Moq tests using ExpectSet() with It.Is<T>() aren't behaving as... expected.

Question

I've isolated the behaviour into the following test case. I'd be grateful to anyone who can tell me how to expect/verify a property set for a List<T> property - it appears there's something going on inside It.Is<T>(predicate) that isn't making a whole lot of sense to me right now. Sample code will run as a console app from VS2008 - you'll need to add a reference to Moq 2.6 (I'm on 2.6.1014.1) - please try uncommenting the different ExpectSet statements to see what's happening...

using System;
using Moq;
using System.Collections.Generic;

namespace MoqDemo {

    public interface IView {
        List<string> Names { get; set; }
    }

    public class Controller {
        private IView view;

        public Controller(IView view) {
            this.view = view;
        }

        public void PopulateView() {
            List<string> names = new List<string>() { "Hugh", "Pugh", "Barney McGrew" };
            view.Names = names;
        }

        public class MyApp {
            public static void Main() {
                Mock<IView> mockView = new Mock<IView>();

                // This works - and the expectation is verifiable.
                mockView.ExpectSet(mv => mv.Names);

                // None of the following can be verified.
                // mockView.ExpectSet(mv => mv.Names, It.Is<Object>(o => o != null));
                // mockView.ExpectSet(mv => mv.Names, It.Is<List<string>>(names => names.Count == 3));
                // mockView.ExpectSet(mv => mv.Names, It.IsAny<IList<String>>());

                Controller controller = new Controller(mockView.Object);
                controller.PopulateView();
                try {
                    mockView.VerifyAll();
                    Console.WriteLine("Verified OK!");
                } catch (MockException ex) {
                    Console.WriteLine("Verification failed!");
                    Console.WriteLine(ex.Message);
                }
                Console.ReadKey(false);
            }
        }
    }
}

Answer 1

The second parameter of ExpectSet() is the value you're expecting. You can't use It.Is<T> in this case as there's no overload that takes a predicate - though it would be nice ;) Here's a (simplified) excerpt from your sample, illustrating the use of a value:

var mockView = new Mock<IView>();
var list = new List<string> { "Hugh", "Pugh", "Barney McGrew" };

mockView.ExpectSet(mv => mv.Names, list);

mockView.Object.Names = list;

Hope that helps.

Edit: fixed typo.

Answer 2

I'm not using the very latest version of Moq, so I don't have an overload of ExpectSet that takes two parameters, but I've had some success with this pattern:

mockView.ExpectSet(mv => mv.Names).Callback(n => Assert.That(n != null));

The Assert (from NUnit) call in the callback will throw an exception if the value assigned to .Names doesn't match the predicate. It does make it hard to trace when a test fails, though. I agree that the ability to pass an It.Is or It.IsAny as the second parameter would be handy.

Answer 3

Moq doesn't provide an overload receiving It.IsAny as it's effectively the same as calling ExpectSet without passing an expected value ;)

Answer 4

BTW, It.Is is not supported on ExpectSet. Your code compiles just because they are regular method invocations when used as values (as opposed to expressions), whereas when used in an Expect expression they are pre-processed by Moq and given specific meaning (rather than the null/default value that all It.Is members actually return).

You could use the stub behavior on the given property (mockView.Stub(mv => mv.Names)) and later assert directly for its value after execution.