string code = GetRndStr();
        //将验证码存入session中
        context.Session["yzm"] = code;

 //一，先判断验证码
            if (txtYZM.Text.Trim().ToLower() == Session["yzm"].ToString().ToLower() && Session["yzm"]!=null)
            {
                //判断验证码成功后，删除session
                Session.Remove("yzm");
                //二，再判断账号密码

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

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "image/jpeg";
        string code = GetRndStr();
        //将验证码存入session中
        context.Session["yzm"] = 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();
        ; i < ; i++)
        {
            code += random.Next();
        }
        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();
        ; i < ; 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）
        );//生4个随机中文汉字编码
        ];
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        ; i < ; 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
        ] { ", "a", "b", "c", "d", "e", "f" };
        Random rnd = new Random();
        object[] bytes = new object[strlength];

        ; i < strlength; i++)
        {
            //区位码第1位
            , );
            string str_r1 = rBase[r1].Trim();
            //区位码第2位
            rnd = new Random(r1 * unchecked((int)DateTime.Now.Ticks) + i);
            int r2;
            )
            {
                r2 = rnd.Next(, );
            }
            else
            {
                r2 = rnd.Next(, );
            }
            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
            , );
            string str_r3 = rBase[r3].Trim();

            //区位码第4位
            rnd = new Random(r3 * unchecked((int)DateTime.Now.Ticks) + i);
            int r4;
            )
            {
                r4 = rnd.Next(, );
            }
            )
            {
                r4 = rnd.Next(, );
            }
            else
            {
                r4 = rnd.Next(, );
            }
            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
            );
            );

            //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)
    {
        ;
        if (type == "ch")
        {
            step = ;//中文字符，边界值做大
        }
         + step));
        System.Drawing.Bitmap image = );
        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();

        ; i < ; 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(), x1, y1, x2, y2);//根据坐标画线
        }

        ; i < checkCode.Length; i++)
        {
            );
            );

            Font f = , System.Drawing.FontStyle.Bold);
            Brush b = new System.Drawing.SolidBrush(c[cindex]);
            ;
            ) %  == )
            {
                ii = ;
            }
            g.DrawString(checkCode.Substring(i, ), f, b,  + (i * ( + step)), ii);

        }
        g.DrawRectangle(), , , image.Width - , image.Height - );
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        return image;
    }
}