验证码ASP.NET

验证码的：
  private void Page_Load(object sender, System.EventArgs e)
  {
   if(!this.IsPostBack)
   {
    string tmp = RndNum(4);
    HttpCookie a = new HttpCookie("ImageV",tmp);  
    Response.Cookies.Add(a);
    this.ValidateCode(tmp);

   }
  }
  private void ValidateCode(string VNum)
  {
   Bitmap Img=null;
   Graphics g=null;
   MemoryStream ms=null;
   int gheight=VNum.Length*12;
   Img=new Bitmap(gheight,25);
   g=Graphics.FromImage(Img);
   //背景颜色
   g.Clear(Color.White);
   //文字字体
   Font f=new Font("Arial Black",10);
   //文字颜色
   SolidBrush s=new SolidBrush(Color.Black);
   g.DrawString(VNum,f,s,3,3);
   ms=new MemoryStream();
   Img.Save(ms,ImageFormat.Jpeg);
   Response.ClearContent();
   Response.ContentType="image/Jpeg";
   Response.BinaryWrite(ms.ToArray());
  
   g.Dispose();
   Img.Dispose();
   Response.End();
  }
  private string RndNum(int VcodeNum)
  {
   string Vchar="0,1,2,3,4,5,6,7,8,9,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[] VcArray=Vchar.Split(new Char [] {','});
   string VNum="";
   int temp=-1;

   Random rand=new Random();
  
   for(int i=1;i<VcodeNum+1;i++)
   {
    if(temp!=-1)
    {
     rand=new Random(i*temp*unchecked((int)DateTime.Now.Ticks));
    }
  
    int t=rand.Next(35);
    if(temp!=-1&&temp==t)
    {
     return RndNum(VcodeNum);
    }
    temp=t;
    VNum+=VcArray[t];
   }
   return VNum;
  }

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你把生成图片的程序专门做一个页面如：
picture.aspx :<img src="picture.aspx">
你如果是带参数生成的话，那么：
<img src="picture.aspx?id=424">思路就是这样