.net vb自定义控件求救 - .NET技术 / 组件/控件开发

重写Render如下：
Protected Overrides Sub Render(ByVal writer As System.Web.UI.HtmlTextWriter)
  CreateChildControls()
  MyBase.Render(writer)
  writer.Write("<script type=""text/javascript"">" _
  & "function PreView(FileName){" _
  & "var ext=FileName.substring(FileName.lastIndexOf("".""),FileName.length);" _
  & "ext=ext.toLowerCase();" _
  & "if((ext!='.jpg')&&(ext!='.gif')&&(ext!='.jpeg')&&(ext!='.png')&&(ext!='.bmp')){" _
  & "alert(""你选择的不是有效的图片文件！"");}else{" _
  & "document.getElementById(""PreView"").innerHTML=""<img src='""+FileName+""' style='width:100px;height:100px; border:0px;' alt=''/>"";}" _
  & "}" _
  & "</script>")
  End Sub

重写CreateChildControls如下：
 Private Sub BuildControl()
  Dim HIF As New HtmlInputFile
  Dim HIB As New HtmlInputButton
  HIF.ID = UniqueID & "_UpFile