protected void Page_Load(object sender, EventArgs e)  
{  
    //获取GB2312编码页（表）  
    /** 
     * 生成中文验证验码所要使用的方法 
     * 注，生成中文验证码时要改变一下生成验证码图片的宽度 
     * var imageCode = new System.Drawing.Bitmap((int)Math.Ceiling((code.Length * 22.5)), 23); //定义图片的宽度和高度 
     **/ 
 
    //var gb = Encoding.GetEncoding("gb2312");  
 
    ////调用函数产生4个随机中文汉字编码  
    //object[] bytes = CreateRegionCode(4);  
 
    ////根据汉字编码的字节数组解码出中文汉字  
    //var sbCode = new StringBuilder().Append(gb.GetString((byte[])Convert.ChangeType(bytes[0], typeof(byte[]))))  
    //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[1], typeof(byte[]))))  
    //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[2], typeof(byte[]))))  
    //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[3], typeof(byte[]))));  
    //CreateCheckCodeImage(sbCode.ToString());  
 
    CreateCheckCodeImage(GenerateCheckCode());      //生成数字英文所要使用的方法  
} 
 
#region 生成汉字验证码  
/// <summary>  
/// 此函数在汉字编码范围内随机创建含两个元素的十六进制字节数组，每个字节数组代表一个汉字，并将四个字节数组存储在object数组中。   
/// </summary>  
/// <param name="strLength">代表需要产生的汉字个数</param>  
/// <returns></returns>  
static object[] CreateRegionCode(int strLength)  
{  
    var rBase = new[]  
        {  
            "0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f" 
        };  
 
    var random = new Random();  
    var bytes = new object[strLength];  
 
    /*每循环一次产生一个含两个元素的十六进制字节数组，并将其放入bject数组中 
     每个汉字有四个区位码组成 
     区位码第1位和区位码第2位作为字节数组第一个元素 
     区位码第3位和区位码第4位作为字节数组第二个元素 
    */ 
 
    for (int i = 0; i < strLength; i++)  
    {  
        //区位码第1位  
        var r1 = random.Next(11, 14);  
        var str_r1 = rBase[r1].Trim();  
 
        random = new Random(r1 * unchecked((int)DateTime.Now.Ticks) + i);   //更换随机数发生器的种子避免产生重复值   
 
        var r2 = 0;  
        if (r1 == 13)  
            r2 = random.Next(0, 7);  
        else 
            r2 = random.Next(0, 16);  
 
        var str_r2 = rBase[r2].Trim();  
 
        //区位码第3位  
        random = new Random(r2 * unchecked((int)DateTime.Now.Ticks) + i);  
        var r3 = random.Next(10, 16);  
        var str_r3 = rBase[r3].Trim();  
 
        //区位码第4位  
        random = new Random(r3 * unchecked((int)DateTime.Now.Ticks) + i);  
        var r4 = 0;  
        if (r3 == 10)  
            r4 = random.Next(1, 16);  
        else if (r3 == 15)  
            r4 = random.Next(0, 15);  
        else 
            r4 = random.Next(0, 16);  
 
        var str_r4 = rBase[r4].Trim();  
 
        var byte1 = Convert.ToByte(str_r1 + str_r2, 16);  
        var byte2 = Convert.ToByte(str_r3 + str_r4, 16);  
        //将两个字节变量存储在字节数组中  
        var str_r = new[] { 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;  
} 
#endregion 
 
#region 生成图片  
void CreateCheckCodeImage(string code)  
{  
    var imageCode = new System.Drawing.Bitmap((int)Math.Ceiling((code.Length * 12.5)), 23); //定义图片的宽度和高度  
    var g = Graphics.FromImage(imageCode);  //加载图片到画布上  
 
    try 
    {  
        var random = new Random();  
        g.Clear(Color.White); //清空图片背景色  
 
