Setup return value fails in a constellation with nullable ints

[Kiechlus]

Using moq version 4.16.1

Setting a return value does not work when an optional nullable int? is setup as It.IsAny<int>(). However, the compiler does not complain neither. Finding out what was the problem took us quite some time, so this can be a real annoyance.

public class MockedClass
{
    public virtual string MockedMethod(int? pageIndex = null) => "test";
}

public class ClassToTest
{
    private readonly MockedClass mockedProperty = null;

    public ClassToTest(MockedClass mockedProperty)
    {
        this.mockedProperty = mockedProperty;
    }

    public string SomeMethod()
    {
        return mockedProperty.MockedMethod();
    }
}

[Fact]
public void Test()
{
    // Works as expected
    var mock = new Mock<MockedClass>();
    mock.Setup(x => x.MockedMethod(It.IsAny<int?>())).Returns("foobar");
    var classToTest = new ClassToTest(mock.Object);
    Assert.Equal("foobar", classToTest.SomeMethod()); // This works

    // Does not work
    var mock2 = new Mock<MockedClass>();
    mock2.Setup(x => x.MockedMethod(It.IsAny<int>())).Returns("foobar"); // Setting up as It.IsAny<int>
    var classToTest2 = new ClassToTest(mock2.Object);
    Assert.Equal("foobar", classToTest2.SomeMethod()); // This throws with null != "foobar"
}

A similar issue was reported on #444 but could not be reproduced.

[stakx]

Actually, I take that back, this is a (admittedly subtle) user mistake, not a bug in Moq: You've set up MockedMethod to return "foobar" when it gets called with any int value... but the method gets called with null, which is not a valid int value; therefore the call to MockedMethod quite correctly doesn't match your setup. And since you haven't set mock2.CallBase = true, the base implementation (which would return "test") gets ignored, too.