How to test a mocked object when that object requires an attribute

Today I needed to unit test a method that checked for and required a specific attribute on the class. I ended up with a fairly simple solution, the decorator pattern. Simply create a decorator for the type that needs the attribute, put the attribute on the decorator. Pass you mocked class to a new instance of the decorator when you pass to the method being tested. I sounds like more work then it is, of course it may not always make sense to go this route. Anyway, here’s the code.

using NUnit.Framework;
using NUnit.Framework.SyntaxHelpers;
using Rhino.Mocks;

namespace MockingClassesWithRequiredAttributes
{
    [TestFixture]
    public class Class1
    {
        [Test]
        public void Test()
        {
            MockRepository mocks = new MockRepository();
            IRun mockedRun = mocks.DynamicMock<IRun>();
            using (mocks.Record())
            {
                mockedRun.Run();
                LastCall.On(mockedRun);
            }
            using (mocks.Playback())
            {
                IRequireCategoryAttribute(new MockDecorator(mockedRun));
            }
        }

        static void IRequireCategoryAttribute(IRun run)
        {
            object[] attributes = run.GetType().GetCustomAttributes(
                typeof(System.ComponentModel.CategoryAttribute), false);
            Assert.That(attributes.Length, Is.GreaterThan(0));
            run.Run();
        }
    }

    [System.ComponentModel.Category]
    class MockDecorator : IRun
    {
        private readonly IRun run;
        
        public MockDecorator(IRun run)
        {
            run = run;
        }

        public void Run()
        {
            _run.Run();
        }
    }

    public interface IRun
    {
        void Run();
    }
}


  One downside to this approach is the MockDecorator class that is just for testing. Any other approaches you’ve used?