but not with respect to await result. And of course, you can use awaits in SomeHardWorkAsync without trashing the asynchronicity between SomeHardWorkAsync and DoSomeOtherWorkInTheMeantime.
The GUI example I've shown way above just takes advantage of handling the continuation as something that happens after the task completes, while ignoring the Task created in the async method (there really isn't much of a difference between using async void and async Task when you ignore the result). So for example, to fire-and-forget your method, you could use code like this:
async void Fire(string filename)
{
var result = await ProcessFileAsync(filename);
DoStuffWithResult(result);
}
Fire("MyFile");
This will cause DoStuffWithResult to execute as soon as result is ready, while the method Fire itself will return immediately after executing ProcessFileAsync (up to the first await or any explicit return someTask).
This pattern is usually frowned upon - there really isn't any reason to return void out of an async method (apart from event handlers); you could just as easily return Task (or even Task<T> depending on the scenario), and let the caller decide whether he wants his code to execute synchronously in respect to yours or not.
Again,
async Task FireAsync(string filename)
{
var result = await ProcessFileAsync(filename);
DoStuffWithResult(result);
}
Fire("MyFile");
does the same thing as using async void, except that the caller can decide what to do with the asynchronous task. Perhaps he wants to launch two of those in parallel and continue after all are done? He can just await Task.WhenAll(Fire("1"), Fire("2")). Or he just wants that stuff to happen completely asynchronously with respect to his code, so he'll just call Fire("1") and ignore the resulting Task (of course, ideally, you at the very least want to handle possible exceptions).

Converting a 3rd party API call into Async

I'm reading left and right about Async and Task but I still don't get how one can wrap an existing 3rd party call into an async method.
Mainly the 3rd party dll offers a method to call one of their APIs
var client = new FooClient();
var response = client.CallMethod(param1, message);
if (response.RestException != null)
{
status = response.RestException.Message;
return false;
}
else
return true
and this call as expected will block the current thread till it returns the response.
So I looked at Task and other ways to make this call Async but still get the response back cause based on that the application will take different actions.
I kind of feel Task is the way to go but their DLL does NOT have an CallMethodAsSync method so I don't know what to do at this point.
Any help will be much appreciated !!

The simplest way to do this is to just dump the call into a Task.Run, and then await the return of the Task.Run. This will offload the intensive work to another thread, allowing the UI thread to continue. Here's a simple example of a method that has to wait for another time-intensive method before it can return:
static void Main()
{
Console.WriteLine("Begin");
var result = BlockingMethod("Hi!");
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
static bool BlockingMethod(string someText)
{
Thread.Sleep(2000);
return someText.Contains("SomeOtherText");
}
As we can see, the BlockingMethod method has a Thread.Sleep(2000) as its first statement. This means that the thread running the calling method Main has to wait a full two seconds before it can get the result of BlockingMethod. If the thread running the Main method is dealing with UI repainting, then this means that we get a UI that appears unresponsive/locked for a full two seconds. We can offload the work of waiting for BlockingMethod onto another thread like so:
First we mark our calling method as 'async' because this tells the compiler to generate something resembling a state machine for all of the 'await's in the async method:
static async void Main()
{
Console.WriteLine("Begin");
var result = BlockingMethod("Hi!");
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
Next we turn the return type 'void' into Task. This allows it to be awaited by other threads (you can leave it as async void if you don't care about that, but async methods with a return type need to return Task):
static async Task Main()
{
Console.WriteLine("Begin");
var result = BlockingMethod("Hi!");
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
Now our method is allowed to run asynchronously, and it's allowed to be awaited by other methods. But this hasn't solved our problem yet, because we're still synchronously waiting for the blocking method. So we move our blocking method out to another thread by calling Task.Run, and awaiting its result:
static async Task Main()
{
Console.WriteLine("Begin");
await Task.Run(() => BlockingMethod("Hi!"));
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
But this presents us with a problem: our blocking method was returning something that we wanted to use later on in our method, but Task.Run returns void! In order to get access to the variable returned by the blocking method running in the other thread, we have to capture a local variable in a closure:
static async Task Main()
{
Console.WriteLine("Begin");
bool result = true;
await Task.Run(() => result = BlockingMethod("Hi!"));
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
So, in summary, what we did was we took a normal synchronous method (the calling method that we can rewrite, rather than the third party API that we can't rewrite) and we changed it so that it was marked as async and it returned a Task. If we wanted it to return a result, then it'd need to be a generically typed Task. After that, we wrapped the call to the blocking method in a Task.Run -- which creates a new thread for the blocking method to run on -- then we gave it an Action (in lambda syntax) to run. In that Action, we referenced a variable defined in the calling function (this is the closure) which can capture the result of the blocking method.
Now, we're asynchronously waiting for a synchronous, blocking method somewhere else. It doesn't matter that the blocking method isn't inherently asynchronous, because we're letting it run synchronously in another thread, and allowing our thread to just await its result.
If any of this was unclear, then please comment. Asynchrony is a bit confusing at first, but it's a godsend for responsive UIs.
EDIT:
In response to a comment by Scott Chamberlain, Task.Run also has an overload which returns the type of the method its running (supplying it a Func, rather than an Action). So we can simply use:
static async Task MainAsync()
{
Console.WriteLine("Begin");
bool result = await Task.Run(() => BlockingMethod("Hi!"));
Console.WriteLine("Result: " + result);
Console.ReadLine();
}
Keep in mind -- as Scott Chamberlain points out -- that there's no inherent performance benefit to running work in another thread. In many cases, it may actually cause worse performance, as setting up and tearing down threads is expensive. Task-based asynchrony is only useful to keep a busy thread (e.g. the UI thread) unblocked, so it can respond to requests, or when properly sub-dividing work through use of e.g. Parallel.ForEach.