asp.Netログイン認証コード

<form method="POST" action="CheckUser.aspx" id="fromLogin">            
            <table>
                <tr>
                    <td><img src="ValidCode.aspx" alt='    ，       。'ondblclick=" this.src = 'ValidCode.aspx?flag=' + Math.random() " border="1" height="44" width="126"/></td>
                </tr>                
            </table>
        </form>

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;
using System.Web.UI;

namespace CST.WebApp
{
    public partial class ValidCode : Page
    {
        private readonly Random ran = new Random();

        protected void Page_Load(object sender, EventArgs e)
        {
            string str = getRandomValidate(5);
            Session["check"] = str;
            //            Session，    ，    ，     cookie
            getImageValidate(str);
        }

        //       ,      
        private string getRandomValidate(int len)
        {
            int num;
            int tem;
            string rtuStr = "";
            for (int i = 0; i < len; i++)
            {
                num = ran.Next();
                if (i%2 == 0)
                {
                    tem = num%10 + '0'; //     
                }
                else
                {
                    tem = num%26 + 'A'; //    
                }
                rtuStr += Convert.ToChar(tem).ToString() + " ";
            }
            return rtuStr;
        }

        //    
        private void getImageValidate(string strValue)
        {
            //string str = "OO00"; //      O，      0
            //int width = Convert.ToInt32(strValue.Length*12); //      
            // var img = new Bitmap(width, 141);
            var img = new Bitmap(150, 42);
            Graphics gfc = Graphics.FromImage(img); //  Graphics  ，    
            gfc.Clear(Color.White);
            drawLine(gfc, img);
            //    ，    Font
            var font = new Font("Brush Script Std", 16);
            var brush =
                new LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.DarkOrchid, Color.Blue, 1.5f,
                                        true);
            gfc.DrawString(strValue, font, brush, 20, 9);
            drawPoint(img);
            gfc.DrawRectangle(new Pen(Color.DarkGray), 0, 0, img.Width - 1, img.Height - 1);
            //        
            var ms = new MemoryStream();
            img.Save(ms, ImageFormat.Gif);
            //  Http 
            Response.ClearContent();
            Response.ContentType = "image/gif";
            Response.BinaryWrite(ms.ToArray());
            //Dispose
            gfc.Dispose();
            img.Dispose();
            Response.End();
        }

        private void drawLine(Graphics gfc, Bitmap img)
        {
            //   10  ,     ，      ，        
            for (int i = 0; i < 10; i++)
            {
                int x1 = ran.Next(img.Width);
                int y1 = ran.Next(img.Height);
                int x2 = ran.Next(img.Width);
                int y2 = ran.Next(img.Height);
                gfc.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2); //          ，         
            }
        }

        private void drawPoint(Bitmap img)
        {
            int col = ran.Next(); //             
            for (int i = 0; i < 100; i++)
            {
                int x = ran.Next(img.Width);
                int y = ran.Next(img.Height);
                img.SetPixel(x, y, Color.FromArgb(col));
            }
        }
    }
}