C# is as 强制转换

1.is：检查对象类型是否兼容制定类型。返回true,false

　　bool x = a is b;

2.as: 效率高于is

因为B为父类，子类可以转换为父类，故b不等于空，无输出

public class main
    {

        public static void Main ()
        {
            A a =new A ();
            B b = a as B;
            if (b == null) {
                Console.WriteLine ("－－－");
            }
        }
    }

    class A:B{ }

    class B{ }

与上面情况相反，输出---

public class main
    {

        public static void Main ()
        {
            A a =new A ();
            B b = a as B;
            if (b == null) {
                Console.WriteLine ("－－－");
            }
        }
    }

    class A{ }

    class B:A{ }