可输入下拉框（完美版），asp.net

最近很是郁闷，一个可输入的下拉框搞了我好几天，开始的时候在网上找到一个服务器控件，用了一下，发现满足不了自己的需求。于是乎花了几个小时改了改，终于成形。这个控件和别人不同有以下两点：
 1,一般的dropdownlist都会默认选中第一条，这样的话，在选择第一条的时候就没有事件，而往往这时候我们需要触发ongchange事件，于是我在它的foucus事件中强行改变它的selectedIndex。
2，输入的内容如果和下拉框中的某项相同时，会自动选中下拉框中那一条。

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;
using System.Collections;

namespace ExtendWebControls
{

 [ToolboxData("<{0}:DropDownListExtend runat=\"server\" />")]
 public class DropDownListExtend : System.Web.UI.WebControls.TextBox
 {
  private Hashtable _values;
  private DropDownList _DropDownList;

  public DropDownListExtend()
  {
   _values = new Hashtable();
   _DropDownList = new DropDownList();
  }

  public Hashtable Values
  {
   get{return _values;}
   set{_values = value;}
  }

  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:" + spanWidth.ToString() + "px;OVERFLOW:hidden;WIDTH:18px\">");

   _DropDownList.Width = Unit.Parse(sWidth.ToString() + "px");
   _DropDownList.Style.Add("MARGIN-LEFT", "-" + spanWidth.ToString() + "px");
   _DropDownList.ID = base.ID + "_Select";
   _DropDownList.Attributes.Add("onchange", "this.parentNode.nextSibling.value=this.value");
   _DropDownList.Attributes.Add("onfocus", ""+this.getFocusScript()+"");
   
   if(_values.Count > 0)
   {
    foreach(string key in _values.Keys)
    {
     ListItem item = new ListItem();

     item.Value = key;
     item.Text = _values[key].ToString();

     _DropDownList.Items.Add(item);
    }
   }
   _DropDownList.RenderControl(output);

   output.Write("</span>");

   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>");
  }

  private string getFocusScript()
  {
   string strScript = "\n";
   strScript += "var isExist = -2;\n";
   strScript += "var obj = event.srcElement;\n";
   strScript += "var str = this.parentNode.nextSibling.value;\n";
   strScript += "var ary = obj.options;\n";
   strScript += "for(var i=0;i<ary.length;i++){\n";
   strScript += " if(str == ary[i].text){\n";
   strScript += "  isExist = i;\n";
   strScript += "  break;\n";
   strScript += " }\n";
   strScript += "}\n";
   strScript += "if(isExist != -2){\n";
   strScript += " obj.selectedIndex = isExist;\n";
   strScript += "}\n";
   strScript += "else{\n";
   strScript += " obj.selectedIndex = -1;\n";
   strScript += "}\n";
    
   return strScript;
  }
 }
}