下面是我写的一个扩展combobox控件的例子
using System;
using System.Collections;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Windows.Forms;

namespace ComboBoxEx
{
 public class ComboBoxEx:ComboBox
 {
  ListBox lst=new ListBox();
  public ComboBoxEx()
  {
   lst.SelectionMode=SelectionMode.MultiExtended;
   this.DrawMode=DrawMode.OwnerDrawFixed;//只有设置这个属性为OwnerDrawFixed才可能让重画起作用
   lst.KeyUp+=new KeyEventHandler(lst_KeyUp);
   lst.MouseUp+=new MouseEventHandler(lst_MouseUp);
   lst.KeyDown+=new KeyEventHandler(lst_KeyDown);
  }
  #region Property
  [Description("选定项的值"),Category("Data")]
  public ListBox.SelectedObjectCollection SelectedItems
  {
   get
   {
    return lst.SelectedItems;
   }
  }

  #endregion

  #region override
  protected override void OnKeyUp(KeyEventArgs e)
  {
   base.OnKeyDown(e);
   bool Pressed=(e.Control && ((e.KeyData & Keys.A)==Keys.A));
   if(Pressed)
   {
    for(int i=0;i<lst.Items.Count;i++)
     lst.SetSelected(i,true);
   }
  }
  protected override void OnMouseDown(MouseEventArgs e)
  {
   this.DroppedDown=false;
   
  }
  protected override void OnMouseUp(MouseEventArgs e)
  {
   this.DroppedDown=false;
   lst.Focus();
  }

  protected override void OnDropDown(EventArgs e)
  {
   lst.Items.Clear();
   lst.Show();
   lst.ItemHeight=this.ItemHeight;
   lst.BorderStyle=BorderStyle.FixedSingle;
   lst.Size=new Size(this.DropDownWidth,this.ItemHeight*(this.MaxDropDownItems-1)-(int)this.ItemHeight/2);
   lst.Location=new Point(this.Left,this.Top+this.ItemHeight+6);
   lst.BeginUpdate();
   for(int i=0;i<this.Items.Count;i++)   
    lst.Items.Add(this.Items[i]);
   lst.EndUpdate();
   
   this.Parent.Controls.Add(lst);
  }

  #endregion
  private void lst_KeyUp(object sender, KeyEventArgs e)
  {
   this.OnKeyUp(e);
  }

  private void lst_MouseUp(object sender, MouseEventArgs e)
  {
   try
   {
    this.Text="";
    for(int i=0;i<lst.SelectedItems.Count;i++)
    {
     if(i==0)
      this.Text=lst.SelectedItems[i].ToString();
     else  
     {
      this.Text=this.Text+","+lst.SelectedItems[i].ToString();
     }
    }
   }
   catch
   {
    this.Text="";
   }
   bool isControlPressed=(Control.ModifierKeys==Keys.Control);
   bool isShiftPressed=(Control.ModifierKeys==Keys.Shift);
   if(isControlPressed || isShiftPressed)
    lst.Show();
   else
    lst.Hide();
  }

  private void lst_KeyDown(object sender, KeyEventArgs e)
  {
   switch(e.KeyData)
   {
    case Keys.Down:
     if(lst.SelectedItems.Count!=0)
     {
      this.Text=lst.SelectedItem.ToString();
     }
     else
      this.Text=this.Items[0].ToString();
     break;
    case Keys.Up:
     if(lst.SelectedItems.Count!=0)
     {
      this.Text=lst.SelectedItem.ToString();
     }
     else
      this.Text=this.Items[0].ToString();
     break;
   }
  }
 }
}
如果有什么不理解的可以加我ＱＱ，不过在加的时候要说明原因．ＱＱ：３４７９５１７３