        //画图片的背景噪音线  
        for (int i = 0; i < 25; i++)  
        {  
            var x1 = random.Next(imageCode.Width);  
            var x2 = random.Next(imageCode.Width);  
            var y1 = random.Next(imageCode.Height);  
            var y2 = random.Next(imageCode.Height);  
 
            g.DrawLine(new Pen(Color.Silver), new Point(x1, y1), new Point(x2, y2));  
        }  
 
        var font = new System.Drawing.Font("Arial", 12F, System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic);  
        var brush = new System.Drawing.Drawing2D.LinearGradientBrush(  
            new Rectangle(0, 0, imageCode.Width, imageCode.Height),  
            Color.Blue, Color.DarkRed, 1.2F, true);  
        g.DrawString(code, font, brush, 2, 2);  
 
        //画图片的前景噪音点  
        for (int i = 0; i < 100; i++)  
        {  
            var x = random.Next(imageCode.Width);  
            var y = random.Next(imageCode.Height);  
            imageCode.SetPixel(x, y, Color.FromArgb(random.Next()));  
        }  
 
        //画图片的边框线  
        g.DrawRectangle(new Pen(Color.Silver), 0, 0, imageCode.Width - 1, imageCode.Height - 1);  
        var ms = new System.IO.MemoryStream();  
        imageCode.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);  
 
        Response.ClearContent();  
        Response.ContentType = "image/Jpeg";  
        Response.BinaryWrite(ms.ToArray());  
    }  
    finally 
    {  
        g.Dispose();  
        imageCode.Dispose();  
    }  
 
} 
#endregion 
 
#region 生成数据验证码  
private string GenerateCheckCode()  
{  
    int number;  
    char code;  
    string checkCode = String.Empty;  
 
    var random = new Random();  
 
    for (int i = 0; i < 5; i++)  
    {  
        number = random.Next();  
 
        if (number % 2 == 0)  
            code = (char)('0' + (char)(number % 10));  
        else 
            code = (char)('A' + (char)(number % 26));  
 
        checkCode += code.ToString();  
    }   
    return checkCode;  
}  
#endregion 
    protected void Page_Load(object sender, EventArgs e)
    {
        //获取GB2312编码页（表）
        /**
         * 生成中文验证验码所要使用的方法
         * 注，生成中文验证码时要改变一下生成验证码图片的宽度
         * var imageCode = new System.Drawing.Bitmap((int)Math.Ceiling((code.Length * 22.5)), 23); //定义图片的宽度和高度
         **/

        //var gb = Encoding.GetEncoding("gb2312");

        ////调用函数产生4个随机中文汉字编码
        //object[] bytes = CreateRegionCode(4);

        ////根据汉字编码的字节数组解码出中文汉字
        //var sbCode = new StringBuilder().Append(gb.GetString((byte[])Convert.ChangeType(bytes[0], typeof(byte[]))))
        //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[1], typeof(byte[]))))
        //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[2], typeof(byte[]))))
        //    .Append(gb.GetString((byte[])Convert.ChangeType(bytes[3], typeof(byte[]))));
        //CreateCheckCodeImage(sbCode.ToString());

        CreateCheckCodeImage(GenerateCheckCode());      //生成数字英文所要使用的方法
    }

    #region 生成汉字验证码
    /// <summary>
    /// 此函数在汉字编码范围内随机创建含两个元素的十六进制字节数组，每个字节数组代表一个汉字，并将四个字节数组存储在object数组中。
    /// </summary>
    /// <param name="strLength">代表需要产生的汉字个数</param>
    /// <returns></returns>
    static object[] CreateRegionCode(int strLength)
    {
        var rBase = new[]
            {
                "0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"
            };

        var random = new Random();
        var bytes = new object[strLength];

