アルファベットの検証コードを生成

 private string GenerateCheckCode()
    {
        int number;
        char code;
        string checkCode = string.Empty;
        Random random = new Random();
        for (int i = 0; i < 4; i++)
        {
            number = random.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);
        Graphics g = Graphics.FromImage(image);

      try 
	{//       	        
		  Random random = new Random();
        g.Clear(Color .White );
//         
        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);
        }
        Font font = new System.Drawing.Font("Arial",12,(System.Drawing.FontStyle .Bold ));
        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);
//         
        for (int i = 0; i < 100; i++)
        {
            int x = random.Next(image.Width);
            int y = random.Next(image.Height);
            image.SetPixel(x,y ,Color .FromArgb (random .Next ()));

        }
//       
        g.DrawRectangle (new Pen (Color.Silver ),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 ());
	}
	catch (Exception ms)
	{
		
		Response .Write (ms.Message );
	}
    finally 
   {
          g.Dispose ();
          image.Dispose ();
   }
 }

 <script type="text/javascript">
        function change() {
            var imgNode = document.getElementById("vimg");
            imgNode.src = "Code.aspx?t=" + (new Date()).valueOf();  //                       
        }
    </script>

<asp:Image ID="vimg" runat="server" Height ="18px" Width ="60px" src="Code.aspx" /><input type="button" value="     " onclick="change()" />