When unit testing, how do I mock a return null from async method?

问题

Normally, I mock my repo like so:

var repository = new Mock<ISRepository>();
repository.Setup(r => r.GetMemberAsync(email))
    .Returns(Task.FromResult(new Member
    {
        FirstName = firstName,
        LastName = lastName
    }));

But, in my code, I check to see if the member is not found, i.e. GetMemberAsync returns null. How do I mock this?

I tried:

var repository = new Mock<ISRepository>();
repository.Setup(r => r.GetMemberAsync(email))
    .Returns(Task.FromResult<object>(null));

but I get a compile error.

回答1:

You get a compiler error because you return a task that doesn't match the type the async method returns. You should return Task<Member> instead of simply Task<object>:

repository.Setup(r => r.GetMemberAsync(email)).Returns(Task.FromResult<Member>(null));

回答2:

It is also possible to return the result without using the Task class.

repository
    .Setup(r => r.GetMemberAsync(email))
    .ReturnsAsync((Member)null);

回答3:

Old question but you can also do this which I think it cleaner:

Assuming the default value of your object is null you can also use:

default(<insert object type here>)

e.g.

default(Member)
default(List<string>)
etc.

Full Example:

var myRepo = new Mock<IMyRepo>();
myRepo 
    .Setup(p => p.GetAsync("name"))
    .ReturnsAsync(default(List<string>));

来源：https://stackoverflow.com/questions/33356808/when-unit-testing-how-do-i-mock-a-return-null-from-async-method