asp.net fileupload控件上传图片并预览图片

 <form id="form1" runat="server">
 <div>
 <asp:fileupload id="fileupload1" runat="server" />
 <asp:button id="button1" runat="server" text="上传" width="54px" onclick="button1_click" />
 <asp:label id="label1" runat="server" text="" style="color: red"></asp:label>
 <asp:image runat="server" id="image1" style="z-index: 102; left: 20px; position: absolute;
  top: 49px" width="73px" />
 </div>
 </form>

using system;
using system.data;
using system.configuration;
using system.web;
using system.web.security;
using system.web.ui;
using system.web.ui.webcontrols;
using system.web.ui.webcontrols.webparts;
using system.web.ui.htmlcontrols;

namespace web.file
{
 public partial class webform1 : system.web.ui.page
 {
 protected void page_load(object sender, eventargs e)
 {

 }
 #region 文件上传
 /// <summary>
 /// 文件上传
 /// </summary>
 protected void button1_click(object sender, eventargs e)
 {
  if (fileupload1.filename == "")
  {
  this.label1.text = "上传文件不能为空";
  return;
  }

  bool fileisvalid = false;
  //如果确认了上传文件，则判断文件类型是否符合要求 
  if (this.fileupload1.hasfile)
  {
  //获取上传文件的后缀 
  string fileextension = system.io.path.getextension(this.fileupload1.filename).tolower();
  string[] restrictextension = { ".gif", ".jpg", ".bmp", ".png" };
  //判断文件类型是否符合要求 
  for (int i = 0; i < restrictextension.length; i++)
  {
   if (fileextension == restrictextension[i])
   {
   fileisvalid = true;
   }
   //如果文件类型符合要求,调用saveas方法实现上传,并显示相关信息 
   if (fileisvalid == true)
   {
   //上传文件是否大于10m
   if (fileupload1.postedfile.contentlength > (10 * 1024 * 1024))
   {
    this.label1.text = "上传文件过大";
    return;
   }
   try
   {
    this.image1.imageurl = "~/file/" + fileupload1.filename;
    this.fileupload1.saveas(server.mappath("~/file/") + fileupload1.filename);
    this.label1.text = "文件上传成功!";
   }
   catch
   {
    this.label1.text = "文件上传失败！";
   }
   finally
   {

   }
   }
   else
   {
   this.label1.text = "只能够上传后缀为.gif,.jpg,.bmp,.png的文件";
   }
  }
  }
 }
 #endregion
 }
}

<!--因为fileupload 控件上传最大为4m,如果要上传更大文件,改下maxrequestlength的大小-->
<configuration>
 <system.web>
 <compilation debug="true" targetframework="4.0" />
 <httpruntime requestvalidationmode="2.0" maxrequestlength="10485760" executiontimeout="3600" apprequestqueuelimit="10000"/>
 </system.web>
</configuration>