Rhino.Mocks produces InvalidCastException when returning polymorphic object

Question

I'm using Rhino.Mocks 3.6 for the first time. I'm trying to create a stub for an interface that returns an inherited type (B). When I try to do this, it will generate an InvalidCastException trying to convert some proxy object to the base class (A).

For example:

class A {}

class B : A {}

interface IMyInterface
{
    A GetA();
}

// Create a stub
var mocks = new MockRepository();
var stub = mocks.Stub<IMyInterface>();
Expect.Call( stub.GetA() ).Return( new B() );

// This will throw an InvalidCastException
var myA = stub.GetA();

It seems to me that the problem is that it's generating proxy classes that do not have the same inheritance structure as the existing classes. However, it seems to me like a fairly common situation to return a subclass of the type specified by the method signature.

I've tried a few variations, but I can't get this to work. Any ideas?

Answer 1

Use mocks.Record to set up your mocked objects, use mocks.PlayBack to run your tests.

public class A { }

public class B : A { }

public interface IMyInterface
{
    A GetA();
}

[TestFixture]
public class RhinoTestFixture
{
    [Test]
    public void TestStub()
    {
        // Create a stub
        var mocks = new MockRepository();
        IMyInterface stub;
        using (mocks.Record())
        {
            stub = mocks.Stub<IMyInterface>();
            stub.Expect(x => stub.GetA()).Return((new B()));
        }

        using (mocks.Playback())
        {
            var myA = stub.GetA();
            Assert.IsNotNull(myA);
        }
    }
}