        /*每循环一次产生一个含两个元素的十六进制字节数组，并将其放入bject数组中
         每个汉字有四个区位码组成
         区位码第1位和区位码第2位作为字节数组第一个元素
         区位码第3位和区位码第4位作为字节数组第二个元素
        */

        for (int i = 0; i < strLength; i++)
        {
            //区位码第1位
            var r1 = random.Next(11, 14);
            var str_r1 = rBase[r1].Trim();

            random = new Random(r1 * unchecked((int)DateTime.Now.Ticks) + i);   //更换随机数发生器的种子避免产生重复值

            var r2 = 0;
            if (r1 == 13)
                r2 = random.Next(0, 7);
            else
                r2 = random.Next(0, 16);

            var str_r2 = rBase[r2].Trim();

            //区位码第3位
            random = new Random(r2 * unchecked((int)DateTime.Now.Ticks) + i);
            var r3 = random.Next(10, 16);
            var str_r3 = rBase[r3].Trim();

            //区位码第4位
            random = new Random(r3 * unchecked((int)DateTime.Now.Ticks) + i);
            var r4 = 0;
            if (r3 == 10)
                r4 = random.Next(1, 16);
            else if (r3 == 15)
                r4 = random.Next(0, 15);
            else
                r4 = random.Next(0, 16);

            var str_r4 = rBase[r4].Trim();

            var byte1 = Convert.ToByte(str_r1 + str_r2, 16);
            var byte2 = Convert.ToByte(str_r3 + str_r4, 16);
            //将两个字节变量存储在字节数组中
            var str_r = new[] { 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;
    }
    #endregion

    #region 生成图片
    void CreateCheckCodeImage(string code)
    {
        var imageCode = new System.Drawing.Bitmap((int)Math.Ceiling((code.Length * 12.5)), 23); //定义图片的宽度和高度
        var g = Graphics.FromImage(imageCode);  //加载图片到画布上

        try
        {
            var random = new Random();
            g.Clear(Color.White); //清空图片背景色

            //画图片的背景噪音线
            for (int i = 0; i < 25; i++)
            {
                var x1 = random.Next(imageCode.Width);
                var x2 = random.Next(imageCode.Width);
                var y1 = random.Next(imageCode.Height);
                var y2 = random.Next(imageCode.Height);

                g.DrawLine(new Pen(Color.Silver), new Point(x1, y1), new Point(x2, y2));
            }

            var font = new System.Drawing.Font("Arial", 12F, System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic);
            var brush = new System.Drawing.Drawing2D.LinearGradientBrush(
                new Rectangle(0, 0, imageCode.Width, imageCode.Height),
                Color.Blue, Color.DarkRed, 1.2F, true);
            g.DrawString(code, font, brush, 2, 2);

            //画图片的前景噪音点
            for (int i = 0; i < 100; i++)
            {
                var x = random.Next(imageCode.Width);
                var y = random.Next(imageCode.Height);
                imageCode.SetPixel(x, y, Color.FromArgb(random.Next()));
            }

            //画图片的边框线
            g.DrawRectangle(new Pen(Color.Silver), 0, 0, imageCode.Width - 1, imageCode.Height - 1);
            var ms = new System.IO.MemoryStream();
            imageCode.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);

            Response.ClearContent();
            Response.ContentType = "image/Jpeg";
            Response.BinaryWrite(ms.ToArray());
        }
        finally
        {
            g.Dispose();
            imageCode.Dispose();
        }

    }
    #endregion

    #region 生成数据验证码
    private string GenerateCheckCode()
    {
        int number;
        char code;
        string checkCode = String.Empty;

        var random = new Random();

        for (int i = 0; i < 5; i++)
        {
            number = random.Next();

            if (number % 2 == 0)
                code = (char)('0' + (char)(number % 10));
            else
                code = (char)('A' + (char)(number % 26));

            checkCode += code.ToString();
        }
        return checkCode;
    }
    #endregion

使用验证码页面：

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<form id="formCode" runat="server">  
<div>  
    <img src="CheckCode.aspx" mce_src="CheckCode.aspx" alt="验证码" />  
</div>  
</form> 
    <form id="formCode" runat="server">
    <div>
        <img src="CheckCode.aspx" mce_src="CheckCode.aspx" alt="验证码" />
    </div>
    </form>