生成噪点背景验证码代码

using System;
using System.Web;
using System.Drawing;
using System.Web.SessionState;
public class ValidateCode : IHttpHandler,IRequiresSessionState {

    public void ProcessRequest(HttpContext context)
    {
        //System.Threading.Thread.Sleep(5000);
        context.Response.ContentType = "image/jpeg";
        string code = GetRndStr();
        context.Session["code"] = code;
        using (Bitmap img = CreateImages(code, "ch"))
        {
            img.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
        }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
    /// <summary>
    /// 数字随机数
    /// </summary>
    /// <returns></returns>
    private string GetRndNum()
    {
        string code = string.Empty;
        Random random = new Random();
        for (int i = 0; i < 4; i++)
        {
            code += random.Next(9);
        }
        return code;
    }
    /// <summary>
    ///  英文随机
    /// </summary>
    /// <returns></returns>
    private string GetRndStr()
    {
        string Vchar = "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";
        string[] VcArray = Vchar.Split(',');
        string checkCode = string.Empty;
        Random rand = new Random();
        for (int i = 0; i < 4; i++)
        {
            int t = rand.Next(VcArray.Length);
            checkCode += VcArray[t];
        }
        return checkCode;
    }
    /// <summary>
    /// 中文随机
    /// </summary>
    /// <returns></returns>
    private string GetRndCh()
    {
        System.Text.Encoding gb = System.Text.Encoding.Default;//获取GB2312编码页（表）
        object[] bytes = CreateRegionCode(4);//生4个随机中文汉字编码
        string[] str = new string[4];
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        for (int i = 0; i < 4; i++)
        {
            //根据汉字编码的字节数组解码出中文汉字
            str[i] = gb.GetString((byte[])Convert.ChangeType(bytes[i], typeof(byte[])));
            sb.Append(str[i].ToString());
        }
        return sb.ToString();
    }
    /// <summary>
    /// 产生随机中文字符
    /// </summary>
    /// <param name="strlength"></param>
    /// <returns></returns>
    private static object[] CreateRegionCode(int strlength)
    {
        //定义一个字符串数组储存汉字编码的组成元素
        string[] rBase = new String[16] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" };
        Random rnd = new Random();
        object[] bytes = new object[strlength];

        for (int i = 0; i < strlength; i++)
        {
            //区位码第1位
            int r1 = rnd.Next(11, 14);
            string str_r1 = rBase[r1].Trim();
            //区位码第2位
            rnd = new Random(r1 * unchecked((int)DateTime.Now.Ticks) + i);
            int r2;
            if (r1 == 13)
            {
                r2 = rnd.Next(0, 7);
            }
            else
            {
                r2 = rnd.Next(0, 16);
            }
            string str_r2 = rBase[r2].Trim();

            //区位码第3位
            rnd = new Random(r2 * unchecked((int)DateTime.Now.Ticks) + i);//更换随机种子
            int r3 = rnd.Next(10, 16);
            string str_r3 = rBase[r3].Trim();

            //区位码第4位
            rnd = new Random(r3 * unchecked((int)DateTime.Now.Ticks) + i);
            int r4;
            if (r3 == 10)
            {
                r4 = rnd.Next(1, 16);
            }
            else if (r3 == 15)
            {
                r4 = rnd.Next(0, 15);
            }
            else
            {
                r4 = rnd.Next(0, 16);
            }
            string str_r4 = rBase[r4].Trim();
            //定义两个字节变量存储产生的随机汉字区位码
            byte byte1 = Convert.ToByte(str_r1 + str_r2, 16);
            byte byte2 = Convert.ToByte(str_r3 + str_r4, 16);

            //将两个字节变量存储在字节数组中
            byte[] str_r = new byte[] { byte1, byte2 };

            //将产生的一个汉字的字节数组放入object数组中
            bytes.SetValue(str_r, i);
        }
        return bytes;
    }
    /// <summary>
    /// 画图片的背景图+干扰线 
    /// </summary>
    /// <param name="checkCode"></param>
    /// <returns></returns>
    private Bitmap CreateImages(string checkCode, string type)
    {
        int step = 0;
        if (type == "ch")
        {
            step = 5;//中文字符，边界值做大
        }
        int iwidth = (int)(checkCode.Length * (13 + step));
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(iwidth, 22);
        Graphics g = Graphics.FromImage(image);
        g.Clear(Color.White);//清除背景色
        Color[] c = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };//定义随机颜色
        string[] font = { "Verdana", "Microsoft Sans Serif", "Comic Sans MS", "Arial", "宋体" };
        Random rand = new Random();

        for (int i = 0; i < 50; i++)
        {
            int x1 = rand.Next(image.Width);
            int x2 = rand.Next(image.Width);
            int y1 = rand.Next(image.Height);
            int y2 = rand.Next(image.Height);
            g.DrawLine(new Pen(Color.LightGray, 1), x1, y1, x2, y2);//根据坐标画线
        }

        for (int i = 0; i < checkCode.Length; i++)
        {
            int cindex = rand.Next(7);
            int findex = rand.Next(5);

            Font f = new System.Drawing.Font(font[findex], 10, System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(c[cindex]);
            int ii = 4;
            if ((i + 1) % 2 == 0)
            {
                ii = 2;
            }
            g.DrawString(checkCode.Substring(i, 1), f, b, 3 + (i * (12 + step)), ii);

        }
        g.DrawRectangle(new Pen(Color.Black, 0), 0, 0, image.Width - 1, image.Height - 1);
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        return image;
    }
}

注意：在一般处理程序里面使用session必须实现IRequiresSessionState接口，命名空间using System.Web.SessionState;

看不清图片，点击异步换张图片

<img id="imgCode" src="ValidateCode.ashx" onclick="this.src='ValidateCode.ashx?ran='+ new Date()" style="cursor:pointer" title="看不清？" />

Jquery实现 

$("#imgCode").arr("src","ValidateCode.ashx?ran=" + new Date());