How to use await on methods in interfaces

Question

When implementing against an interface (because of mocking, remoting or similiar) using the await keyword and having an interface with methods returning Task<> :

interface IFoo {     Task<BigInteger> CalculateFaculty(int value); } 

the compiler comes up with an error:

The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'

Which is a bit unusual considering the return type is "Task". This problem is kind of frustrating and forces me to "fall back" using continuation style or providing an extra proxy around this interface (and therefore for almost every interface which is not really feasible for me)

Does anyone have a good idea on how to solve this?

Answer 1

The message is not about the interface, but about the calling method. You need to mark the method containing the await keyword with the async modifier:

public interface IFoo {     Task<int> AwaitableMethod(); }  class Bar {     static async Task AsyncMethod() // marked as async!     {         IFoo x;         await x.AwaitableMethod();     } }