async/await - when to return a Task vs void?

问题

Under what scenarios would one want to use

public async Task AsyncMethod(int num)

instead of

public async void AsyncMethod(int num)

The only scenario that I can think of is if you need the task to be able to track its progress.

Additionally, in the following method, are the async and await keywords unnecessary?

public static async void AsyncMethod2(int num)
{
    await Task.Factory.StartNew(() => Thread.Sleep(num));
}

回答1

Normally, you would want to return a Task. The main exception should be when you need to have a void return type (for events). If there's no reason to disallow having the caller await your task, why disallow it? 一般来说，你都会想要返回一个Task，例外是你想要为事件返回void。
async methods that return void are special in another aspect: they represent top-level async operations, and have additional rules that come into play when your task returns an exception. The easiest way is to show the difference is with an example:

static async void f()
{
    await h();
}

static async Task g()
{
    await h();
}

static async Task h()
{
    throw new NotImplementedException();
}

private void button1_Click(object sender, EventArgs e)
{
    f();
}

private void button2_Click(object sender, EventArgs e)
{
    g();
}

private void button3_Click(object sender, EventArgs e)
{
    GC.Collect();
}

f's exception is always "observed". An exception that leaves a top-level asynchronous method is simply treated like any other unhandled exception.

g's exception is never observed.

When the garbage collector comes to clean up the task, it sees that the task resulted in an exception, and nobody handled the exception. When that happens, the TaskScheduler.UnobservedTaskException handler runs. You should never let this happen. To use your example,

public static async void AsyncMethod2(int num)
{
    await Task.Factory.StartNew(() => Thread.Sleep(num));
}

Yes, use async and await here, they make sure your method still works correctly if an exception is thrown.

For more information see: https://docs.microsoft.com/en-us/archive/msdn-magazine/2013/march/async-await-best-practices-in-asynchronous-programming

I agree with your recommendations, but note that in the final release of async (VS2012), the exception from g【实际说的是f】 will be passed to the SynchronizationContext. –
Stephen Cleary
Aug 27, 2012 at 16:06

@StephenCleary I have VS2012 installed (the final release, not the RC), and if I call g, I don't get the regular unhandled exception dialog box (which based on your comment I understand would happen on your system), and I do see the UnobservedTaskException handler run. –
user743382
Aug 28, 2012 at 6:51

I meant f instead of g in my comment.

The exception from f is passed to the SynchronizationContext. g will raise UnobservedTaskException, but UTE no longer crashes the process if it's not handled.

There are some situations where it's acceptable to have "asynchronous exceptions" like this that are ignored. –
Stephen Cleary
Aug 28, 2012 at 13:07

回答2

I have come across this very useful article about async and void written by Jérôme Laban: https://jaylee.org/archive/2012/07/08/c-sharp-async-tips-and-tricks-part-2-async-void.html

The bottom line is that an async+void can crash the system and usually should be used only on the UI side event handlers.

The reason behind this is the Synchronization Context used by the AsyncVoidMethodBuilder, being none in this example. When there is no ambient Synchronization Context, any exception that is unhandled by the body of an async void method is rethrown on the ThreadPool. While there is seemingly no other logical place where that kind of unhandled exception could be thrown, the unfortunate effect is that the process is being terminated, because unhandled exceptions on the ThreadPool effectively terminate the process since .NET 2.0. You may intercept all unhandled exception using the AppDomain.UnhandledException event, but there is no way to recover the process from this event.

When writing UI event handlers, async void methods are somehow painless because exceptions are treated the same way found in non-async methods; they are thrown on the Dispatcher. There is a possibility to recover from such exceptions, with is more than correct for most cases. Outside of UI event handlers however, async void methods are somehow dangerous to use and may not that easy to find.

回答3

The problem with calling async void is that

you don’t even get the task back. You have no way of knowing when the function’s task has completed. —— Crash course in async and await | The Old New Thing

Here are the three ways to call an async function:

async Task<T> SomethingAsync() { ... return t; }
async Task SomethingAsync() { ... }
async void SomethingAsync() { ... }
In all the cases, the function is transformed into a chain of tasks. The difference is what the function returns.

In the first case, the function returns a task that eventually produces the t.

In the second case, the function returns a task which has no product, but you can still await on it to know when it has run to completion.

The third case is the nasty one. The third case is like the second case, except that you don't even get the task back. You have no way of knowing when the function's task has completed.

The async void case is a "fire and forget": You start the task chain, but you don't care about when it's finished. When the function returns, all you know is that everything up to the first await has executed. Everything after the first await will run at some unspecified point in the future that you have no access to.

async/await when to return a Task vs void? 这里也提到了async exception

async/await - when to return a Task vs void?