//输出图片按钮之间的空白图片
  protected virtual void WriteSpace(HtmlTextWriter writer)
  {
   writer.AddAttribute(HtmlTextWriterAttribute.Src, DefaultImagePath+"HSpace2.gif");
   writer.RenderBeginTag(HtmlTextWriterTag.Img);
   writer.RenderEndTag();
  }
 
  protected override void Render(HtmlTextWriter writer)
  {
//输出具有一行两个cell的表格其中第一个cell放置htmlinputfile,第二个cell放upload/delete/download/inputhidden按钮

   writer.AddAttribute(HtmlTextWriterAttribute.Cellpadding, "0");
   writer.AddAttribute(HtmlTextWriterAttribute.Cellspacing, "0");
   if(this.Width!=Unit.Empty)
    writer.AddAttribute(HtmlTextWriterAttribute.Width, Width.ToString());
   writer.AddAttribute(HtmlTextWriterAttribute.Border, "0");
   //table
   writer.RenderBeginTag(HtmlTextWriterTag.Table);
   writer.RenderBeginTag(HtmlTextWriterTag.Tr);
   //cell1
//使第二cell的按钮充满单元格,由于inputfile的宽度为100%这样通过设置最外层table的width可以精确控制控件的宽度
   writer.AddAttribute(HtmlTextWriterAttribute.Width, "98%");
   writer.RenderBeginTag(HtmlTextWriterTag.Td);
//定义inputfile的客户端id
   this.FileUpload.Attributes.Add("id",this.ClientID+"__file");
   this.FileUpload.RenderControl(writer);
   writer.RenderEndTag();
   //cell2,为了使后三个按钮不换行输出nowrap属性
   writer.AddAttribute(HtmlTextWriterAttribute.Nowrap,"Yes");
   writer.RenderBeginTag(HtmlTextWriterTag.Td);
   //hiden input file
   this.TxtFileName.Attributes.Add("id",this.ClientID);
   this.TxtFileName.RenderControl(writer);
   //upload btn
   WriteSpace(writer);
//绑定upload客户端事件
   string onClick = this.BtnUpLoad.Attributes["onclick"]==null?string.Empty:this.BtnUpLoad.Attributes["onclick"];
   this.BtnUpLoad.Attributes["onclick"] = string.Format("if(!({0}_obj.CheckUpload())) return false;",this.ClientID) + onClick;
   this.BtnUpLoad.RenderControl(writer);

   //delete btn
   if(this.AllowDelete) WriteSpace(writer); 
//绑定delete客户端事件
   onClick = (this.BtnDelete.Attributes["onclick"]==null)?string.Empty:this.BtnDelete.Attributes["onclick"];
   this.BtnDelete.Attributes["onclick"] = string.Format("if(!({0}_obj.CheckDelete())) return false;",this.ClientID)+ onClick;
   this.BtnDelete.RenderControl(writer);


   //download btn
   WriteSpace(writer);
//绑定download客户端事件
   this.BtnDownload.Attributes["onclick"]=string.Format("if(!({0}_obj.CheckDownload(this))) return false;",this.ClientID);
   this.BtnDownload.RenderControl(writer);
   writer.RenderEndTag();
   
   writer.RenderEndTag();
   writer.RenderEndTag();
  }
  
  protected override void CreateChildControls()
  {
   this.Controls.Clear();
   //button hidden ctl0首先创建inputhidden因为其它控件会用到些控件.
   this._txtFileName = new System.Web.UI.HtmlControls.HtmlInputHidden();
   this.Controls.Add(this._txtFileName);

   //file upload ctl1创建上传文件控件
   this._fileUpload = new System.Web.UI.HtmlControls.HtmlInputFile();
//充满第一个单元格
   this._fileUpload.Style.Add("WIDTH","100%");
//应用样式
   if(this.CSS!=string.Empty)
    this._fileUpload.Attributes.Add("class",this.CSS);
   this.Controls.Add(this._fileUpload);

   //button upload上传控件
   this._btnUpLoad = new ImageButton();
   this._btnUpLoad.ImageUrl = this.UploadImg;
   if(this.UploadFileNameType != FileNameType.AutoGenerate)
    this._btnUpLoad.EnableWarning = true;
   else
    this._btnUpLoad.EnableWarning = false;
   this._btnUpLoad.AlternateText = this.UploadAlt;
   this._btnUpLoad.WarningMessage = "如果文件已经存在将被覆盖，确定吗？";
   this._btnUpLoad.CausesValidation = false;
//绑定上传事件
   this._btnUpLoad.Click += new ImageClickEventHandler(Upload_Clicked);
   this.Controls.Add(this._btnUpLoad);

   //button delete创建删除按钮
   this._btnDelete = new ImageButton();
   this._btnDelete.ImageUrl = this.DeleteImg;
   this._btnDelete.EnableWarning = true;
   this._btnDelete.AlternateText = this.DeleteAlt;
   this._btnDelete.WarningMessage = "确定要删除吗？";
//不触发验证
   this._btnDelete.CausesValidation = false;
//绑定删除事件
   this._btnDelete.Click +=new ImageClickEventHandler(Delete_Clicked);
   if(!this.AllowDelete)
    this._btnDelete.Visible = false;
   this.Controls.Add(this._btnDelete);

   //button download
   this._btnDownload = new HyperLink();
   this._btnDownload.Style.Add("cusor","hand");
   this._btnDownload.ImageUrl = this.DownloadImg;
   this._btnDownload.ToolTip = this.DownloadAlt;
   this._btnDownload.NavigateUrl = this.FullFileName;
   this._btnDownload.Target = "_blank";
   this._btnDownload.Text = "download";

   this.Controls.Add(this._btnDownload);


//子控件创建成功
   ChildControlsCreated = true;
  }