SealMethod.cs

    public static class SealMethod
    {
        public static void MainFunc()
        {
            MiddleClass a = new MiddleClass();
            a.SomeWeirdFunction();

            Bottom b = new Bottom();
            b.SomeWeirdFunction();
        }
    }

    public class TopClass
    {
        private string ThisIsOnlyAccessibleForTopClass { get; set; }
        private string ThisIsOnlyAccessibleForTopClassAndMiddleClass { get; set; }
        protected virtual string OnlyMiddleClass()
        {
            return ThisIsOnlyAccessibleForTopClassAndMiddleClass = "As this is a MiddleClass class, I am accessible here.. That is good!";
        }

        public virtual void SomeWeirdFunction()
        {
            ThisIsOnlyAccessibleForTopClass = "I can access this here as it is grand father's private member!";
        }
    }

    public class MiddleClass : TopClass
    {
        public override void SomeWeirdFunction()
        {
            Console.WriteLine(base.OnlyMiddleClass()); // As this is a MiddleClass class, I am accessible here.. That is good!
            Console.WriteLine(OnlyMiddleClass()); // Prohibited
        }

        protected override sealed string OnlyMiddleClass() => "Prohibited";
    }

    public class Bottom : MiddleClass
    {
        public override void SomeWeirdFunction()
        {
            Console.WriteLine(base.OnlyMiddleClass()); // Prohibited
            Console.WriteLine(OnlyMiddleClass()); // Prohibited
        }
    }