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Is having a return type of Task enough to make a method run asynchronously?

I have a simple method that does a complicated string operation and returns the result. As you can see, the return type of this method is Task<string>. Therefore, I can use Task.FromResult(result) to return the value of my string.
public Task<string> ComplexOperation()
{
string result = // Do something complex
return Task.FromResult(result);
}
Then, I can use the await keyword to call this method.
public static async Task Main(string[] args)
{
var myResult = await ComplexOperation();
}
Because I am awaiting CompelxOperation(), (waiting for the Task to return on completion) will this method run asynchronously?

Based on your comments you need to do something like this:
public Task<string> ComplexOperation()
{
return Task.Run(() => /* Do something complex */);
}
You can play with it like this:
public static async Task Main()
{
Console.WriteLine("Before" );
var myResultTask = ComplexOperation();
Console.WriteLine("After task creation");
var result = await myResultTask;
Console.WriteLine("After async await");
}
public Task<string> ComplexOperation()
{
Console.WriteLine("Creation");
return Task.Run(() =>
{
Console.WriteLine("In before work");
Thread.Sleep(500); //simulate work;
Console.WriteLine("In after work");
return "Done";
});
}
And compare the behavior with switching to your implementation when you just return Task.FromResult. Also it does not makes much sense in such test example, TBH.

Just as you flagged as async the Main method to get an asynchronous method you need to flag the method as asynchronous with the async keyword, and make the result a Task.
But in the framework Tasks are used for 2 related but different concepts, parallelization and asynchronous programming.
What you are doing is use parallelization.
So you are just running some synchronous code on a different thread.
Based on your example i think what you need is to use parallelization, which can speed up some complex calculation or work by use many threads to work in parallel.
The concept behind asynchronous code is to free the thread while you are waiting for external resources (like some data from a web service), to allow other work to be done in the meantime.
So, if you need to do complex work with local resources it's better use tasks without async logic, instead when working with remote resources you can go for asynchronous operations.

Semantically the method ComplexOperation is asynchronous because it returns an awaitable type, and in order to follow the guidelines it should be named ComplexOperationAsync.
Asynchronous methods in TAP include the Async suffix after the operation name for methods that return awaitable types, such as Task, Task<TResult>, ValueTask, and ValueTask<TResult>.
But it is not a well behaved asynchronous method. An asynchronous method is expected to return an incomplete Task immediately, allowing the caller to await the task asynchronously without been blocked. From the docs:
An asynchronous method that is based on TAP can do a small amount of work synchronously, such as validating arguments and initiating the asynchronous operation, before it returns the resulting task. Synchronous work should be kept to the minimum so the asynchronous method can return quickly.
The ComplexOperation method does exactly the opposite: it forces the calling thread to perform the complex operation, and finally it hands back a completed task. For all intents and purposes this operation is not asynchronous at all. It is 100% synchronous and 0% asynchronous, plus some overhead (to add insult to injury). So don't do this, and if there is some library that does it, don't use the library. It is simply bad practice.
Clarification: Since the term asynchronous method can mean different things in different contexts, I should clarify that the context of this answer is the perspective of the consumer, who sees the signature of the method as a contract, and builds expectations based on the visible contract and not on the invisible implementation. In this context the method ComplexOperation is asynchronous. If we switch perspective and focus on the implementation, the method ComplexOperation is not asynchronous. My statement «it is simply bad practice» refers to the practice of breaking the contract, and providing a synchronous implementation to a method with asynchronous contract. I am not criticizing the use of Task.FromResult method per se. I am criticizing it only when it follows a complex/lengthy/latent operation, that according to the method's contract should be made asynchronous.
P.S. I am thankful to #DmytroMukalov for providing (in the chat) the helpful distinction between asynchrony by contract and by implementation.

