protected override void RenderContents(HtmlTextWriter output)
        {
            output.RenderBeginTag(HtmlTextWriterTag.Div);
            output.AddAttribute(HtmlTextWriterAttribute.Type, "text");
            output.AddAttribute(HtmlTextWriterAttribute.Id, this.ClientID);
            output.AddAttribute(HtmlTextWriterAttribute.Name, this.ClientID);
            output.AddAttribute(HtmlTextWriterAttribute.Value, this.Text);
            output.AddAttribute("OnBlur", "ClientCallback();");
            this.AddAttributesToRender(output);
            output.RenderBeginTag(HtmlTextWriterTag.Input);
            output.RenderEndTag();
            output.RenderEndTag();
        }

protected override void OnPreRender(EventArgs e)
        {
            //Page.ClientScript.RegisterClientScriptInclude("UtilityFunctions",
 "JScript.js");
            Page.ClientScript.RegisterStartupScript(typeof(Page), 
"ControlFocus", "document.getElementById('" + this.ClientID + "').focus();", true);
            Page.ClientScript.RegisterStartupScript(typeof(Page),
"ClientCallback","function ClientCallback() {"+"args=document.getElementById
('"+this.ClientID+"').value;"+Page.ClientScript.GetCallbackEventReference
(this,"args","CallbackHandler",null,"ErrorHandler",true)+"}");
            //向服务器发送请求，由服务器端生成回调的客户端脚本。
        } 

#region ICallbackEventHandler Members
        public string RaiseCallbackEvent(string eventArgument)
        {
            int result;
            if (!Int32.TryParse(eventArgument, out result))
                throw new Exception("The method is not implemented.");
            return "Valid Data";
        }
        #endregion

    最后，在jscript.js文件中写好相应的回调处理函数即可：

var args;
var ctx;
function ValidateText(ctl)
{
   if(ctl.value=='')
   {
      alert("Please enter a value");
      ctl.focus();
   }
}
function CallbackHandler(args,ctx)
{
    alert("The data is valid");
}
function ErrorHandler(args,ctx)
{
    alert("The data is not a number");
}