Moq with Task await

I think you need to return the Task from the DoSomething mock

this._mockService.Setup(x => x.DoSomething(It.IsAny<CredentialDataList>(), It.IsAny<string>()))
    .Returns(Task.FromResult<int>(0));

DoSomething returns null instead of returning a Task, and so you get an exception when awaiting it. You need to specify when building the mock that it should return a Task.

In this case it seems that you can simply return an already completed task using Task.FromResult so the mock setup should look like this:

this._mockService.Setup(...).Returns(Task.FromResult(false));

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Beginning with the next version of .Net (4.6) you can use Task.CompletedTask like this:

this._mockService.Setup(...).Returns(Task.CompletedTask);

You can reduce the amount of clutter in the code by using ReturnsAsync

this._mockService.Setup(...).ReturnsAsync(false);

This way you can remove the Task.FromResult part of the code