gistfile1.cs

    public class BaseClass
    {
        public string STRING { get { return "Base"; } }
    }

    public class ChildClass : BaseClass
    {
        public new string STRING { get { return "Child"; } }
    }

gistfile2.cs

    [TestFixture]
    public class tests
    {
        [Test]
        public void NOT_WHAT_YOU_THOUGHT()
        {
            BaseClass bc = new ChildClass(); 
            Assert.That(bc.STRING == "Base");
        }

        [Test]
        public void STILL_NOT_WHAT_YOU_THOUGHT()
        {
            BaseClass bc = new ChildClass();
            ChildClass cc = bc as ChildClass;
            Assert.IsNotNull(cc);
            Assert.That(cc.STRING == "Child");
            
        }

        [Test]
        public void WTF()
        {
            Assert.That(test_method(new ChildClass()) == "Base");
        }

        string test_method(BaseClass baseClass)
        {
            return baseClass.STRING; 
        }
    }