Implementation code

/// <summary>
    /// 生成驗證碼圖片，保存session名稱VerificationCode
    /// </summary>
    public static void CreateVerificationCode()
    {
        int number;
        string checkCode = string.Empty;
        //隨機數(shù)種子
        Random randoms = new Random();
        for (int i = 0; i < 4; i++) //校驗碼長度為4
        {
            //隨機的整數(shù)
            number = randoms.Next();
            //字符從0-9,A-Z中隨機產(chǎn)生,對應(yīng)的ASCII碼分別為
            //48-57,65-90
            number = number % 36;
            if (number < 10)
            {
                number += 48;
            }
            else
            {
                number += 55;
            }
            checkCode += ((char)number).ToString();
        }
        //在session中保存校驗碼
        System.Web.HttpContext.Current.Session["VerificationCode"] = checkCode;
        //若校驗碼為空，則直接返回
        if (checkCode == null || checkCode.Trim() == String.Empty)
        {
            return;
        }
        //根據(jù)校驗碼的長度確定輸出圖片的長度
        System.Drawing.Bitmap image = new System.Drawing.Bitmap(55, 20);//(int)Math.Ceiling(Convert.ToDouble(checkCode.Length * 15))
        //創(chuàng)建Graphics對象
        Graphics g = Graphics.FromImage(image);
        try
        {
            //生成隨機數(shù)種子
            Random random = new Random();
            //清空圖片背景色
            g.Clear(Color.White);
            //畫圖片的背景噪音線 10條
            //---------------------------------------------------
            for (int i = 0; i < 10; i++)
            {
                //噪音線起點坐標(x1,y1),終點坐標(x2,y2)
                int x1 = random.Next(image.Width);
                int x2 = random.Next(image.Width);
                int y1 = random.Next(image.Height);
                int y2 = random.Next(image.Height);
                //用銀色畫出噪音線
                g.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);
            }
            //---------------------------------------------------
            //Brush b = Brushes.Silver;
            //g.FillRectangle(b, 0, 0, image.Width, image.Height);
            //---------------------以上兩種任選其一------------------------------
            //輸出圖片中校驗碼的字體: 12號Arial,粗斜體
            Font font = new Font("Arial", 12, (FontStyle.Bold | FontStyle.Italic));
            //線性漸變畫刷
            LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.Purple, 1.2f, true);
            g.DrawString(checkCode, font, brush, 2, 2);
            //畫圖片的前景噪音點 50個
            for (int i = 0; i < 50; 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.Peru), 0, 0, image.Width - 1, image.Height - 1);
            //創(chuàng)建內(nèi)存流用于輸出圖片
            using (MemoryStream ms = new MemoryStream())
            {
                //圖片格式指定為png
                image.Save(ms, ImageFormat.Jpeg);
                //清除緩沖區(qū)流中的所有輸出
                System.Web.HttpContext.Current.Response.ClearContent();
                //輸出流的HTTP MIME類型設(shè)置為"image/Png"
                System.Web.HttpContext.Current.Response.ContentType = "image/Jpeg";
                //輸出圖片的二進制流
                System.Web.HttpContext.Current.Response.BinaryWrite(ms.ToArray());
            }
        }
        finally
        {
            //釋放Bitmap對象和Graphics對象
            g.Dispose();
            image.Dispose();
        }
    }

Create an aspx page

 <%@ Page Language="C#" AutoEventWireup="true" CodeFile="AuthCode.aspx.cs" Inherits="AuthCode" %>

 <%Help.CreateVerificationCode(); %>

Add HTML code, quote

 <div class="positionR">
     <label>驗證碼：</label>
     <span class="style1"> *</span>
     <input type="text" class="yanZm" runat="Server" reg="^.+$" id="txtAuthCode" tip="請輸入驗證碼!" />
     <img class="yanZm_img lazy"  src="/static/imghw/default1.png"  data-src="AuthCode.aspx"    alt="" id="imgAuthCode" />
 </div>

How to achieve refresh?

     <script type="text/javascript">
         $("#imgAuthCode").click(function () {
             $(this).attr("src", "AuthCode.aspx?code=" + (new Date()).getTime());
         });
     </script>

Rendering

For more ASP.NET small examples of implementing verification codes and refreshing verification codes, please pay attention to the PHP Chinese website!