c# this关键字用法

/// <summary>
    /// 扩展类 用于为原始类扩展方法  
    /// </summary>
   public static class AM_Extends
    {
        /// <summary>
        /// 为string类扩展了一个child方法，实现某功能
        /// </summary>
        /// <param name="str"></param>
        /// <param name="new_str"></param>
        public static void Child( this string str,string new_str)
        {
            object obj = str;
            str=new_str;
        }
    }

1 private void Form1_Load(object sender, EventArgs e)
2         {
3             string st1 = "123";
4             string st2 = "";
5             string st3 = "";
6             st3 = st2.Child(st1);//st3的值为“123”
7         }