Calling Async Method from Sync Method

I see a lot of questions about calling asyc methods from sync ones, like here: How to call asynchronous method from synchronous method in C#?
I understand the solutions however, experimentally, I am seeing that I don't need to add a .Wait() or Task.Run(async () => await MyAsyncMethod()) for it to run synchronously. I have a get method that pulls information from a DB.
My code is below and when I run, it waits at var task = GetItemAsync() and I get the correct result from the DB. I find it hard to believe that the DB call could come back before the code would move on which seems to imply that it ran synchronously. Am I missing something?
public object GetItem()
{
var task = GetItemAsync();
var result = task.Result;
return result
}

See The docs on Task<>.Result
System.Threading.Tasks.Task.Result is a blocking call that waits until the task finishes. So it has a built in wait call essentially.
In the remarks section of that microsoft doc:
Accessing the property's get accessor blocks the calling thread until the asynchronous operation is complete; it is equivalent to calling the Wait method.
It's unfortunate that Microsoft did not reflect this in the name of the accessor because it's an extremely critical side effect of this accessor.

The only right way to call awaitable(method which returns Task) method is to await it. But you can await only within the method which returns Task. In legacy code it happens you cannot change the method signature because you have too many reference and you need to basically refactor entire code base. So you have these options:
.Result
.Wait()
.GetAwaiter().GetResult()
Neither of those are good. All of them are going to block the thread. But if you cannot change the method signature to await the execution, I would pick the last option - GetAwaiter().GetResult() becasue it will not wrap the exception into AggregateException

Should I use Task.Run to wait tasks in a synchronous context?

I have an ASPX page which I cannot convert to async but which uses some async methods in a synchronous context. The way it invokes them is like so:
public void MySyncMethod()
{
var myTask = Task.Run(() => _myField.DoSomethingAsync());
myTask.Wait();
//use myTask.Result
}
Is there any difference between doing that and the following as far as async/await and/or blocking goes?
public void MySyncMethod()
{
var myTask = _myField.DoSomethingAsync(); //just get the Task direct, no Task.Run
myTask.Wait();
//use myTask.Result
}
I assume a previous developer added the Task.Run for a reason. But I am having issues which accessing things in HttpContext as the work is being run on a different thread.
Is there a reason to use Task.Run here?

Is there any difference between doing that and the following as far as
async/await and/or blocking goes?
Yes the first block of code uses a thread pool thread then waits for this to return, so your using two threads not one. They both block.
I assume a previous developer added the Task.Run for a reason.
Yes, blocking (directly) on async code from an ASP.Net context is a bad idea and can cause deadlocks. So you second block of code is more efficent (in thread usage) but suffers from serious deadlock issues.
The correct solution here is to make public void MySyncMethod() async itself (public async Task MySyncMethod()). Both these solutions have drawbacks and the only real way out is to make the whole call stack async. If you can do this, do it.
If you can't call an async method from another async method then Task.Run is the way to go. See How to call asynchronous method from synchronous method in C#? for more details.
If you want HttpContext inside your thread have a read though Using HttpContext in Async Task I would definitely favour:
Make every thing async
Or the Read the values from the context then pass them
Options of those answers and keep in mind
First off, you're not creating a copy of the object, you're just
copying the reference to the object.HttpContext isn't a
struct.....etc

The internal workings of asynchronous code based on async/await is fundamentally different than tasks started by Task.Run. async/await tasks are promise based and depend on the caller cooperating with returning the execution back to the asynchronous method when appropriate. Tasks started by Task.Run however are usually started on a parallel thread taken from the thread pool and do not depend on the caller's cooperation to continue execution when appropriate.
This constellation leads to the problem that you can not treat a promise based task the same as the other tasks, since the promise based task might wait for the callers cooperation to return the execution, which might never occur since the other task is executed independently and might wait for the caller. The result is a deadlock.
The solution is a specific Task.Run overload that will create a proxy for an existing task-based method that allows proper execution of a promise based task. It is safe to call Wait on this proxy. That's why the other developer used this construct. He could have also simplified the call and avoided an anonymous method like this:
var myTask = Task.Run(_myField.DoSomethingAsync);

Task.Run is used to run code asynchronously.
Be clear that it returns Task and needs to be awaited. Here's an example:
Task myTask = Task.Run(() => DoSomething());
await myTask;

Why use async when I have to use await?

I've been stuck on this question for a while and haven't really found any useful clarification as to why this is.
If I have an async method like:
public async Task<bool> MyMethod()
{
// Some logic
return true;
}
public async void MyMethod2()
{
var status = MyMethod(); // Visual studio green lines this and recommends using await
}
If I use await here, what's the point of the asynchronous method? Doesn't it make the async useless that VS is telling me to call await? Does that not defeat the purpose of offloading a task to a thread without waiting for it to finish?

