Asp.Net验证码

protected void Page_Load(object sender, EventArgs e)     {         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.LightSteelBlue);         //文字字体          Font f = new Font("Arial Black", 10);         //文字颜色          SolidBrush s = new SolidBrush(Color.RoyalBlue);         g.DrawString(VNum, f, s, 3, 3);         ms = new MemoryStream();         Img.Save(ms, ImageFormat.Jpeg);         Response.ClearContent();         Response.ContentType = "images/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;     }