using System;
class Client
{
public static void Main ()
{
A a = new A();
B b = new B();
a.b = b;
b.a = a;
}
}
class A
{
public B b;
}
class B
{
public A a;
}
class Client
{
public static void Main ()
{
A a = new A();
B b = new B();
a.b = b;
b.a = a;
}
}
class A
{
public B b;
}
class B
{
public A a;
}

using System;
class Client
{
public static void Main ()
{
Base b = new Base();
Derived d = new Derived();
b.d = d;
Console.WriteLine(b.d.m);
}
}
class Base
{
public int n = 9;
public Derived d;
}
class Derived : Base
{
public int m = 10;
}
class Client
{
public static void Main ()
{
Base b = new Base();
Derived d = new Derived();
b.d = d;
Console.WriteLine(b.d.m);
}
}
class Base
{
public int n = 9;
public Derived d;
}
class Derived : Base
{
public int m = 10;
}

注意看上面这个图里面表示出来的继承的关系。