I recently discovered a rather annoying limitation in Moq: you cannot setup expectations on the ToString method.  For a good discussion of the issue, check out Sean’s post.  His solution was to add ToString explicitly to the interface you are mocking, but I don’t want to dirty up my interfaces unnecessarily.  Fortunately, Moq does allow you to create mocks that implement multiple interfaces, so you could move the ToString method to a dummy interface, and use Mock<T>.As to setup expectations on the dummy interface.  That’s the approach I took, but I wrapped it up in a nice extension method:

public static class MoqExtensions
{
    public static ISetup<IToStringable, string> SetupToString<TMock>(this Mock<TMock> mock) where TMock : class
    {
        return mock.As<IToStringable>().Setup(m => m.ToString());
    }
    
    //Our dummy nested interface.
    public interface IToStringable
    {
        /// <summary>
        /// ToString.
        /// </summary>
        /// <returns></returns>
        string ToString();
    }
}

Now I can setup expectations on ToString by simply using:

[Test]
public void ExpectationOnToStringIsMet()
{
    var widget = new Mock<IWidget>();
    widget.SetupToString().Returns("My value").Verifiable();

    Assert.That(widget.Object.ToString(), Is.EqualTo("My value"));

    widget.Verify();
}