Does that not defeat the purpose of offloading a task to a thread without waiting for it to finish?
Yes, of course. But that's not the purpose of await/async. The purpose is to allow you to write synchronous code that uses asynchronous operations without wasting threads, or more generally, to give the caller a measure of control over the more or less asynchronous operations.
The basic idea is that as long as you use await and async properly, the whole operation will appear to be synchronous. This is usually a good thing, because most of the things you do are synchronous - say, you don't want to create a user before you request the user name. So you'd do something like this:
var name = await GetNameAsync();
var user = await RemoteService.CreateUserAsync(name);
The two operations are synchronous with respect to each other; the second doesn't (and cannot!) happen before the first. But they aren't (necessarily) synchronous with respect to their caller. A typical example is a Windows Forms application. Imagine you have a button, and the click handler contains the code above - all the code runs on the UI thread, but at the same time, while you're awaiting, the UI thread is free to do other tasks (similar to using Application.DoEvents until the operation completes).
Synchronous code is easier to write and understand, so this allows you to get most of the benefits of asynchronous operations without making your code harder to understand. And you don't lose the ability to do things asynchronously, since Task itself is just a promise, and you don't always have to await it right away. Imagine that GetNameAsync takes a lot of time, but at the same time, you have some CPU work to do before it's done:
var nameTask = GetNameAsync();
for (int i = 0; i < 100; i++) Thread.Sleep(100); // Important busy-work!
var name = await nameTask;
var user = await RemoteService.CreateUserAsync(name);
And now your code is still beautifuly synchronous - await is the synchronization point - while you can do other things in parallel with the asynchronous operations. Another typical example would be firing off multiple asynchronous requests in parallel but keeping the code synchronous with the completion of all of the requests:
var tasks = urls.Select(i => httpClient.GetAsync(i)).ToArray();
await Task.WhenAll(tasks);
The tasks are asynchronous in respect to each other, but not their caller, which is still beautifuly synchronous.
I've made a (incomplete) networking sample that uses await in just this way. The basic idea is that while most of the code is logically synchronous (there's a protocol to be followed - ask for login->verify login->read loop...; you can even see the part where multiple tasks are awaited in parallel), you only use a thread when you actually have CPU work to do. Await makes this almost trivial - doing the same thing with continuations or the old Begin/End async model would be much more painful, especially with respect to error handling. Await makes it look very clean.

If I use await here, what's the point of the asynchronous method?
await does not block thread. MyMethod2 will run synchronously until it reaches await expression. Then MyMethod2 will be suspended until awaited task (MyMethod) is complete. While MyMethod is not completed control will return to caller of MyMethod2. That's the point of await - caller will continue doing it's job.
Doesn't it make the async useless that VS is telling me to call await?
async is just a flag which means 'somewhere in the method you have one or more await'.
Does that not defeat the purpose of offloading a task to a thread
without waiting for it to finish?
As described above, you don't have to wait for task to finish. Nothing is blocked here.
NOTE: To follow framework naming standards I suggest you to add Async suffix to asynchronous method names.

An async method is not automatically executed on a different thread. Actually, the opposite is true: an async method is always executed in the calling thread. async means that this is a method that can yield to an asynchronous operation. That means it can return control to the caller while waiting for the other execution to complete. So asnync methods are a way to wait for other asynchronoous operations.
Since you are doing nothing to wait for in MyMethod2, async makes no sense here, so your compiler warns you.
Interestingly, the team that implemented async methods has acknowledged that marking a method async is not really necessary, since it would be enough to just use await in the method body for the compiler to recognize it as async. The requirement of using the async keyword has been added to avoid breaking changes to existing code that uses await as a variable name.

