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Before we dive all the way into “asynchronous OOP”, let’s address one fairly common question: how does one deal with inheritance of asynchronous methods? What about an “asynchronous interface”?

Fortunately, async does work well with inheritance (and interfaces). Remember that async is an implementation detail, so interfaces can’t be defined with async. To define an asynchronous method in an interface, you just need to define a method with the same signature, minus the async keyword:

interface IMyInterface
  Task MyMethodAsync();

You can then implement it using async:

sealed class MyClass : IMyInterface
  public async Task MyMethodAsync()

If you have an implementation that isn’t async, you can use TaskCompletionSource<T> or one of its shorthand forms such as Task.FromResult to implement the asynchronous method signature synchronously.

Similarly, if you have a base class method that returns Task or Task<T> (which may be asynchronous or synchronous), you can override it with an asynchronous or synchronous method.

Update (2014-12-01): For more details, see Recipe 10.1 in my Concurrency Cookbook.

Next time, we’ll take a look at asynchronous constructors.