Following is the code. create a class lib add the ref to NUnit framework 2.5.3.9345 and Moq.dll 4.0.0.0 and paste the following code. Try running it on my machine it throws

TestCase 'MoqTest.TryClassTest.IsMessageNotNull' failed: Moq.MockException : Expected invocation on the mock at least once, but was never performed: v => v.Model = It.Is(value(Moq.It+<>c__DisplayClass21[MoqTest.GenInfo]).match) at Moq.Mock.ThrowVerifyException(IProxyCall expected, Expression expression, Times times, Int32 callCount) at Moq.Mock.VerifyCalls(Interceptor targetInterceptor, MethodCall expected, Expression expression, Times times) at Moq.Mock.VerifySet[T](Mock1 mock, Action1 setterExpression, Times times, String failMessage) at Moq.Mock1.VerifySet(Action`1 setterExpression) Class1.cs(22,0): at MoqTest.TryClassTest.IsMessageNotNull()

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Moq;
using NUnit.Framework;

namespace MoqTest
{
    [TestFixture]
    public class TryClassTest
    {
        [Test]
        public void IsMessageNotNull()
        {
            var mockView = new Mock<IView<GenInfo>>();
            mockView.Setup(v => v.ModuleId).Returns("");

            TryPresenter tryPresenter = new TryPresenter(mockView.Object);
            tryPresenter.SetMessage(new object(), new EventArgs());
            // mockView.VerifySet(v => v.Message, Times.AtLeastOnce());
            mockView.VerifySet(v => v.Model = It.Is<GenInfo>(x => x != null));
        }
    }

    public class TryPresenter
    {
        private IView<GenInfo> view;
        public TryPresenter(IView<GenInfo> view)
        {
            this.view = view;
        }

        public void SetMessage(object sender, EventArgs e)
        {
            this.view.Model = null;
        }
    }

    public class MyView : IView<GenInfo>
    {
        #region Implementation of IView<GenInfo>

        public string ModuleId
        {
            get; set;
        }

        public GenInfo Model
        {
            get; set;
        }

        #endregion
    }

    public interface IView<T>
    {
        string ModuleId { get; set; }
        T Model { get; set; }
    }

    public class GenInfo
    {
        public String Message { get; set; }
    }
}

And if you change one line mockView.VerifySet(v => v.Model = It.Is(x => x != null)); to mockView.VerifySet(v => v.Model, Times.AtLeastOnce()); it works fine.

I think Exception is incorrect.