Moq设置单元测试

Question

I have a simple method like:

public class MyService : IMyService
{
    public int Add(int x, int y)
    {
        return x + y;
    }
}

public interface IMyService
{
    int Add(int x, int y);
}

and I wrote a unit test with for that method:

public void PassingTest()
{
    var mock = new Mock<IMyService>();

    mock.Setup(x => x.Add(It.IsAny<int>(), It.IsAny<int>())).Returns<int, int>((x, y) => { return x + y; });

    var svc = mock.Object;

    var result = svc.Add(3, 3);
    var result2 = svc.Add(2, 5);
    result.Should().Be(6);
    result2.Should().Be(7);
}

Is this code is okay? Why should I must write Returns statement which is almost the same like whole method ?

Answer 1

You would only do something like this if IMyService was to be used as a dependency to another class that you wanted to test. If IMyService is the system under test then you wouldn't as you don't mock the system you are trying to test. You use an actual instance of the target class.

Let's say in a very simplified example, we have a class like this.

public class SomeClass {
    public SomeClass(IMyService service) {
        this.service = service;
    }
    private readonly IMyService service;

    public int SomeMethodThatUsesMyService(int input) {
        int someConstant = 10;

        var result = service.Add(someConstant, input);
        return result;
    }
}

If the implementation of IMyService does some processing that is IO dependent and is very difficult to test on its own. Then a mocking IMyService would make sense for trying to test that class.

[Fact]
public void Given_Input_SomeMethodThatUsesMyService_Should_Increase_By_Ten() {
    //Arrange
    var expected = 14;

    var mock = new Mock<IMyService>();

    mock.Setup(x => x.Add(It.IsAny<int>(), It.IsAny<int>())).Returns<int, int>((x, y) => { return x + y; });

    var svc = mock.Object;

    var sut = new SomeClass(svc);

    //Act
    var result = sut.SomeMethodThatUsesMyService(4);

    //Assert
    result.Should().Be(expected);
}