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改了改冬眠的蛤蟆里面的验证码程序，绝对可用。

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Imaging;//该类将用于生成验证码图片
public partial class ValidateCode1 : System.Web.UI.Page
{
    private static Pen BorderColor = Pens.DarkGray;static string[] FontItems = new string[] {       
                                                      "Arial", 
                                                      "Helvetica", 
                                                      "Geneva", 
                                                      "sans-serif", 
                                                      "Verdana"
                                             };
    static Brush[] BrushItems = new Brush[]  {       
                                                      Brushes.OliveDrab,
                                                      Brushes.ForestGreen,
                                                      Brushes.DarkCyan,
                                                      Brushes.LightSlateGray,
                                                      Brushes.RoyalBlue,
                                                      Brushes.SlateBlue,
                                                      Brushes.DarkViolet,
                                                      Brushes.MediumVioletRed,
                                                      Brushes.IndianRed,
                                                      Brushes.Firebrick,
                                                      Brushes.Chocolate,
                                                      Brushes.Peru,
                                                      Brushes.Goldenrod
                                               };//调用函数将验证码生成图片
    protected void Page_Load(object sender, EventArgs e)
    {
        CreateCheckCodeImage(GenerateCheckCode());
    }//产生五位的随机字符串作为验证码
    private string GenerateCheckCode()
    {
        string checkCode = String.Empty;

        checkCode 

= Guid.NewGuid().ToString().Substring(0, 4);//用于客户端校验码比较
        Session["CheckCode"] = checkCode;
        return checkCode;
    }//将验证码生成图片显示
    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
        {
            //生成随机生成器 
            Random random = new Random();//清空图片背景色 
            g.Clear(Color.White);/// 随机取一个字体
            int fontIndex = random.Next(0, FontItems.Length);
            FontStyle fontStyle = GetFontStyle(random.Next(0, 2));
            Font font = new Font(FontItems[fontIndex], 12, fontStyle);/// 随机取一个笔刷
            int brushIndex = random.Next(0, BrushItems.Length);
            Brush brush = BrushItems[brushIndex];
            g.DrawString(checkCode, font, brush, 2, 2);//画图片的前景噪音点 
            for (int i = 0; i < 30; 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(BorderColor, 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();
        }

    }

/**/
    /// <summary>
    /// 取一个字体的样式
    /// </summary>
    /// <param name="index"></param>
    /// <returns></returns>
    private FontStyle GetFontStyle(int index)
    {
        switch (index)
        {
            case 0:
                return FontStyle.Bold;
            case 1:
                return FontStyle.Italic;
            default:
                return FontStyle.Regular;
        }
    }
}