接口如何知道调用哪个类方法?这是正确的代码吗?或不
namespace IntExample
{
    interface Iinterface
    {
       void add();
       void sub();
    }
    public partial class Form1 : Form,Iinterface
    {
        public Form1()
        {
            InitializeComponent();
        }
        public void add()
        {
            int a, b, c;
            a = Convert.ToInt32(txtnum1.Text);
            b = Convert.ToInt32(txtnum2.Text);
            c = a + b;
            txtresult.Text = c.ToString();
        }
        public void sub()
        {
            int a, b, c;
            a = Convert.ToInt32(txtnum1.Text);
            b = Convert.ToInt32(txtnum2.Text);
            c = a - b;
            txtresult.Text = c.ToString();
        }
        private void btnadd_Click(object sender, EventArgs e)
        {
            add();
        }
        private void button2_Click(object sender, EventArgs e)
        {
            sub();
        }
        class cl2 : Form1,Iinterface
        {
            public void add()
            {
                int a, b, c;
                a = Convert.ToInt32(txtnum1.Text);
                b = Convert.ToInt32(txtnum2.Text);
                c = a + b;
                txtresult.Text = c.ToString();
            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
        }