Asp.Nnet验证码实现

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Web;

/// <summary>
///
/// </summary>
public class GenerateCheckCode
{
    private static string chkCode;

    public static MemoryStream GenCheckCode()
    {
        //颜色列表，用于验证码、噪线、噪点
        Color[] color ={ Color.Tomato, Color.OrangeRed, Color.Olive, Color.Gold, Color.GreenYellow,
                           Color.Blue, Color.LawnGreen, Color.Lime, Color.MediumSpringGreen, Color.Aqua, Color.RoyalBlue, Color.MediumBlue,
                           Color.BlueViolet, Color.MediumOrchid, Color.Fuchsia, Color.DeepPink, Color.HotPink };

        //字体列表，用于验证码
        string[] font = { "Times New Roman", "MS Mincho", "Book Antiqua", "Gungsuh", "PMingLiU" };

        //验证码的字符集，去掉了一些容易混淆的字符
        char[] character ={ '2', '3', '4', '5', '6', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G',
                              'H', 'J', 'K', 'L', 'M', 'N', 'P', 'R', 'S', 'T', 'W', 'X', 'Y' };
        Random rnd = new Random();

        chkCode = string.Empty;

        //生成验证码字符串
        for (int i = 0; i < 4; i++)
        {
            chkCode += character[rnd.Next(character.Length)];
        }

        HttpContext.Current.Session["checkcode"] = chkCode;

        Bitmap bmp = new Bitmap(100, 40);
        Graphics g = Graphics.FromImage(bmp);

        g.Clear(Color.White);

        //画噪线
        for (int i = 0; i < 10; i++)
        {
            int x1 = rnd.Next(100);
            int y1 = rnd.Next(40);
            int x2 = rnd.Next(100);
            int y2 = rnd.Next(40);
            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 = 0; i < chkCode.Length; i++)
        {
            string fnt = font[rnd.Next(font.Length)];
            Font ft = new Font(fnt, 18);
            Color clr = color[rnd.Next(color.Length)];
            g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * 20 + 8, (float)8);
        }

        //画噪点
        for (int i = 0; i < 100; i++)
        {
            int x = rnd.Next(bmp.Width);
            int y = rnd.Next(bmp.Height);
            Color clr = color[rnd.Next(color.Length)];
            bmp.SetPixel(x, y, clr);
        }

        //画边框
        Color col = color[rnd.Next(color.Length)];
        g.DrawLine(new Pen(color[rnd.Next(color.Length)]), 0, 0, bmp.Width - 1, 0);
        g.DrawLine(new Pen(color[rnd.Next(color.Length)]), 0, 0, 0, bmp.Height - 1);
        g.DrawLine(new Pen(color[rnd.Next(color.Length)]), bmp.Width - 1, 0, bmp.Width - 1, bmp.Height - 1);
        g.DrawLine(new Pen(color[rnd.Next(color.Length)]), 0, bmp.Height - 1, bmp.Width - 1, bmp.Height - 1);

        //清除该页输出缓存，设置该页无缓存
        HttpContext.Current.Response.Buffer = true;
        HttpContext.Current.Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0);
        HttpContext.Current.Response.Expires = 0;
        HttpContext.Current.Response.CacheControl = "no-cache";
        HttpContext.Current.Response.AddHeader("Pragma", "No-Cache");
        HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);

        //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);
            HttpContext.Current.Response.ClearContent();
            HttpContext.Current.Response.ContentType = "image/Png";
            return ms;
        }
        catch (Exception)
        {
            return null;
        }
        finally
        {
            //显式释放资源
            bmp.Dispose();
            g.Dispose();
        }
    }
}

使用方法如下：

新建名为 GenerateCheckCode.aspx 的文件，文件的内容为：

<%@ Page Language="C#" %>
<% Response.BinaryWrite(GenerateCheckCode.GenCheckCode().ToArray());%>

在需要验证码的地方放置的代码为：
<a href="javascript:void(0);" onfocus="javascript:blur();" onclick="document.getElementById('img1').src='Control/CheckCode.aspx?id='+ (new Date().getTime().toString(36));"><img id="img1" style="float: left; margin-top: 1px; margin-left: 3px;" class="imgcode" title="看不清，换一张！" src="Control/CheckCode.aspx" /> </a>

以上js代码为的是实现验证码的刷新。
验证码输入框中的字符和 Session[“checkcode”] 的值比较是否相同来确定验证是否通过。