B b = new B();
  if (b.GetType().GetInterface("ISay") != null)
            {
                //如果实现了接口“ISay”,...
            }

B b = new B();
 if (b is ISay)
            {
                Console.WriteLine("类B实现了接口ISay");
            }

   B b = new B();
  ISay say = b as ISay;
            if (say != null)
            {
                Console.WriteLine("类B实现了接口ISay");
            }