Sealed Modifier

Prevents derivation of classes or overriding of methods.

public sealed class Circle : Shape

you cannot derive a class from Circle

public class Circle : Shape
{
  public sealed override void Draw()
  {
  .....
  }
}

seal can be applied to only overriden methods.

Why ?

Sealed classes are slightly faster because of some run time optimizations.

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Related Subjects : interface, Polymorphism, Abstract