Tasks on type/interface contracts - guidelines? [duplicate]

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• Interface naming convention for method returning Task [closed] 1 answer

Simple question: Are there any guidelines, either published by Microsoft itself or a common practice regarding to declaring tasks on type and interface contracts?

So far, I've seen many different approaches:

Synchronous and asynchronous using TAP (most .NET types):

 void DoSomething();
 Task DoSomethingAsync();

TAP only:

 Task DoSomething(); // notice, no 'Async' member naming

Split contracts:

 interface IDoSomething {

 }

 interface IDoSomethingSync : IDoSomething {
     void DoSomething();
 }

 interface IDoSomethingAsync: IDoSomething {
     Task DoSomething();
 }

...or finally, just making a contract without any tasks and leave it up to the consumer.

Is there an official way to declare this? For a while, I sticked to the first example since it's the most common one in the .NET framework, but even there it's sometimes not consistent because of legacy reasons.