Sealed Classes in C# - ablealias/asp.net GitHub Wiki

Classes can be declared as sealed by putting the keyword sealed before the class definition. A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes prevent derivation. Because they can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster.

public sealed class D
{
    // Class members here.
}