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.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Text;
using System.Drawing.Imaging;
using System.IO;
using System.Drawing;

public partial class image : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string tep = generateCheckCode(4);
        HttpCookie a = new HttpCookie("image",tep);
        Response.Cookies.Add(a);
        //this.validatecode(tep);
        this.CreateCheckCodeImage(tep);

    }
    //生成随机数
    private string generateCheckCode(int length)
    {
        char[] constant ={ '0','1', '2', '3', '4', '5', '6', '7', '8', '9',
            'a', 'b', 'c', 'd', 'e','f','g','h','i','j','k','u','m','l','o','p','q','r','s','t','u','v','w','x','y','z',
        'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',};

        StringBuilder checkcodes = new StringBuilder(62);
        Random rd = new Random();
        for (int i = 0; i < length; i++)
        {
            checkcodes.Append(constant[rd.Next(62)]);
        }
        return checkcodes.ToString();
    }

    //生成验证码图片
    //方法一
    private void validatecode(string code)
    {
        Bitmap img = null;
        Graphics g = null;
        MemoryStream ms = null;

        int gwidth = code.Length * 12;
        img = new Bitmap(gwidth, 25);
        g = Graphics.FromImage(img);
        //背景颜色
        g.Clear(Color.White);
        Font f = new Font("Arial Black", 10); //文字字体
        SolidBrush s = new SolidBrush(Color.Black); //文字颜色

        g.DrawString(code, f, s,3,3);
        ms = new MemoryStream();
        img.Save(ms, ImageFormat.Jpeg);
        Response.ClearContent();
        Response.ContentType = "image/jpge";
        Response.BinaryWrite(ms.ToArray());
        g.Dispose();
        img.Dispose();
        Response.End();

    }
    //方法二
    public 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);

            /**/
            ///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.Silver), x1, y1, x2, y2);
            }

            Font font = new System.Drawing.Font("Arial", 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 < 100; 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.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());
        }
        finally
        {
            g.Dispose(); image.Dispose();
        }
    }
}

  要在 a.aspx 页面中生成图片验证码，在a 页面中放一个image 图像控件，在pageload中 写入它的imageurl，如上面的代码页面为b.aspx 则为 imageurl=b.aspx

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