Write your own async method

I would like to know how to write your own async methods the "correct" way.
I have seen many many posts explaining the async/await pattern like this:
http://msdn.microsoft.com/en-us/library/hh191443.aspx
// Three things to note in the signature:
// - The method has an async modifier.
// - The return type is Task or Task<T>. (See "Return Types" section.)
// Here, it is Task<int> because the return statement returns an integer.
// - The method name ends in "Async."
async Task<int> AccessTheWebAsync()
{
// You need to add a reference to System.Net.Http to declare client.
HttpClient client = new HttpClient();
// GetStringAsync returns a Task<string>. That means that when you await the
// task you'll get a string (urlContents).
Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
// You can do work here that doesn't rely on the string from GetStringAsync.
DoIndependentWork();
// The await operator suspends AccessTheWebAsync.
// - AccessTheWebAsync can't continue until getStringTask is complete.
// - Meanwhile, control returns to the caller of AccessTheWebAsync.
// - Control resumes here when getStringTask is complete.
// - The await operator then retrieves the string result from getStringTask.
string urlContents = await getStringTask;
// The return statement specifies an integer result.
// Any methods that are awaiting AccessTheWebAsync retrieve the length value.
return urlContents.Length;
}
private void DoIndependentWork()
{
resultsTextBox.Text += "Working........\r\n";
}
This works great for any .NET Method that already implements this functionality like
System.IO opertions
DataBase opertions
Network related operations (downloading, uploading...)
But what if I want to write my own method that takes quite some time to complete where there just is no Method I can use and the heavy load is in the DoIndependentWork method of the above example?
In this method I could do:
String manipulations
Calculations
Handling my own objects
Aggregating, comparing, filtering, grouping, handling stuff
List operations, adding, removing, coping
Again I have stumbled across many many posts where people just do the following (again taking the above example):
async Task<int> AccessTheWebAsync()
{
HttpClient client = new HttpClient();
Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
await DoIndependentWork();
string urlContents = await getStringTask;
return urlContents.Length;
}
private Task DoIndependentWork()
{
return Task.Run(() => {
//String manipulations
//Calculations
//Handling my own objects
//Aggregating, comparing, filtering, grouping, handling stuff
//List operations, adding, removing, coping
});
}
You may notice that the changes are that DoIndependentWork now returns a Task and in the AccessTheWebAsync task the method got an await.
The heavy load operations are now capsulated inside a Task.Run(), is this all it takes?
If that's all it takes is the only thing I need to do to provide async Method for every single method in my library the following:
public class FooMagic
{
public void DoSomeMagic()
{
//Do some synchron magic...
}
public Task DoSomeMagicAsync()
{
//Do some async magic... ?!?
return Task.Run(() => { DoSomeMagic(); });
}
}
Would be nice if you could explain it to me since even a high voted question like this:
How to write simple async method? only explains it with already existing methods and just using asyn/await pattern like this comment of the mentioned question brings it to the point:
How to write simple async method?

Actual Answer
You do that using TaskCompletionSource, which has a Promise Task that doesn't execute any code and only:
"Represents the producer side of a Task unbound to a delegate, providing access to the consumer side through the Task property."
You return that task to the caller when you start the asynchronous operation and you set the result (or exception/cancellation) when you end it. Making sure the operation is really asynchronous is on you.
Here is a good example of this kind of root of all async method in Stephen Toub's AsyncManualResetEvent implementation:
class AsyncManualResetEvent
{
private volatile TaskCompletionSource<bool> _tcs = new TaskCompletionSource<bool>();
public Task WaitAsync() { return _tcs.Task; }
public void Set() { _tcs.TrySetResult(true); }
public void Reset()
{
while (true)
{
var tcs = _tcs;
if (!tcs.Task.IsCompleted ||
Interlocked.CompareExchange(ref _tcs, new TaskCompletionSource<bool>(), tcs) == tcs)
return;
}
}
}
Background
There are basically two reasons to use async-await:
Improved scalability: When you have I/O intensive work (or other inherently asynchronous operations), you can call it asynchronously and so you release the calling thread and it's capable of doing other work in the mean time.
Offloading: When you have CPU intensive work, you can call it asynchronously, which moves the work off of one thread to another (mostly used for GUI threads).
So most of the .Net framework's asynchronous calls support async out of the box and for offloading you use Task.Run (as in your example). The only case where you actually need to implement async yourself is when you create a new asynchronous call (I/O or async synchronization constructs for example).
These cases are extremely rare, which is why you mostly find answers that
"Only explains it with already existing methods and just using async/await pattern"
You can go deeper in The Nature of TaskCompletionSource

