abstract class AbstractClass
    {
        protected void NotModifiedMethod() { }

        protected abstract void AbstractMethod();

        protected virtual void VirtualMethod() { }
    }

    class ConcreteClass : AbstractClass
    {
        //This HAS to be overriden because it's abstract
        protected override void AbstractMethod() { }

        //This CAN be overriden because it's virtual
        protected override void VirtualMethod()
        {
            base.VirtualMethod();
        }

        //You CAN NOT do this, because NotModifiedMethod is not
        //marked as abstract or virtual
        protected override void NotModifiedMethod() { }
    }