/// <summary>

    ///生成验证码

    /// </summary>

    public class VerifyCode

    {

        /// <summary>

        /// 生成验证码

        /// </summary>

        /// <returns></returns>

        public byte[] GetVerifyCode()

        {

            int codeW = ;

            int codeH = ;

            int fontSize = ;

            string chkCode = string.Empty;

            //颜色列表，用于验证码、噪线、噪点

            Color[] color = { Color.Black, Color.Red, Color.Blue, Color.Green, Color.Orange, Color.Brown, Color.Brown, Color.DarkBlue };

            //字体列表，用于验证码

            string[] font = { "Times New Roman" };

            //验证码的字符集，去掉了一些容易混淆的字符

            char[] character = { '', '', '', '', '', '', '', 'a', 'b', 'd', 'e', 'f', 'h', 'k', 'm', 'n', 'r', 'x', 'y', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'R', 'S', 'T', 'W', 'X', 'Y' };

            Random rnd = new Random();

            //生成验证码字符串

            for (int i = ; i < ; i++)

            {

                chkCode += character[rnd.Next(character.Length)];

            }

            //写入Session、验证码加密

            HttpContext.Current.Session("session_verifycode", MD5PassWordProvider.CutMD5(chkCode.ToLower(),));

            //创建画布

            Bitmap bmp = new Bitmap(codeW, codeH);

            Graphics g = Graphics.FromImage(bmp);

            g.Clear(Color.White);

            //画噪线

            for (int i = ; i < ; i++)

            {

                int x1 = rnd.Next(codeW);

                int y1 = rnd.Next(codeH);

                int x2 = rnd.Next(codeW);

                int y2 = rnd.Next(codeH);

                Color clr = color[rnd.Next(color.Length)];

                g.DrawLine(new Pen(clr), x1, y1, x2, y2);

            }

            //画验证码字符串

            for (int i = ; i < chkCode.Length; i++)

            {

                string fnt = font[rnd.Next(font.Length)];

                Font ft = new Font(fnt, fontSize);

                Color clr = color[rnd.Next(color.Length)];

                g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * , (float));

            }

            //将验证码图片写入内存流，并将其以 "image/Png" 格式输出

            MemoryStream ms = new MemoryStream();

            try

            {

                bmp.Save(ms, ImageFormat.Png);

                return ms.ToArray();

            }

            catch (Exception)

            {

                return null;

            }

            finally

            {

                g.Dispose();

                bmp.Dispose();

            }

        }

    }

//MD5加密

 #region 将普通字符串转成MD5字符串

    /// <summary>

    /// 将普通字符串转成MD5字符串

    /// </summary>

    public static class MD5PassWordProvider

    {

        public static string Hash(string message)

        {

            if (string.IsNullOrEmpty(message))

            {

                return string.Empty;

            }

            else

            {

                MD5 md5 = MD5.Create();

                byte[] source = Encoding.UTF8.GetBytes(message);

                byte[] result = md5.ComputeHash(source);

                StringBuilder buffer = new StringBuilder(result.Length);

                for (int i = ; i < result.Length; i++)

                {

                    buffer.Append(result[i].ToString("x"));

                }

                return buffer.ToString();

            }

        }

        /// <summary>

        /// MD5加密验证码

        /// </summary>

        public static string CutMD5(string str, int code)

        {

            string strEncrypt = string.Empty;

            if (code == )

            {

                strEncrypt = Hash(str).Substring(, );

            }

            return strEncrypt;

        }

    }

    #endregio

/// <summary>

        /// 生成验证码

        /// </summary>

        [HttpGet]

        public ActionResult VerifyCode()

        {

            return File(new VerifyCode().GetVerifyCode(), @"image/Gif");

        }

<img id="login_verifycode" class="login_verifycode" src="~/Login/VerifyCode" alt="点击切换验证码"

                                 title="点击切换验证码"/>

//样式

.login_verifycode{

    float: right;

    margin-top: 1px;

    display:inline-block;

    text-align:center;

    cursor:pointer;

    width: 100px;

    height: 36px;

    padding-right: ;

    padding-left: ;

    /*border: 1px solid #d5d5d5;*/

    font-size: 14px;

    color: #;

    background: white;

    margin-left:-4px;

    border-top-right-radius: 5px;

    border-bottom-right-radius: 5px;

}

//js

//点击切换验证码

        $("#login_verifycode").click(function () {

            $("#code").val('');//验证码的输入

            $("#login_verifycode").attr("src", contentPath + "/Login/VerifyCode?time=" + Math.random());

        });