使用C＃6进行一行方法调用以获得等待方法？

Question

In Jon Skeet's C# 6 post entitled Clean Event Handlers Invocation with C# 6 he shows how you can now write

public void OnFoo()
{
    Foo?.Invoke(this, EventArgs.Empty);
}

instead of

public void OnFoo()
{
    EventHandler handler = Foo;
    if (handler != null)
    {
        handler(this, EventArgs.Empty);
    }   
}

Is there any way to do a one-liner like that if we're talking about an awaitable method? If I try

await Foo?.Invoke();

I get the following compile error:

The awaiter type System.Runtime.CompilerServices.TaskAwaiter? must have suitable IsCompleted and GetResult members (CS4011).

Note: I am using Mono (Xamarin Studio for OSX v5.9.5), so the compiler results can vary from what one might get using Visual Studio.

Answer 1

@i3arnon Has a good solution, however this is another way that you could do it that wouldn't require adding a field or property.

await (Foo?.Invoke() ?? Task.CompletedTask);

or if Task<T>

var i = await (Foo?.Invoke() ?? Task.FromResult<T>(default(T)))

Answer 2

Not if you're trying to use the null propagation operator (ie ?. ).

When you use this operator the result must be of a nullable type, since it may or may not invoke the method (depending on whether Foo is null ). Reference types are nullable but value types (structs) are not so for them this operator returns Nullable<T> ( T? ).

When you await something it compiles into calling GetAwaiter on that awaitable (usually a Task ) and getting an awaiter (usually TaskAwaiter ) that has IsCompleted , GetResult and OnCompleted . If a type doesn't have all these then it's not awaitable and you can't await it.

So in your case, you call Foo?.Invoke() and get back a Nullable<TaskAwaiter> (ie TaskAwaiter? ) since TaskAwaiter is a struct and TaskAwaiter? doesn't have the required methods (unlike TaskAwaiter itself) it can't be awaited (it wouldn't have been possible even if TaskAwaiter was a class since you can't await a null ).

However, you can simply initialize Foo with an efficient dummy registration so it wouldn't ever be null and you wouldn't need to check it:

Func<Task> Foo = () => Task.CompletedTask;

public void OnFoo()
{
    await Foo();
}