Orphaned async tasks in Azure Functions

Orphaned async tasks in Azure Functions - c#

What happens if in an Azure Function an async Task is started but orphaned before it finishes (it is never awaited), e.g.:
[Function("FuncAsync")]
public static async Task<HttpResponseData> FuncAsync(
[HttpTrigger(AuthorizationLevel.Function, "post", Route = "FuncAsync")]
HttpRequestData req,
FunctionContext context)
{
var obj = FactoryClass.GetObject(); // return some object with async LongTaskAsync method
obj.LongTaskAsync(); // async Task LongTaskAsync()
return req.CreateResponse(HttpStatusCode.OK);
}
The intention here was (I guess) to initiate a long running process and instantly return from the function.
I assume it is a bad practice but seems to work in a legacy code. I suspect there's no guarantee for the life of that async task and the azure process can be randomly winded up, when no function entry point is running/triggered.
For a console application if a running async Task is orphaned (and keeps running while the process terminates) it is abandoned/killed (and no exception is thrown).
class Program
{
public static async Task RunFuncAsync()
{
try
{
Console.WriteLine("Task started.");
await Task.Delay(10 * 1000);
Console.WriteLine("Task finished."); // this is never executed
}
catch (Exception e)
{
Console.WriteLine("Exception: " + e.Message); // this is never executed
}
}
static async Task MainAsync(string[] args)
{
var t = RunFuncAsync(); // no awaiting - purposefuly
await Task.Delay(5 * 1000);
Console.WriteLine("Exiting.");
}
static void Main(string[] args)
{
MainAsync(args).GetAwaiter().GetResult();
}
}
Output:
Task started.
Exiting.
C:\TestTasks_Console.exe (process 6528) exited with code 0.
To automatically close the console when debugging stops, enable Tools->Options->Debugging->Automatically close the console when debugging stops.
Press any key to close this window . . .

What happens if in an Azure Function an async Task is started but
orphaned before it finishes ?
The answer for the above question which you have is correct that it will get terminated. Azure Functions have many features that make our work a lot easier. However, stateless Azure Functions are not suitable for long running operations which would require to store a state of progress.
For such cases the Durable Functions are the best option. With this the restrictions on execution times no longer apply.
The task-based programming model and async/await are well suited for mapping workflows of Durable Functions.
If you check the Azure Function best practices, you will also find that we should avoid long running function and our function should be stateless.
Another option is to go for the WebJobs. You can use Azure WebJobs to execute custom jobs as background tasks within an Azure Web App
In conclusion, we can say that stateless Azure Functions not recommended for long running operations as you may get timeout issues or like in your case may get terminated.

Related

Azure functions - Parallel tasks seems not to run simultaneously

I have a question referencing the usage of concurrently running tasks in Azure Functions, on the consumption plan.
One part of our application allows users to connect their mail accounts, then downloads messages every 15 minutes. We have azure function to do so, one for all users. The thing is, as users count increases, the function need's more time to execute.
In order to mitigate a timeout case, I've changed our function logic. You can find some code below. Now it creates a separate task for each user and then waits for all of them to finish. There is also some exception handling implemented, but that's not the topic for today.
The problem is, that when I check some logs, I see executions as the functions weren't executed simultaneously, but rather one after one. Now I wonder if I made some mistake in my code, or is it a thing with azure functions that they cannot run in such a scenario (I haven't found anything suggesting it on the Microsoft sites, quite the opposite actually)
PS - I do know about durable functions, however, for some reason I'd like to resolve this issue without them.
My code:
List<Task<List<MailMessage>>> tasks = new List<Task<List<MailMessage>>>();
foreach (var account in accounts)
{
using (var cancellationTokenSource = new CancellationTokenSource(TimeSpan.FromMinutes(6)))
{
try
{
tasks.Add(GetMailsForUser(account, cancellationTokenSource.Token, log));
}
catch (TaskCanceledException)
{
log.LogInformation("Task was cancelled");
}
}
}
try
{
await Task.WhenAll(tasks.ToArray());
}
catch(AggregateException aex)
{
aex.Handle(ex =>
{
TaskCanceledException tcex = ex as TaskCanceledException;
if (tcex != null)
{
log.LogInformation("Handling cancellation of task {0}", tcex.Task.Id);
return true;
}
return false;
});
}
log.LogInformation($"Zakończono pobieranie wiadomości.");
private async Task<List<MailMessage>> GetMailsForUser(MailAccount account, CancellationToken cancellationToken, ILogger log)
{
log.LogInformation($"[{account.UserID}] Rozpoczęto pobieranie danych dla konta {account.EmailAddress}");
IEnumerable<MailMessage> mails;
try
{
using (var client = _mailClientFactory.GetIncomingMailClient(account))
{
mails = client.GetNewest(false);
}
log.LogInformation($"[{account.UserID}] Pobrano {mails.Count()} wiadomości dla konta {account.EmailAddress}.");
return mails.ToList();
}
catch (Exception ex)
{
log.LogWarning($"[{account.UserID}] Nie udało się pobrać wiadomości dla konta {account.EmailAddress}");
log.LogError($"[{account.UserID}] {ex.Message} {ex.StackTrace}");
return new List<MailMessage>();
}
}
Output:

