Inheritance in C# models the "is-a" relationship; that is, the derived class offers all the behavior of the base class, plus more. Because of the is-a relationship, derived instances are type compatible with the base class.

e.g.
class A {} 
class B:A { } 
class Program { 
  static void Main(string[] args) { 
    B b = new B(); 
    ConvertClass(b); 
  } 
  static void ConvertClass(A a) { 
  } 
}