Modifiers: virtual, override, new, abstract, sealed, internal

public abstract class A
   {
      public abstract void Do();
      public virtual void DrawLine() { Console.WriteLine("A:DrawLine"); }
      public virtual void DrawPoint() { Console.WriteLine("A:DrawPoint"); }
      public virtual void DoWork() { Console.WriteLine("A:DoWork"); }
   }

   public class B : A
   {
      public override void Do() { Console.WriteLine("B:DO"); }
      public new void DrawLine() { Console.WriteLine("B:DrawLine"); }
      public override void DrawPoint() { Console.WriteLine("B:DrawPoint"); }
      public override void DoWork() { Console.WriteLine("B:DoWork"); }
   }

   public class C : B
   {
      public sealed override void DoWork() { Console.WriteLine("C:DoWork"); }
   }

   public class D : C
   {
      // Attempting to override DoWork causes compiler error CS0239.
      //public override void DoWork() {}
      public new void DoWork() { Console.WriteLine("D:DoWork"); }
   }

   // Only accessible within the same assembly
   internal class MyInternalClass
   {
      public string Value { get; set; }
   }

   //public sealed class MySealedClass { }
   //public class Derived : MySealedClass { } // error CS0509: '': cannot derive from sealed type

   interface I
   {
      void M();
   }
   abstract class AC : I
   {
      public abstract void M();
   }

   public class Base
   {
      public virtual void DoWork(int param) { Console.WriteLine("Base:DoWork(int)"); }
   }

   public class Derived : Base
   {
      public override void DoWork(int param) { Console.WriteLine("Derived:DoWork(int)"); }
      public void DoWork(double param) { Console.WriteLine("Derived:DoWork(double)"); }
   }

   A a = new B();
   a.DrawLine(); // output A:DrawLine
   a.DrawPoint(); // output B:DrawPoint
   a.Do(); // output B:DO

   D d = new D();
   d.DoWork(); // output D:DoWork
   C c = (C)d;
   c.DoWork(); // output C:DoWork
   B b = (B)d;
   b.DoWork(); // output C:DoWork

   int val = 5;
   Derived der = new Derived();
   der.DoWork(val);  // Calls DoWork(double).
   ((Base)der).DoWork(val);  // Calls DoWork(int) on Derived.