ASP.NET验证码

1、创建一个生成验证码图片的page

protected void Page_Load(object sender, EventArgs e)
{
    CreateCheckCodeImage(GenerateCheckCode());
}

private string GenerateCheckCode()
{
    int number;
    char code;
    string checkCode = String.Empty;

    System.Random 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();
    }

    Response.Cookies.Add(new HttpCookie("CheckCode", checkCode));

    return checkCode;
}

private void CreateCheckCodeImage(string checkCode)
{
    if (checkCode == null || checkCode.Trim() == String.Empty)
        return;

    System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length * 12.5)), 22);
    Graphics g = Graphics.FromImage(image);

    try
    {               
        Random random = new Random();   

        //清空图片背景色           
        g.Clear(Color.White);

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

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

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

        //画图片的前景噪音点
        for (int i = 0; i < 100; i++)
        {
            int x = random.Next(image.Width);
            int y = random.Next(image.Height);

            image.SetPixel(x, y, Color.FromArgb(random.Next()));
        }

        //画图片的边框线
        g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
        Response.ClearContent();
        Response.ContentType = "image/Gif";
        Response.BinaryWrite(ms.ToArray());
    }
    finally
    {
        g.Dispose();
        image.Dispose();
    }
}

2、调用验证码的前台代码

假如以上验证码生成器页面名为：CheckCode.aspx，那么在登录页面中使用“<img>” 这个 HTML 元素来显示生成的验证码图片：
<img id="imgVerify" style="vertical-align:bottom;" alt="verify code" src="VerifyCode.aspx" />

<a href="#" class="skybluelink" onclick="refreshVerifyCode()" style="vertical-align:bottom;">try a new code</a>

function refreshVerifyCode()
{
    document.getElementById("imgVerify").src = "VerifyCode.aspx?param=" + Math.random();
}

3、调用验证码的后台代码

if (Request.Cookies["CheckCode"] == null)
{
    lbMessage.Text = "Your browser has disabled the cookies, please enable the cookies.";
    lbMessage.Visible = true;
    return;
}

if (String.Compare(Request.Cookies["CheckCode"].Value, txtVerifyCode.Text, true) != 0)
{
    lbMessage.Text = "The verify code is not correct，please enter the right code.";
    lbMessage.Visible = true;
    return;
}