C# async/await chaining with ConfigureAwait(false)

问题

Based on numerous books and blogs including this excellent one here, it is clear that when one writes a dll library exposing helper async methods i.e. the wrapper methods, it is generally considered a best practice to internally complete the I/O task of actual async methods on a threadpool thread like so (pseudo code shown below for brevity and I'm using HttpClient as an example)

public Async Task<HttpResponseMessage> MyMethodAsync(..)
{
    ...
    var httpClient = new HttpClient(..);
    var response = await httpClient.PostAsJsonAsync(..).ConfigureAwait(false);
    ...
    return response;
}

The key here is the usage of ConfigureAwait(false) so that IO task completion occurs on a threadpool thread instead of on the original thread context, thereby potentially preventing deadlocks.

My question is from the perspective of a caller. I'm particularly interested in a scenario where there are layers of method calls between the caller and the above method call, as the following example shows.

CallerA -> Method1Async -> Method2Async -> finally the above MyMethodAsync

Is it enough to have ConfigureAwait(false) on the final method only or should one also ensure Method1Async and Method2Async also internally call their async methods with ConfigureAwait(false)? It seems silly to have it included on all these intermediary methods, especially if Method1Async and Method2Async are simply overloads that end up calling MyMethodAsync. Any thoughts, please enlighten us!

Updated with Example So if I have a library with the following private async method,

private async Task<string> MyPrivateMethodAsync(MyClass myClass)
{
    ...
    return await SomeObject.ReadAsStringAsync().ConfigureAwait(false);
}

should I make sure the following public overloaded methods both also include ConfigureAwait(false) as shown below?

public async Task<string> MyMethodAsync(string from)
{
        return await MyPrivateMethodAsync(new (MyClass() { From = from, To = "someDefaultValue"}).ConfigureAwait(false);
}
public async Task<string> MyMethodAsync(string from, string to)
{
        return await MyPrivateMethodAsync(new (MyClass() { From = from, To = to }).ConfigureAwait(false);
}

回答1:

Definitely not. ConfigureAwait just as it's name suggest configures the await. It only affects the await coupled with it.

ConfigureAwait actually returns a different awaitable type, ConfiguredTaskAwaitable instead of Task which in turn returns a different awaiter type ConfiguredTaskAwaiter instead of TaskAwaiter

If you want to disregard the SynchronizationContext for all your awaits you must use ConfigureAwait(false) for each of them.

If you want to limit the use of ConfigureAwait(false) you can use my NoSynchronizationContextScope (see here) at the very top:

async Task CallerA()
{
    using (NoSynchronizationContextScope.Enter())
    {
        await Method1Async();
    }
}

回答2:

When the task is awaited, it creates a corresponding TaskAwaiter to keep track of the task which also captures the current SynchronizationContext. After the task completes, the awaiter runs the code after the await ( called the continuation) on that captured context.

You can prevent that by calling ConfigureAwait(false), which creates a different kind of awiatable (ConfiguredTaskAwaitable) and its corresponding awaiter (ConfiguredTaskAwaitable.ConfiguredTaskAwaiter) that does not run the continuation on the captured context.

The point is that for each await, a different instance of an awaiter is created, it is not something that is shared between all the awaitables in the method or program. So it's best that you call ConfigureAwait(false) for each await statement.

You can see the source code for the awaiters here.

来源：https://stackoverflow.com/questions/28776408/c-sharp-async-await-chaining-with-configureawaitfalse

标签

.net

task-parallel-library

async-await

io-completion-ports