Casting Mock To Interface Loses Value

[aolszowka]

After upgrading to Moq 4.2.1502.911 we are seeing what we believe is a bug in Moq when attempting to cast a mocked type to an Interface. We've produced a test case below along with a few unit tests.

namespace MoqBug
{
    using System.Collections.Generic;
    using System.Linq;
    using Moq;
    using NUnit.Framework;

    [TestFixture]
    public class FunWithTypes
    {
        [Test]
        public void MoqErrors()
        {
            Mock<ExtendingTypeBase> fake = new Mock<ExtendingTypeBase>(42);
            ExtendingTypeBase realFake = fake.Object;

            // Make sure we're telling the truth that the value is mocked prior to our frobbing
            Assert.That(realFake.ExtendedValue, Is.EqualTo(42));

            Frobber frobber = new Frobber(realFake);

            Assert.That(frobber.WasExtendedType, Is.True);
            // BUGBUG: Seems to have been set back to the default for this type (Moq type is by default LOOSE)
            Assert.That(frobber.ExtendedTypeValue, Is.EqualTo(42), "BUGBUG: Moq Lost the value and set back to default.");
        }

        [Test]
        public void CSharpIsCoolWithIt()
        {
            ExtendingTypeBase real = new ExtendedConcreteType(42);

            // Make sure we're telling the truth that the value is mocked prior to our frobbing
            Assert.That(real.ExtendedValue, Is.EqualTo(42));

            Frobber frobber = new Frobber(real);

            Assert.That(frobber.WasExtendedType, Is.True);
            Assert.That(frobber.ExtendedTypeValue, Is.EqualTo(42));
        }

        [Test]
        public void MoqErrorsTwo()
        {
            var rootMock = new Mock<ExtendingTypeBase>(42);
            ExtendingTypeBase realObject = rootMock.Object;
            IExtendedType realObjectInterfaceType = rootMock.Object;
            Assert.That(realObject.ExtendedValue, Is.EqualTo(42));
            // BUGBUG: Seems to have been set back to the default for this type (Moq type is by default LOOSE)
            Assert.That(realObjectInterfaceType.ExtendedValue, Is.EqualTo(42));
        }

        [Test]
        public void MoqErrorsThree()
        {
            var rootMock = new Mock<ExtendingTypeBase>(42);
            ExtendingTypeBase realObject = rootMock.Object;
            IExtendedType realObjectInterfaceType = rootMock.As<IExtendedType>().Object;
            Assert.That(realObject.ExtendedValue, Is.EqualTo(42));
            // BUGBUG: Seems to have been set back to the default for this type (Moq type is by default LOOSE)
            Assert.That(realObjectInterfaceType.ExtendedValue, Is.EqualTo(42));
        }
    }

    public class Frobber
    {
        List<ISharedType> internalStore;

        public Frobber(params ISharedType[] inputs)
        {
            // Save the Internal Store
            this.internalStore = new List<ISharedType>(inputs);
        }

        public bool WasExtendedType
        {
            get
            {
                return this.internalStore.First() is IExtendedType;
            }
        }

        public int ExtendedTypeValue
        {
            get
            {
                return ((IExtendedType)this.internalStore.First()).ExtendedValue;
            }
        }
    }

    public class ExtendedConcreteType : ExtendingTypeBase
    {
        public ExtendedConcreteType(int extendedValue):
            base(extendedValue)
        {
        }

        public override int CommonValue
        {
            get;
            set;
        }
    }

    public interface ISharedType
    {
        int CommonValue { get; set; }
    }

    public interface IExtendedType
    {
        int ExtendedValue { get; set; }
    }

    public abstract class ExtendingTypeBase : ISharedType, IExtendedType
    {
        public ExtendingTypeBase(int extendedValue)
        {
            this.ExtendedValue = extendedValue;
        }

        public abstract int CommonValue { get; set; }
        public int ExtendedValue { get; set; }
    }
}

Looking at the issues log I think it might be related to #164 and probably the root cause of #156 along with #162. I did a pull of the latest source and the issue is still present, however nothing was readily apparent in my debugging, but I'll admit I did not dig very deep.

The first scenario posted is more or less a very very stripped down version of something we have in our "production" test code, the remaining test cases are what some of our other developers came up with.

[stakx]

Fixed in Moq 4.7.58.

[aolszowka]

Wow I don't know why I wasn't notified of this fix back in 2017 but for some reason thought about coming back to this bug. You are awesome @stakx Doing some amazing work!

[stakx]

@aolszowka:

The issue was already fairly old when I closed it, perhaps GitHub muted it for that reason...?

However it may be... thank you for the kind words! Glad if I could help.