<asp:Image ID="Image1" runat="server"ImageUrl="~/ValidateNum.aspx" />
<asp:LinkButton ID="LinkButton1" runat="server" style="font-size: small; ">看不清，換一張</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位隨機字符串
 CreateImage(validateNum);//將生成的隨機字符串繪成圖片
 Session["ValidateNum"] = validateNum;//保存驗證碼
 }
 }
 //生成隨機字符串
 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(',');//拆分成數組
 string randomNum = "";
 int temp = -1;//記錄上次隨機數的數值，盡量避免產生幾個相同的隨機數

 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;
 }
 //生成圖片
 private void CreateImage(string validateNum)
 {
 if (validateNum == null || validateNum.Trim() == String.Empty)
 return;
 //生成Bitmap圖像
 System.Drawing.Bitmap image = new System.Drawing.Bitmap(validateNum.Length * 12 + 10, 22);
 Graphics g = Graphics.FromImage(image);

 try
 {
 //生成隨機生成器 
 Random random = new Random();

 //清空圖片背景色 
 g.Clear(Color.White);

 //畫圖片的背景噪音線 
 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);

 //畫圖片的前景噪音點 
 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()));
 }

 //畫圖片的邊框線 
 g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

 System.IO.MemoryStream ms = new System.IO.MemoryStream();
 //將圖像保存到指定的流
 image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
 Response.ClearContent();
 Response.ContentType = "image/Gif";
 Response.BinaryWrite(ms.ToArray());
 }
 finally
 {
 g.Dispose();
 image.Dispose();
 }
 }
}