在Winfrom 中，如何实现combox 的列表自动显示ToolTip提示 ？

ToolTip tt = null;

       ComboBox cb = null;

       private void Form1_Load(object sender, EventArgs e)

       {

           cb = new ComboBox();

           cb.Items.Insert(0,"第一");

           cb.Items.Insert(1,"第二");

           cb.Items.Insert(2,"第三");

           cb.Items.Insert(3,"第四");

           cb.DrawMode = DrawMode.OwnerDrawFixed;

           cb.DrawItem+=new DrawItemEventHandler(cb_DrawItem);

           cb.DropDownClosed+=new EventHandler(cb_DropDownClosed);

           this.Controls.Add(cb);

           cb.SelectedIndex = 1;

           tt = new ToolTip();

           tt.SetToolTip(cb, "zj");

       }

void cb_DrawItem(object sender, System.Windows.Forms.DrawItemEventArgs e)

       {

           // 绘制背景

           e.DrawBackground();

           //绘制列表项目

           e.Graphics.DrawString(cb.Items[e.Index].ToString(), e.Font, System.Drawing.Brushes.Black, e.Bounds);

           //将高亮的列表项目的文字传递到toolTip1(之前建立ToolTip的一个实例)

           if ((e.State & DrawItemState.Selected) == DrawItemState.Selected)

               tt.Show(cb.Items[e.Index].ToString(), cb, e.Bounds.X + e.Bounds.Width, e.Bounds.Y + e.Bounds.Height);

           e.DrawFocusRectangle();

       }

       void cb_DropDownClosed(object sender, EventArgs e)

       {

           tt.Hide(cb);

       }