实例成员的受保护访问

public class A

{

    protected int x;

    static void F(A a, B b) {

        a.x = 1;      // Ok

        b.x = 1;      // Ok

    }

}

public class B: A

{

    static void F(A a, B b) {

        a.x = 1;      // Error, must access through instance of B

        b.x = 1;      // Ok

    }

}