using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;

public partial class Tools_CheckCode : System.Web.UI.Page
{
 protected void Page_Load(object sender, EventArgs e)
 {
  this.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
  {
   //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.GreenYellow), x1, y1, x2, y2);
   }

   Font font = new System.Drawing.Font("Verdana", 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 < 80; 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.Red), 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();
  }
 }


}

<img src="/UploadFiles/2021-04-02/CheckCode.aspx">

3.用web handler生成图片。这个其实和前面的意思大致差不多，调用方法也基本和2一样,不同的是，它的验证码是存入Session的。供学习参考。
效果图如下：

ValidateImageHandler.ashx


%@ WebHandler Language="C#" Class="ValidateImageHandler" %>

using System;
using System.Web;
using System.Web.SessionState;
using System.Drawing;
using System.Drawing.Imaging;
using System.Text;

/// <summary>
/// ValidateImageHandler 生成网站验证码功能
/// </summary>
public class ValidateImageHandler : IHttpHandler, IRequiresSessionState
{
 int intLength = 5;    //长度
 string strIdentify = "Identify"; //随机字串存储键值，以便存储到Session中
 public ValidateImageHandler()
 {  
 }

 /// <summary>
 /// 生成验证图片核心代码
 /// </summary>
 /// <param name="hc"></param>
 public void ProcessRequest(HttpContext hc)
 {
  //设置输出流图片格式
  hc.Response.ContentType = "image/gif";
  
  Bitmap b = new Bitmap(200, 60);
  Graphics g = Graphics.FromImage(b);
  g.FillRectangle(new SolidBrush(Color.YellowGreen), 0, 0, 200, 60);
  Font font = new Font(FontFamily.GenericSerif, 48, FontStyle.Bold, GraphicsUnit.Pixel);
  Random r = new Random();

  //合法随机显示字符列表
  string strLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
  StringBuilder s = new StringBuilder();
  
  //将随机生成的字符串绘制到图片上
  for (int i = 0; i < intLength; i++)
  {
   s.Append(strLetters.Substring(r.Next(0, strLetters.Length - 1), 1));
   g.DrawString(s[s.Length - 1].ToString(), font, new SolidBrush(Color.Blue), i * 38, r.Next(0, 15));
  }

  //生成干扰线条
  Pen pen = new Pen(new SolidBrush(Color.Blue), 2);
  for (int i = 0; i < 10; i++)
  {
   g.DrawLine(pen, new Point(r.Next(0, 199), r.Next(0, 59)), new Point(r.Next(0, 199), r.Next(0, 59)));
  }
  b.Save(hc.Response.OutputStream, ImageFormat.Gif);
  hc.Session[strIdentify] = s.ToString(); //先保存在Session中，验证与用户输入是否一致
  hc.Response.End();
 
 }
 
 /// <summary>
 /// 表示此类实例是否可以被多个请求共用(重用可以提高性能)
 /// </summary>
 public bool IsReusable
 {
  get
  {
   return true;
  }
 }
}



以上就是ASP.NET3种验证码，各有各的优缺点，希望大家可以熟练实现不同验证码的功能。