自定义可输入下拉框.

一、在vs2003环境下测试良好，在vs2005中不可以（主要表现在输入框在下层，下拉框在上层，设置z－index也不行）
二、程序源码：
using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;
using System.Drawing.Design;
using System.Drawing;
using System.Web;
using System.Collections;

namespace WebControls
{
 /// <summary>
 /// WebCustomControl1 的摘要说明。
 /// </summary>
 [ToolboxBitmapAttribute(typeof(DropDownListExtend), "DropDownListExtend.bmp")]
 [DefaultProperty("Text"),
  ToolboxData("<{0}:DropDownListExtend runat=server></{0}:DropDownListExtend>")]
 public class DropDownListExtend : System.Web.UI.WebControls.WebControl
 {
  private TextBox tb;
  private DropDownList ddl;
  private ArrayList items;

  public DropDownListExtend()
  {
   tb=new TextBox();
   ddl=new DropDownList();
   items=new ArrayList();
   
  }

  public void Add(System.Web.UI.WebControls.ListItem li)
  {
   ddl.Items.Add(li);
   items.Add(new string[]{li.Text,li.Value});
   
   ViewState[this.ClientID]=items;
   
  }
  private void ReInit()
  {
   
   this.ddl.Items.Clear();
   if(ViewState[this.ClientID]!=null)
   {
    ArrayList a=ViewState[this.ClientID] as ArrayList;
    for(int i=0;i<a.Count;i++)
    {
     string[] array=a[i] as string[];
     ddl.Items.Add(new ListItem(array[0],array[1]));
    }
    
   }
   
   ddl.Items.Insert(0,new ListItem("请选择","-1"));
   if(ViewState[this.ClientID+"_text"]!=null)
   {
    this.tb.Text=ViewState[this.ClientID+"_text"].ToString();
   }
   
  }
  public void Insert(int index,ListItem li)
  {
   ddl.Items.Insert(index,li);
   items.Insert(index,new string[]{li.Text,li.Value});
   ViewState[this.ClientID]=items;
  
  }
  [Bindable(true),
  Category("Appearance"),
  DefaultValue("")]
  public object DataSource
  {
   get
   {
    return ddl.DataSource;
   }
   set
   {
    ddl.DataSource=value;
   }
  }

  [Bindable(true),
  Category("Appearance"),
  DefaultValue("")]
  public string DataTextField
  {
   get
   {
    return ddl.DataTextField;
   }
   set
   {
    ddl.DataTextField=value;
   }
  }

  [Bindable(true),
  Category("Appearance"),
  DefaultValue("")]
  public string DataValueField
  {
   get
   {
    return ddl.DataValueField;
   }
   set
   {
    ddl.DataValueField=value;
   }
  }

  [Bindable(true),
   Category("Appearance"),
   DefaultValue("")]
  public string Text
  {
   get
   {
    string temp=System.Web.HttpContext.Current.Request.Form[this.ClientID+"_tb"];
    ViewState[this.ClientID+"_text"]=temp;
    return temp;
   }

   set
   {
    ViewState[this.ClientID+"_text"]=value;
    tb.Text = value;
   }
  }

  /// <summary>
  /// 将此控件呈现给指定的输出参数。
  /// </summary>
  /// <param name="output"> 要写出到的 HTML 编写器 </param>
  protected override void Render(HtmlTextWriter output)
  {
   int iWidth = Convert.ToInt32(base.Width.Value);
   if(iWidth == 0)
   {
    iWidth = 102;
    base.Width = Unit.Parse("102px");
   }

   int sWidth = iWidth + 16;
   int spanWidth=sWidth-18;
   

   output.Write("<div style=\"POSITION:relative\">");
   output.Write("<span style=\"MARGIN-LEFT:" + iWidth.ToString() + "px;OVERFLOW:hidden;WIDTH:16px\">");

   this.ddl.ID=this.ClientID+"_ddl";
   ddl.Width = Unit.Parse(sWidth.ToString() + "px");
   ddl.Style.Add("MARGIN-LEFT", "-" + spanWidth.ToString() + "px");
   ddl.Attributes.Add("onchange", "this.parentNode.nextSibling.value=this.options[this.selectedIndex].text");

      this.ReInit();
   ddl.RenderControl(output);
   
   
   

   output.Write("</span>");
   tb.ID=this.ClientID+"_tb";
   tb.Width=sWidth;
   tb.ToolTip="请选择或者输入!";
   tb.Style.Add("MARGIN-LEFT", "-" + sWidth.ToString() + "px");
   tb.RenderControl(output);
//
//   base.Style.Clear();
//   base.Width = Unit.Parse(iWidth.ToString() + "px");
//   base.Style.Add("left", "0px");
//   base.Style.Add("POSITION", "absolute");
//
//   base.Render(output);

   output.Write("</div>");
  }
 }
}


三、放DropDownListExtend.bmp图片到工程目录并设置生成属性为嵌入的资源（更改控件的图标）
四、在AssemblyInfo.cs中引用命名空间
  using System.Web.UI;

加入这句
[assembly: TagPrefix("DropDownListExtend", "DDLExtend")]
更改其生成代码的默认前缀