Azure functions in a consumption plan scales out automatically. Problem is that the load needs to be high enough to trigger the scale out.
What is probably happening is that the scaling is not being triggered, therefore everything runs on the same instance, therefore the calls run sequentially.
There is a discussion on this with some code to test it here: https://learn.microsoft.com/en-us/answers/questions/51368/http-triggered-azure-function-not-scaling-to-extra.html

The compiler will give you a warning for GetMailsForUser:
CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(…)' to do CPU-bound work on a background thread.
It's telling you it will run synchronously, which is the behaviour you're seeing. In the warning message there's a couple of recommendations:
Use await. This would be the most ideal solution, since it will reduce the resources your Azure Function uses. However, this means your _mailClientFactory will need to support asynchronous APIs, which may be too much work to take on right now (many SMTP libraries still do not support async).
Use thread pool threads. Task.Run is one option, or you could use PLINQ or Parallel. This solution will consume one thread per account, and you'll eventually hit scaling issues there.

If you want to identify which Task is running in which Function instance etc. use invocation id ctx.InvocationId.ToString(). May be prefix all your logs with this id.
Your code isn't written such that it can be run in parallel by the runtime. See this: Executing tasks in parallel
You can also get more info about the trigger using trigger meta-data. Depends on trigger. This is just to get more insight into what function is handling what message etc.

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?

BackgroundService implementation that prevents AspNetCore to start/stop correctly

I am using a BackgroundService object in an aspnet core application.
Regarding the way the operations that run in the ExecuteAsync method are implemented, the Aspnet core fails to initialize or stop correctly. Here is what I tried:
I implemented the abstract ExecuteAsync method the way it is explained in the documentation.
the pipeline variable is an IEnumerable<IPipeline> that is injected in the constructor.
public interface IPipeline {
Task Start();
Task Cycle();
Task Stop();
}
...
protected override async Task ExecuteAsync(CancellationToken stoppingToken) {
log.LogInformation($"Starting subsystems");
foreach(var engine in pipeLine) {
try {
await engine.Start();
}
catch(Exception ex) {
log.LogError(ex, $"{nameof(engine)} failed to start");
}
}
log.LogInformation($"Runnning main loop");
while(!stoppingToken.IsCancellationRequested) {
foreach(var engine in pipeLine) {
try {
await engine.Cycle();
}
catch(Exception ex) {
log.LogError(ex, $"{engine.GetType().Name} error in Cycle");
}
}
}
log.LogInformation($"Stopping subsystems");
foreach(var engine in pipeLine) {
try {
await engine.Stop();
}
catch(Exception ex) {
log.LogError(ex, $"{nameof(engine)} failed to stop");
}
}
}
Because of the current development state of the project, there are many "nop" Pipeline that contains an empty Cycle() operation that is implemented this way:
public async Task Cycle() {
await Task.CompletedTask;
}
What I noticed is:
If at least one IPipeline object contains an actual asynchronous method (await Task.Delay(1)), then everything runs smoothly and I can stop the service gracefully using CTRL+C.
If all IPipeline objects contains await Task.CompletedTask;,
Then on one hand, aspnetcore fails to initialize correctly. I mean, there is no "Now listening on: http://[::]:10001 Application started. Press Ctrl+C to shut down." on the console.
On the other, when I hit CTRL+C, the console shows "Application is shutting down..." but the cycle loop continues to run as if the CancellationToken was never requested to stop.
So basically, if I change a single Pipeline object to this:
public async Task Cycle() {
await Task.Delay(1);
}
Then everything is fine, and I dont understand why. Can someone explain me what I did not understood regarding Task processing ?

