随机产生验证码

Code
protected void Page_Load(object sender, EventArgs e)
    {
        this.CreateCheckCodeImage(GenerateCheckCode());
    }
    private string GenerateCheckCode()
    {
        int number;
        char code;
        string checkCode = string.Empty;

        Random radom = new Random();

        for (int i = 0; i < 6; i++)
        {
            number = radom.Next();
            if (number % 2 == 0)
            {
                code = (char)('0' + (char)(number % 10));
            }
            else
            { 
               code = (char)('A'+(char)(number%26));
            }
            checkCode += code.ToString();
        }
        Response.Cookies.Add(new HttpCookie("CheckCode",checkCode));
        return checkCode;
    }

    private void CreateCheckCodeImage(string checkCode)
    {
        if (checkCode == null || checkCode.Trim() == String.Empty)
        {
            return;
        }
        System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling(checkCode.Length*12.5),22);
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(image);
        try
        {
            //生成随机生成器
            Random random = new Random();

            //清空图片背景色
            g.Clear(Color.White);

            //画图片的背景噪音线
            for (int i = 0; i < 25; i++)
            {
                int x1 = random.Next(image.Width);
                int x2 = random.Next(image.Width);
                int y1 = random.Next(image.Height);
                int y2 = random.Next(image.Height);

                g.DrawLine(new Pen(Color.GreenYellow), x1, y1, x2, y2);
            }

            Font font = new System.Drawing.Font("Verdana", 12, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
            g.DrawString(checkCode, font, brush, 2, 2);

            //画图片的前景噪音点
            for (int i = 0; i < 80; i++)
            {
                int x = random.Next(image.Width);
                int y = random.Next(image.Height);

                image.SetPixel(x, y, Color.FromArgb(random.Next()));
            }

            //画图片的边框线
            g.DrawRectangle(new Pen(Color.Red), 0, 0, image.Width - 1, image.Height - 1);

            System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            Response.ClearContent();
            Response.ContentType = "image/Gif";
            Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            image.Dispose();
        }
    }

<form id="form1" runat="server">
    <div>
    <asp:TextBox runat="server" ID="txt"></asp:TextBox>
     <img src="pic.aspx" alt="点击更换图片!" onclick="this.src=this.src+'?'"/>
     <br />
     <asp:Button runat="server" ID="bt" onclick="bt_Click" Text="提交"/><asp:Label runat="server" ID="msg"></asp:Label>
    </div>
    </form>
//C#