第一.两个页面Login.aspx和gif.aspx。

第二.Login.aspx页面的image控件的URL指定为gif.aspx。

第三.gif.aspx页面代码如下：

private void Page_Load(object sender, System.EventArgs e)
  {
   string strNum = RandNum(4); 
   Session["Test"] = strNum;
   ValidateCode(strNum);   
   ViewState["Test"] = strNum; 
  }

/// <summary>
  /// 产生随机数
  /// </summary>
  /// <param name="iNum">种子</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;

//这里可以对产生的随机数，进行加密处理。
  }

/// <summary>
  /// 重画图像生成
  /// </summary>
  /// <param name="strNum">验证码</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页面验证如下：

string strTemp = Session["Test"].ToString();
   Session.Abandon();
   ViewState["TestNum"] = strTemp;
   if ( this.TextBox1.Text == ViewState["TestNum"].ToString() )
   {
    this.Label1.Text = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
   }