第一.兩個頁面login.aspx和gif.aspx。

第二.login.aspx頁面的image控件的url指定為gif.aspx。

第三.gif.aspx頁面代碼如下：

private void page_load(object sender, system.eventargs e)
  {
   string strnum = randnum(4); 
   session["test"] = strnum;
   validatecode(strnum);   
   viewstate["test"] = strnum; 
  }

/// <summary>
  /// 產生隨機數
  /// </summary>
  /// <param name="inum">種子</param>
  /// <returns></returns>
  /// <author>tianzm</author>
  /// <history>2005-06-18</history>

private string randnum(int inum)
  {
   string strchar = "0,1,2,3,4,5,6,7,8,9,0,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z";
   string[] strary = strchar.split(',');
   string strnum = "";   
   system.random obj = new random();
   for ( int i=0; i<inum; i++)
   {   
    int index = obj.next(strary.length);
    strnum = strnum + strary[index];    
   }
   return strnum;

//這里可以對產生的隨機數，進行加密處理。
  }

/// <summary>
  /// 重畫圖像生成
  /// </summary>
  /// <param name="strnum">驗證碼</param>
  /// <author>tianzm</author>
  /// <history>2005-06-18</history>

  private void validatecode(string strnum)
  {
   system.drawing.bitmap img = null;
   system.drawing.graphics g = null;
   system.io.memorystream ms = null;
   int iwidth = convert.toint16(strnum.length * 11.5);
   img = new bitmap(iwidth,20);
   g = graphics.fromimage(img);
   g.drawstring(strnum,(new font("arial",10)),(new solidbrush(color.blue)),3,3);
   ms = new memorystream();
   img.save(ms,imageformat.png);
   response.clearcontent();
   response.contenttype = "image/png";
   response.binarywrite(ms.toarray());
   g.dispose();
   img.dispose();
   response.end();
  }  

在login.aspx頁面驗證如下：

string strtemp = session["test"].tostring();
   session.abandon();
   viewstate["testnum"] = strtemp;
   if ( this.textbox1.text == viewstate["testnum"].tostring() )
   {
    this.label1.text = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
   }