疑惑的virtual method

 class A
    {
       
       public virtual void fun(int i)
        {
            Console.WriteLine(i);
        }
       public void fun2(A a)
        {
            a.fun(1);
            fun(5);
        }
    }
    class B:A
    {
       
       public override void fun(int i)
        {
           base.fun(i+1);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            A a = new A();
            B b = new B();
            a.fun2(b);
            b.fun2(a);

        }
    }