Question

Task or Task<TResult> object is awaitable, so we can use await key on those whose return value is Task or Task<TResult>.
Task or Task<TResult> are the most frequently-used awaitable object.

We also can define our own awaitable object.The object should has below qualification.

1. It has a GetAwaiter() method (instance method or extension method);
2. Its GetAwaiter() method returns an awaiter. An object is an awaiter
   if:
   • It implements INotifyCompletion or ICriticalNotifyCompletion
     interface;
   • It has an IsCompleted, which has a getter and returns a Boolean;
   • it has a GetResult() method, which returns void, or a result.

My question is that why Microsoft didn’t provide a interface to constrain these awaitable object?
The current method to implement awaitable object is a little complicated.

Answer 1

It is best answered in Lucian Wischik’s blog post Why must async methods return Task?

In summary (and I am not doing the blog post justice, you should read it), the issue is that Task already exists, so introducing an interface would mean

• All the internal methods would need to be changed to the interface, a break change and thus almost impossible for the framework people to willingly do.
• As a programmer you would constantly need to decide if you want to return Task or the interface, a decision that doesn’t matter much.
• The compiler would always need a concrete type, so even if you returned an interface from a method then it would still be compiled as Task.

The impact from the above is so massive that it doesn’t make sense to provide an interface.