Question

Async Sub like this:

Dim f As Func(Of Task) = Async Sub()
                         End Sub

Produces compiler error:

error BC36670: Nested sub does not have a signature that is compatible with delegate ‘System.Func(Of System.Threading.Tasks.Task)’.

Equivalent C# code compiles fine:

Func<Task> f = async () => { };

Rewriting Async Sub into Async Function make code works.

Why does Async Sub() is not convertible to delegate types with return value of type Task?

Answer 1

VB.NET Sub is equivalent to C# returning void. There’s a difference between async void Foo() {} and async Task Foo() {}, and your VB.NET is doing the former, while you want the latter. As you mention, Async Function makes it work, because then it actually does the latter.

Edit: some more details:

C#:

async void Foo() { }

async Task Bar() { }

void Baz()
{
    object dummy;
    dummy = (Action) Foo; // OK
    dummy = (Func<Task>) Foo; // error
    dummy = (Action) Bar; // error
    dummy = (Func<Task>) Bar; // OK
}

However, this gets a bit more confusing when delegates are used, because the delegate equivalents of Foo and Bar look the same: async () => { }.

The only difference is that in VB.NET, the delegates do not look the same, because the Sub or Function keyword remains part of the syntax there.