选中第五个时，自动取消

      private void listBox1_Click(object sender, EventArgs e)
        {
            if (listBox1.SelectedItems.Count >4)
            {
                for (int i = 0; i < listBox1.SelectedItems.Count; i++)
                {
                    if (listBox1.SelectedItem != listBox1.SelectedItems[i])
                    { 
                    listBox1.SetSelected(listBox1.SelectedIndex,false);
                    return;
                    }
                }
            }
        }

------解决方案--------------------
      private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
           if (listBox1.SelectedItems.Count > 4)
           {
               for (int i = 0; i < listBox1.SelectedItems.Count; i++)
               {
                   if (listBox1.SelectedItem != listBox1.SelectedItems[i])
                   {
                       listBox1.SetSelected(listBox1.SelectedIndex, false);
                       return;
                   }
               }
           }
       }

------解决方案--------------------
还没有弄好吗？你可以这样子滴：
  void listBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
           if (listBox1.SelectedItems.Count > 4)
           {
               this.listBox1.SelectedItems.Remove(this.listBox1.SelectedItem);

           }
       }