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
