string s ="23,56,11,76";
 string[] id=ids.Split(new char[',']);

 string s ="23,56,11,76";
 string[] id=ids.Split(new char[',']);

  int index = 0; 
控制index就可以取id[index]的值了啊

------解决方案--------------------

        private void btnFirst_Click(object sender, EventArgs e)
       {
           ShowStr("first");
       }

       private void btnPre_Click(object sender, EventArgs e)
       {
           ShowStr("pre");
       }

       private void btnNext_Click(object sender, EventArgs e)
       {
           ShowStr("next");

       }

       private void btnLast_Click(object sender, EventArgs e)
       {
           ShowStr("last");

       }
       private int pos = 0;
       private void ShowStr(string flag)
       {
           string s = "23,56,11,76";
           string[] id = s.Split(',');
           if (id.Count() > 0)
           {
              
               switch (flag)
               {
                   case "first":
                       pos = 0;
                       break;
                   case "pre":

                       if (pos == 0)
                           pos = id.Length - 1;
                       else
                           pos--;

                       break;
                   case "next":
                       if (pos == id.Length - 1)
                           pos = 0;
                       else
                           pos++;
                       break;
                   case "last":
                       pos = id.Length - 1;
                       break;

               }
               label1.Text = id[pos];
           }
       }