如何用ASP.NET生成图片的缩略图

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.IO;
using System.Drawing.Imaging;

namespace eMeng.Exam
{
/// <summary>
/// Thumbnail 的摘要说明。
/// </summary>
public class Thumbnail : System.Web.UI.Page
{
 protected System.Web.UI.WebControls.Label Label1;
 protected System.Web.UI.WebControls.Button Button1;

 private void Page_Load(object sender, System.EventArgs e)
 {
  // 在此处放置用户代码以初始化页面
  Label1.Text = "<h3>在ASP.NET里轻松实现缩略图</h3>";
  Button1.Text = "上载并显示缩略图";

 }

 #region Web 窗体设计器生成的代码
 override protected void OnInit(EventArgs e)
 {
  //
  // CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
  //
  InitializeComponent();
  base.OnInit(e);
 }
 
 /// <summary>
 /// 设计器支持所需的方法 - 不要使用代码编辑器修改
 /// 此方法的内容。
 /// </summary>
 private void InitializeComponent()
 {   
  this.Button1.Click += new System.EventHandler(this.Button1_Click);
  this.Load += new System.EventHandler(this.Page_Load);

 }
 #endregion

 private void Button1_Click(object sender, System.EventArgs e)
 {
  HttpFileCollection MyFileColl = HttpContext.Current.Request.Files;
  HttpPostedFile MyPostedFile = MyFileColl[0];

  if (MyPostedFile.ContentType.ToString().ToLower().IndexOf("image") < 0)
  {
   Response.Write("无效的图形格式。");
   return;
  }
  GetThumbNail(MyPostedFile.FileName, 100, 100,
   MyPostedFile.ContentType.ToString(), false, MyPostedFile.InputStream);
 }
 private System.Drawing.Imaging.ImageFormat GetImageType(object strContentType)
 {
  if ((strContentType.ToString().ToLower()) == "image/pjpeg")
  {
   return System.Drawing.Imaging.ImageFormat.Jpeg;
  }
  else if ((strContentType.ToString().ToLower()) == "image/gif")
  {
   return System.Drawing.Imaging.ImageFormat.Gif;
  }
  else if ((strContentType.ToString().ToLower()) == "image/bmp")
  {
   return System.Drawing.Imaging.ImageFormat.Bmp;
  }
  else if ((strContentType.ToString().ToLower()) == "image/tiff")
  {
   return System.Drawing.Imaging.ImageFormat.Tiff;
  }
  else if ((strContentType.ToString().ToLower()) == "image/x-icon")
  {
   return System.Drawing.Imaging.ImageFormat.Icon;
  }
  else if ((strContentType.ToString().ToLower()) == "image/x-png")
  {
   return System.Drawing.Imaging.ImageFormat.Png;
  }
  else if ((strContentType.ToString().ToLower()) == "image/x-emf")
  {
   return System.Drawing.Imaging.ImageFormat.Emf;
  }
  else if ((strContentType.ToString().ToLower()) == "image/x-exif")
  {
   return System.Drawing.Imaging.ImageFormat.Exif;
  }
  else if ((strContentType.ToString().ToLower()) == "image/x-wmf")
  {
   return System.Drawing.Imaging.ImageFormat.Wmf;
  }
  else
  {
   return System.Drawing.Imaging.ImageFormat.MemoryBmp;
  }
 }

 private void GetThumbNail(string strFileName, int iWidth, int iheight,
  string strContentType, bool blnGetFromFile, System.IO.Stream ImgStream)
 {
  System.Drawing.Image oImg;
 
  if (blnGetFromFile)
  {
   oImg = System.Drawing.Image.FromFile(strFileName);
  }
  else
  {
   oImg = System.Drawing.Image.FromStream(ImgStream);
  }
  oImg = oImg.GetThumbnailImage(iWidth, iheight, null, IntPtr.Zero);
  string strGuid = System.Guid.NewGuid().ToString().ToUpper();
  string strFileExt = strFileName.Substring(strFileName.LastIndexOf("."));
  Response.ContentType = strContentType;
  MemoryStream MemStream = new MemoryStream();
  oImg.Save(MemStream, GetImageType(strContentType));
  MemStream.WriteTo(Response.OutputStream);
 }

}
}