What's the method signature for passing an async delegate?

Question

I\'ve recently moved back to C# from being in Objective-C land, and the async/await keywords in C# 5 look cool. But I\'m still trying to get a handle on the proper syntax.

Answer 1

If I have a task that I want to be passed but not executed, I can wrap the Task in a Func<>, then call that Func<> to create that task. The await can be used in the normal way.

public class Example {
    public Example(Func<Task> toBeExecutedInTheFuture)
    {
        FutureTask = toBeExecutedInTheFuture;
    }

    public async void ExecuteTaskExample()
    {
        await FutureTask();

        // or alternatively

        var myTask = FutureTask();
        // do work
        await myTask;
    }
}

Answer 2

The return type of the method signatue is Task if there is no return type, or Task<T> if there is a return type.

Tho, I'm not 100% certain if you can have async lambdas like that.

In the method that is consuming the task, you would either 'await' the task or use the properties and methods on Task to get the result.

Answer 3

A function that takes a delegate as a parameter must use a named delegate type; unlike in Objective-C you can't declare an anonymous delegate type inline in the function definition. However, the generics Action<> and Func<> are provided so that you don't have to declare a new type yourself. In the code below I'm assuming the delegate takes a single int as a parameter.

void DoSomethingWithCallback(Func<int,Task> callbackDelegate)
{
    Task t = callbackDelegate(42);
}

If this function doesn't actually do anything with the Task object returned (as with the code shown above), you can instead use Action<int> as the delegate type. If you use Action, you can still declare the delegate async (below) but the implicit Task object returned is ignored.

The lambda syntax for calling the above function is straightforward and the syntax you used in the question is correct. Note that the parameter type doesn't need to be specified here since it can be inferred:

DoSomethingWithCallback(async (intParam) => { this.myint = await Int2IntAsync(intParam); });

You can also pass a method or delegate variable, if you wish, instead of using the lambda syntax:

async Task MyInt2Int(int p) { ... }
Func<int,Task> myDelegate;
void OtherMethod()
{
    myDelegate = MyInt2Int;
    DoSomethingWithCallback(myDelegate); // this ...
    DoSomethingWithCallback(MyInt2Int);  // ... or this.
}