How to mock Imediator pattern controller that returns ActionResult<ICollection<Course>> GetResult(CancellationToken)

Question

I am a totally newbie to Mediator pattern and Moq. I have an API controller that has an async task method and want to mock this method

[Route("api/[controller]")]
[ApiController]
[Authorize]
public class UsersController : UsersControllerBase
{
    private readonly IMediator _mediator;

    public UsersController(IMediator mediator)
    {
        this._mediator = mediator;
    }
    
    [HttpGet("GetUsers")]
    public async Task<ActionResult<ICollection<User>>> GetUsers(CancellationToken cancellationToken)
    {

        var result = await _mediator.Send(new UserGetRequest(), 
                           cancellationToken).ConfigureAwait(false);

        return DataOrProblem(result);
    }
}

I am trying to create or mock this method and not sure how to do it? Here is what I have tried

public class GetUsersTest
{
    //Arrange
    private readonly Mock<IMediator> _mediator = new Mock<IMediator>();
    private readonly Mock<UsersController> _sut;

    public GetUsersTest()
    {
        _sut = new Mock<UsersController>(_mediator);
    }
}

Answer 1

If the controller is the subject under test then do not mock it. Create an actual instance, injecting the mocked mediator.

public class UsersControllerTest {
    
    public async Task GetUsersTests() {
        //Arrange
        Mock<IMediator> mediator = new Mock<IMediator>();
        UsersController sut = new UsersController(mediator.Object);
        
        //... setup the behavior of the mocked mediator
        ICollection<User> expected = //...provide fake data here
        mediator
            .Setup(s => s.Send<UserGetRequest>(It.IsAny<UserGetRequest>(), It.IsAny<CancellationToken>()))
            .ReturnsAsync(expected);

        //Act
        ActionResult<ICollection<User>> result = await sut.GetUsers(CancellationToken.None);

        //Assert
        //assert the expected behavior with what actually happened
    }
}