string str = "101 102 103";
         private void Form8_Paint(object sender, PaintEventArgs e)
         {
             Font font = new Font("宋体",10);
             Point p = new Point(0,0);
             e.Graphics.DrawString(str, font, Brushes.Black, p);
         }

         private void Form8_Click(object sender, EventArgs e)
         {
             str += " 104";
             this.Refresh();
         }

------解决方案--------------------

 List<string> str = new List<string>();
       int i = 100;
       private void button1_Click(object sender, EventArgs e)
       {
           i++;
           str.Add(i.ToString ());
           this.Invalidate();
       }

       private void Form3_Paint(object sender, PaintEventArgs e)
       {
           Graphics  g = e.Graphics;
           for (int i = 0; i < str.Count; i++)
           {
               g.DrawString(str[i], new Font("宋体", 12), Brushes.Black, 10 + 30 * i, 20);
           }
       }

       private void Form3_Load(object sender, EventArgs e)
       {
           str.Clear();
       }