protected void Page_Load(object sender, EventArgs e)
    {

        _Default v = new _Default();

        v.Length = this.length;
        v.FontSize = this.fontSize;
        v.Chaos = this.chaos;
        v.BackgroundColor = this.backgroundColor;
        v.CodeSerial = this.codeSerial;
        v.Colors = this.colors;
        v.Fonts = this.fonts;
        v.Padding = this.padding;
        string code = v.CreateVerifyCode();                //取随机码
        v.CreateImageOnPage(code, this.Context);        // 输出图片

        Response.Cookies.Add(new HttpCookie("CheckCode", code.ToUpper()));// 使用Cookies取验证码的值

    }

    #region 验证码长度(默认6个验证码的长度)
    int length = 4;
    public int Length
    {
        get { return length; }
        set { length = value; }
    }
    #endregion

    #region 验证码字体大小(为了显示扭曲效果，默认40像素，可以自行修改)
    int fontSize = 12;
    public int FontSize
    {
        get { return fontSize; }
        set { fontSize = value; }
    }
    #endregion

    #region 边框补(默认1像素)
    int padding = 2;
    public int Padding
    {
        get { return padding; }
        set { padding = value; }
    }
    #endregion

    #region 是否输出燥点(默认不输出)
    bool chaos = true;
    public bool Chaos
    {
        get { return chaos; }
        set { chaos = value; }
    }
    #endregion

    #region 自定义背景色(默认白色)
    Color backgroundColor = Color.White;
    public Color BackgroundColor
    {
        get { return backgroundColor; }
        set { backgroundColor = value; }
    }
    #endregion

    #region 自定义随机颜色数组
    Color[] colors = { Color.Black, Color.Red, Color.DarkBlue, Color.Green, Color.Orange, Color.Brown, Color.DarkCyan, Color.Purple };
    public Color[] Colors
    {
        get { return colors; }
        set { colors = value; }
    }
    #endregion

    #region 自定义字体数组
    string[] fonts = { "Arial", "Georgia" };
    public string[] Fonts
    {
        get { return fonts; }
        set { fonts = value; }
    }
    #endregion

    #region 自定义随机码字符串序列(使用逗号分隔)
    string codeSerial = "0,1,2,3,4,5,6,7,8,9,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";
    public string CodeSerial
    {
        get { return codeSerial; }
        set { codeSerial = value; }
    }
    #endregion

    #region 生成校验码图片
    public Bitmap CreateImageCode(string code)
    {
        int fSize = FontSize;
        int fWidth = fSize + Padding;

        int imageWidth = (int)(code.Length * fWidth) + 4 + Padding * 2;
        int imageHeight = fSize * 2 + Padding;

        System.Drawing.Bitmap image = new System.Drawing.Bitmap(imageWidth, imageHeight);

        Graphics g = Graphics.FromImage(image);

        g.Clear(BackgroundColor);

        Random rand = new Random();

        //给背景添加随机生成的燥点
        if (this.Chaos)
        {

            //Pen pen = new Pen(ChaosColor, 0);
            int c = Length * 10;

            for (int i = 0; i < c; i++)
            {
                int x = rand.Next(image.Width);
                int y = rand.Next(image.Height);

                //g.DrawRectangle(pen, x, y, 1, 1);
            }
        }

        int left = 0, top = 0, top1 = 1, top2 = 1;

        int n1 = (imageHeight - FontSize - Padding * 2);
        int n2 = n1 / 4;
        top1 = n2;
        top2 = n2 * 2;

        Font f;
        Brush b;

        int cindex, findex;

        //随机字体和颜色的验证码字符
        for (int i = 0; i < code.Length; i++)
        {
            cindex = rand.Next(Colors.Length - 1);
            findex = rand.Next(Fonts.Length - 1);

            f = new System.Drawing.Font(Fonts[findex], fSize, System.Drawing.FontStyle.Bold);
            b = new System.Drawing.SolidBrush(Colors[cindex]);

            if (i % 2 == 1)
            {
                top = top2;
            }
            else
            {
                top = top1;
            }

            left = i * fWidth;

            g.DrawString(code.Substring(i, 1), f, b, left, top);
        }

        //画一个边框 边框颜色为Color.Gainsboro
        g.DrawRectangle(new Pen(Color.Gainsboro, 0), 0, 0, image.Width - 1, image.Height - 1);
        g.Dispose();

        ////产生波形（Add By 51aspx.com）
        //image = TwistImage(image, true, 8, 4);

        return image;
    }
    #endregion

    #region 将创建好的图片输出到页面
    public void CreateImageOnPage(string code, HttpContext context)
    {
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        Bitmap image = this.CreateImageCode(code);

        image.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);

        context.Response.ClearContent();
        context.Response.ContentType = "image/Jpeg";
        context.Response.BinaryWrite(ms.GetBuffer());

        ms.Close();
        ms = null;
        image.Dispose();
        image = null;
    }
    #endregion

    #region 生成随机字符码
    public string CreateVerifyCode(int codeLen)
    {
        if (codeLen == 0)
        {
            codeLen = Length;
        }

        string[] arr = CodeSerial.Split(',');

        string code = "";

        int randValue = -1;

        Random rand = new Random(unchecked((int)DateTime.Now.Ticks));

        for (int i = 0; i < codeLen; i++)
        {
            randValue = rand.Next(0, arr.Length - 1);

            code += arr[randValue];
        }

        return code;
    }
    public string CreateVerifyCode()
    {
        return CreateVerifyCode(0);
    }
    #endregion

调用：<img id="imgVerify" src="Default.aspx?" alt="看不清，点击换一张" onclick="this.src=this.src+'?'" width="80px" height="40px"/>