C# ASP.NET 验证码

<img id="imgvalidatecode" src="../ashx/validatecode.ashx" alt="点击刷新" style="vertical-align: middle;" />

public class ashx_ValidateCode : BaseHttpHandlerNo
{
    public override void ProcessRequest(HttpContext context)
    {
        base.ProcessRequest(context);
        //设置页面不被缓存
        Response.Buffer = true;
        Response.ExpiresAbsolute = System.DateTime.Now.AddSeconds(-1);
        Response.Expires = 0;
        Response.CacheControl = "no-cache";
        Response.AppendHeader("Pragma", "No-Cache");

        string checkCode =ValidateCode.GetRandomCode(4);
        Session["CheckCode"] = checkCode;
        ValidateCode.CreateImage(checkCode,Response);
    }
}

public static string GetRandomCode(int codeCount)
    {
        string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,M,N,P,Q,R,S,T,U,W,X,Y,Z";
        string[] allCharArray = allChar.Split(‘,‘);
        string randomCode = "";
        int temp = -1;

        Random rand = new Random();
        for (int i = 0; i < codeCount; i++)
        {
            if (temp != -1)
            {
                rand = new Random(temp * i * ((int)DateTime.Now.Ticks));
            }

            int t = rand.Next(33);

            while (temp == t)
            {
                t = rand.Next(33);
            }

            temp = t;
            randomCode += allCharArray[t];
        }

        return randomCode;
    }
    public static void CreateImage(string checkCode,HttpResponse response)
    {
        int iwidth = (int)(checkCode.Length * 14);
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 20);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
        System.Drawing.Font f = new System.Drawing.Font("Arial ", 10);//, System.Drawing.FontStyle.Bold);
        System.Drawing.Brush b = new System.Drawing.SolidBrush(System.Drawing.Color.Black);
        System.Drawing.Brush r = new System.Drawing.SolidBrush(System.Drawing.Color.FromArgb(166, 8, 8));

        //g.FillRectangle(new System.Drawing.SolidBrush(Color.Blue), 0, 0, image.Width, image.Height);
        //g.Clear(Color.AliceBlue);//背景色
        g.Clear(System.Drawing.ColorTranslator.FromHtml("#99C1CB"));//背景色

        char[] ch = checkCode.ToCharArray();
        for (int i = 0; i < ch.Length; i++)
        {
            if (ch[i] >= ‘0‘ && ch[i] <= ‘9‘)
            {
                //数字用红色显示
                g.DrawString(ch[i].ToString(), f, r, 3 + (i * 12), 3);
            }
            else
            {   //字母用黑色显示
                g.DrawString(ch[i].ToString(), f, b, 3 + (i * 12), 3);
            }
        }

        //for循环用来生成一些随机的水平线
                    Pen blackPen = new Pen(Color.Black, 0);
                    Random rand = new Random();
                    for (int i=0;i<2;i++)
                    {
                        int y = rand.Next(image.Height);
                        g.DrawLine(blackPen,0,y,image.Width,y);
                    }

        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
        //history back 不重复 
        response.Cache.SetNoStore();//这一句         
        response.ClearContent();
        response.ContentType = "image/Jpeg";
        response.BinaryWrite(ms.ToArray());
        g.Dispose();
        image.Dispose();
    }