<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>

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();}}
}