Explicitly use a Func for asynchronous lambda function when Action overload is available

Question

Reading over this blog post on some of the gotchas of C#5\'s async/await. It mentions in Gotcha #4 something that is quite profound and that I hadn\'t thought of before.

Answer 1

Because Task.Run has signatures of both Task.Run(Func) and Task.Run(Action), what type is the async anonymous function compiled to? An async void or a Func? My gut feeling says it will compile down to an async void purely because its a non-generic type however the C# Compiler might be smart and give Func types preference.

The general rule, even without async, is that a delegate with a return type is a better match than a delegate without a return type. Another example of this is:

static void Foo(Action a) { }
static void Foo(Func f) { }
static void Bar()
{
  Foo(() => { throw new Exception(); });
}

This is unambiguous and calls the second overload of Foo.

Also, is there a way to explicitly declare which overload I wish to use?

A nice way to make this clear is to specify the parameter name. The parameter names for the Action and Func overloads are different.

Task.Run(action: async () => {
  await Task.Delay(1000);
});
Task.Run(function: async () => {
  await Task.Delay(1000);
});