c# - Using a generic type as a return type of an async method

Question

A previous question made me wonder why the following method would raise a compile time error:

The return type of an async method must be void, Task or Task

public async T MyMethodAsync<T>() where T : Task
{
     // Irrelevant code here which returns a Task
}

Since we know at compile time that T is always a Task or a derived type, why won't this work?

Edit

The reason I'm asking is that a method may return a Task or a Task<T>. Let's say the method can return either and I don't want to duplicate code.

Of course this is all theoretical and isn't ment for production purposes.

Edit 2

Found a great article by Lucian Wischik: Why must async return Task

Answer 1

Three problems:

• Just because T is "Task or a derived type" doesn't mean that it's Task or Task<T>. What would you expect if I called MyMethodAsync<MyCustomTask> where MyCustomTask derives from Task?

• The compiler needs to know whether it's building a state machine returning Task or Task<T> when it compiles the method - it uses different helper classes in the different cases

• If an async method has a return type of Task, any return statements can't specify a value; if it has a return type of Task<T> any return statements must specify a value which is implicitly convertible to T. How can that work within MyMethodAsync? It's a bit like saying "my method is either void or returns a T - you can decide when you call it".

It's not clear what you're trying to achieve here, but basically this isn't going to work.