The simplest workaround is to add await Task.Yield(); as line one of ExecuteAsync.
I am not an expert... but the "problem" is that all the code inside this ExecuteAsync actually running synchronously.
If all the "cycles" return a Task that has completed synchronously (as Task.CompletedTask will be) then the while and therefore the ExecuteAsync method never "yield"s.
The framework essentially does a foreach over the registered IHostedServices and calls StartAsync. If your service does not yield then the foreach gets blocked. So any other services (including the AspNetCore host) will not be started. As bootstrapping cannot finish, things like ctrl-C handling etc also never get setup.
Putting await Task.Delay(1) in one of the cycles "releases" or "yields" the Task. This allows the host to "capture" the task and continue. Which allows the wiring up of Cancellation etc to happen.
Putting Task.Yield() at the top of ExecuteAsync is just the more direct way of achieving this and means the cycles do not need to be aware of this "issue" at all.
note: this is usually only really an issue in testing... 'cos why would you have a no-op cycle?
note2: If you are likely to have "compute" cycles (ie they don't do any IO, database, queues etc) then switching to ValueTask will help with perf/allocations.

Where to catch exceptions in an asynchronous Azure function

I've set up an Azure function and I want it to run asynchronously because I expect to have hundreds/thousands/more messages in my queue that will all get dequeued at the same time, so I've implemented it as such below (maybe there's a better way). Or do I need to worry about running code in the functions asynchronously?
Will Azure handle thousands of these functions run at the same time, if thousands of messages in the queue are all dequeued at once? This Azure function article says only a few hundred can run at once
Where is the best place to put the try catch statement? Inside the asynchronous call around my logic or outside the asynchronous call like my code? Or does it matter?
public static class CancelEvent
{
[FunctionName("CancelEvent")]
public static async void RunAsync([ServiceBusTrigger("canceleventqueue", AccessRights.Manage, Connection = "service_bus_key")]string myQueueItem, TraceWriter log, ExecutionContext context)
{
try
{
await Task.Run(() => Processor.ProcessAsync());
}
catch(Exception ex)
{
}
}
}
public class Processor
{
public static void ProcessAsync()
{
// do the work
}
}

TLDR; don't build your Azure Functions to be async.
In my recent experience with azure functions, I found it best NOT to build my functions as asynchronous.
In my situation, I was using a ServiceBusTrigger to receive a message, and then was awaiting a processing method, and was baffled that when bubbling an exception out it would not show up in the azure portal as failed, nor would it capture the exception detail in the Application Insights account, nor would it properly abandon or deadletter the message. I would only discover the exception detail by going to the files\eventlog.xml section in the storage account where I would find a complaint about an unhandled exception bringing the function down to its knees.
After a day spent going down the road of doing a full try/catch and handling message.Abandon() and message.Deadletter() logic myself as well as logging all the AppInsights telemetry manually, I discovered that simply NOT doing async and bubbling out he exception (after first performing error reporting or handling logic) resulted in all the behavior I expected out of the platform.
The 'run history' of the function in the portal correctly showed passed and failed executions, and all the exception detail was captured.
This is a cautionary tale to keep your Azure functions static, synchronous, and simple. The message-handling nature of their design should already be async enough.

Adding await Task.Run around the call to a synchronous method doesn't make much sense. Strive to make ProcessAsync really async (returning Task and non-blocking).
So the better option (note that both methods return Task):
[FunctionName("CancelEvent")]
public static async Task RunAsync([ServiceBusTrigger("canceleventqueue", AccessRights.Manage, Connection = "service_bus_key")]string myQueueItem, TraceWriter log, ExecutionContext context)
{
try
{
await Processor.ProcessAsync();
}
catch (Exception ex)
{
// Try catch will work fine
}
}
public class Processor
{
public static async Task ProcessAsync()
{
// do the work
}
}
and the worse option, but also viable:
[FunctionName("CancelEvent")]
public static void RunAsync([ServiceBusTrigger("canceleventqueue", AccessRights.Manage, Connection = "service_bus_key")]string myQueueItem, TraceWriter log, ExecutionContext context)
{
try
{
Processor.Process();
}
catch (Exception ex)
{
// Try catch will work fine
}
}
public class Processor
{
public static void Process()
{
// do the work
}
}
Azure will run multiple executions in parallel in both cases. It's just that the first option will be more lean in resource usage.
Not all messages will be processed at once, but Azure will scale the amount of parallel executions based on some internal scaling logic.

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?

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