asp.net 生成图形验证码（字母和数字混合）

验证码技术是网站开发过程中比较重要的技术，可以防止非法人员利用注册机或者登陆工具来攻击我们的网站。下面是效果图：

具体实现方法如下：

1、主要思路是：引用Using System.Drawing命名空间，利用Graphics的FromImage方法创建一个画布，同时设置画布的宽和高，然后通过Graphics类 的DrawString方法随机生成的字符串绘制到画布中，绘制验证码的同时，在画布中利用SetPixel方法绘制一些色点，从而防止非法人员利用机器 人来进行登陆。当我们绘制验证码完毕后，在需要验证码的页面中利用Image空间将其显示出来，Image控件显示验证码的HTML源码设置如下：

<asp:Image ID="Image1" runat="server"ImageUrl="~/ValidateNum.aspx" /><asp:LinkButton ID="LinkButton1" runat="server" style="font-size: small; ">看不清，换一张</asp:LinkButton>

2、这里所用到的ValidateNum.aspx页面代码如下：

using System;using System.Collections;using System.Configuration;using System.Data;using System.Linq;using System.Web;using System.Web.Security;using System.Web.UI;using System.Web.UI.HtmlControls;using System.Web.UI.WebControls;using System.Web.UI.WebControls.WebParts;using System.Xml.Linq;using System.Drawing;public partial class ValidateNum : System.Web.UI.Page{    protected void Page_Load(object sender, EventArgs e)    {        if (!IsPostBack)        {            string validateNum = CreateRandomNum(4);//成生4位随机字符串            CreateImage(validateNum);//将生成的随机字符串绘成图片            Session["ValidateNum"] = validateNum;//保存验证码        }    }    //生成随机字符串    private string CreateRandomNum(int NumCount)    {        string allChar = "0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,W,X,Y,Z";        string[] allCharArray = allChar.Split(',');//拆分成数组        string randomNum = "";        int temp = -1;//记录上次随机数的数值，尽量避免产生几个相同的随机数        Random rand = new Random();        for (int i = 0; i < NumCount; i++)        {            if (temp != -1)            {                rand = new Random(i * temp * ((int)DateTime.Now.Ticks));            }            int t = rand.Next(35);            if (temp == t)            {                return CreateRandomNum(NumCount);            }            temp = t;            randomNum += allCharArray[t];        }        return randomNum;    }    //生成图片    private void CreateImage(string validateNum)    {        if (validateNum == null || validateNum.Trim() == String.Empty)            return;        //生成Bitmap图像        System.Drawing.Bitmap image = new System.Drawing.Bitmap(validateNum.Length * 12 + 10, 22);        Graphics g = Graphics.FromImage(image);        try        {            //生成随机生成器             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(validateNum, 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();            //将图像保存到指定的流            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);            Response.ClearContent();            Response.ContentType = "image/Gif";            Response.BinaryWrite(ms.ToArray());        }        finally        {            g.Dispose();            image.Dispose();        }    }}