Would be nice if you could explain it to me: How to write simple async
method?
First, we need to understand what an async method means. When one exposes an async method to the end user consuming the async method, you're telling him: "Listen, this method will return to you quickly with a promise of completing sometime in the near future". That is what you're guaranteeing to your users.
Now, we need to understand how Task makes this "promise" possible. As you ask in your question, why simply adding a Task.Run inside my method makes it valid to be awaited using the await keyword?
A Task implements the GetAwaiter pattern, meaning it returns an object called an awaiter (Its actually called TaskAwaiter). The TaskAwaiter object implements either INotifyCompletion or ICriticalNotifyCompletion interfaces, exposing a OnCompleted method.
All these goodies are in turn used by the compiler once the await keyword is used. The compiler will make sure that at design time, your object implements GetAwaiter, and in turn use that to compile the code into a state machine, which will enable your program to yield control back to the caller once awaited, and resume when that work is completed.
Now, there are some guidelines to follow. A true async method doesn't use extra threads behind the scenes to do its job (Stephan Cleary explains this wonderfully in There Is No Thread), meaning that exposing a method which uses Task.Run inside is a bit misleading to the consumers of your api, because they will assume no extra threading involved in your task. What you should do is expose your API synchronously, and let the user offload it using Task.Run himself, controlling the flow of execution.
async methods are primarily used for I/O Bound operations, since these naturally don't need any threads to be consumed while the IO operation is executing, and that is why we see them alot in classes responsible for doing IO operations, such as hard drive calls, network calls, etc.
I suggest reading the Parallel PFX teams article Should I expose asynchronous wrappers for synchronous methods? which talks exactly about what you're trying to do and why it isn't recommended.

TL;DR:
Task.Run() is what you want, but be careful about hiding it in your library.
I could be wrong, but you might be looking for guidance on getting CPU-Bound code to run asynchronously [by Stephen Cleary]. I've had trouble finding this too, and I think the reason it's so difficult is that's kinda not what you're supposed to do for a library - kinda...
Article
The linked article is a good read (5-15 minutes, depending) that goes into a decent amount of detail about the hows and whys of using Task.Run() as part of an API vs using it to not block a UI thread - and distinguishes between two "types" of long-running process that people like to run asynchronously:
CPU-bound process (one that is crunching / actually working and needs a separate thread to do its work)
Truly asynchronous operation (sometimes called IO-bound - one that is doing a few things here and there with a bunch of waiting time in between actions and would be better off not hogging a thread while it's sitting there doing nothing).
The article touches on the use of API functions in various contexts, and explains whether the associated architectures "prefer" sync or async methods, and how an API with sync and async method signatures "looks" to a developer.
Answer
The last section "OK, enough about the wrong solutions? How do we fix this the right way???" goes into what I think you're asking about, ending with this:
Conclusion: do not use Task.Run in the implementation of the method; instead, use Task.Run to call the method.
Basically, Task.Run() 'hogs' a thread, and is thus the thing to use for CPU-bound work, but it comes down to where it's used. When you're trying to do something that requires a lot of work and you don't want to block the UI Thread, use Task.Run() to run the hard-work function directly (that is, in the event handler or your UI based code):
class MyService
{
public int CalculateMandelbrot()
{
// Tons of work to do in here!
for (int i = 0; i != 10000000; ++i)
;
return 42;
}
}
...
private async void MyButton_Click(object sender, EventArgs e)
{
await Task.Run(() => myService.CalculateMandelbrot());
}
But... don't hide your Task.Run() in an API function suffixed -Async if it is a CPU-bound function, as basically every -Async function is truly asynchronous, and not CPU-bound.
// Warning: bad code!
class MyService
{
public int CalculateMandelbrot()
{
// Tons of work to do in here!
for (int i = 0; i != 10000000; ++i)
;
return 42;
}
public Task<int> CalculateMandelbrotAsync()
{
return Task.Run(() => CalculateMandelbrot());
}
}
In other words, don't call a CPU-bound function -Async, because users will assume it is IO-bound - just call it asynchronously using Task.Run(), and let other users do the same when they feel it's appropriate. Alternately, name it something else that makes sense to you (maybe BeginAsyncIndependentWork() or StartIndependentWorkTask()).