How do you stub an async method using Visual studio's Fakes framework?

Question

The methods I need to stub are called e.g. like this:

List<DocumentInfo> documentInfosToDelete = await _documentInfoRepository.GetListByExternalIdAsync(
    partyLegalEntity, 
    externalId,
    type,
    status);

This works, but generates a compiler warning: 'This async method lacks 'await' operators' etc.

   testService.DocumentInfoRepos.GetListByExternalIdAsyncStringStringDocumentTypeDocumentStatus =
                (async (a, b, c, d) => 
            {
                GetListByExternalIdAsyncCalled = true;
                return new List<DocumentInfo>();
            });

I want to get rid of those warnings, as they would drown out any warnings about actual problems.

Returning new Task(...) hangs. If I understand what I have found on the WWW correctly, Task.Run(...) does something completely different from the await async pattern we are using.

What to do?

Answer 1

Returning new Task(…) hangs, because that creates an unstarted Task. What you can do instead is to create an already completed Task by calling Task.FromResult:

testService.DocumentInfoRepos.GetListByExternalIdAsyncStringStringDocumentTypeDocumentStatus =
        (a, b, c, d) => 
        {
            GetListByExternalIdAsyncCalled = true;
            return Task.FromResult(new List<DocumentInfo>());
        };

This code will behave the same as an async method with no await, but it will not produce a warning.