1.隨機生成驗證碼字元：

        /// <summary>
        /// 產生驗證碼
        /// </summary>
        /// <param name="codeLength">驗證碼長度</param>
        /// <returns></returns>
        public string CreateCode(int codeLength)
        {

            string so = "1,2,3,4,5,6,7,8,9,0,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z
 
";
            string[] strArr = so.Split(',');
            string code = "";
            Random rand = new Random();
            for (int i = 0; i < codeLength; i++)
            {
                code += strArr[rand.Next(0, strArr.Length)];
            }
            return code;
        }

2.根據驗證碼字元繪製圖片、並隨機繪製干擾線：

        /// <summary>
        /// 產生驗證圖片
        /// </summary>
        /// <param name="code">驗證碼</param>
        public void CreateImage(string code)
        {

            Bitmap image = new Bitmap(60, 20);
            Graphics g = Graphics.FromImage(image);
            WebColorConverter ww 
 
= new WebColorConverter();
            g.Clear((Color)ww.ConvertFromString("#FAE264"));

            Random random = new Random();

            //畫圖片的干擾線
            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.Red), x1, x2, y1, y2);
            }

            Font font = new Font("Arial", 15, FontStyle.Bold | FontStyle.Italic);
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(
                new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.Gray, 1.2f, true);
            g.DrawString(code, font, brush, 0, 0);

            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());
            g.Dispose();
            image.Dispose();
        }

使用方法

1.WebForm中使用：

新建VerificationCode.aspx

        protected void Page_Load(object sender, EventArgs e)
        {
            string checkCode = CreateCode(4);
            Session["CheckCode"] = checkCode;
            CreateImage(checkCode);
        }

在其他頁面中引用使用：

<img src="VerificationCode.aspx" align="middle">

2.在MVC中使用：

修改上面生成圖片的方法，將Response.Write修改為返回byte[]物件：

        /// <summary>
        /// 產生驗證圖片
        /// </summary>
        /// <param name="code">驗證碼</param>
        public void CreateImage(string code)
        {

            Bitmap image = new Bitmap(60, 20);
            Graphics g = Graphics.FromImage(image);
            WebColorConverter ww = new WebColorConverter();
            g.Clear((Color)ww.ConvertFromString("#FAE264"));

            Random random = new Random();

            //畫圖片的干擾線
            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.Red), x1, x2, y1, y2);
            }

            Font font = new Font("Arial", 15, FontStyle.Bold | FontStyle.Italic);
            System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.LinearGradientBrush(
                new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.Gray, 1.2f, true);
            g.DrawString(code, font, brush, 0, 0);

           System.IO.MemoryStream ms = new System.IO.MemoryStream();
            image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            return ms.ToArray()

 }

        public ActionResult VerificationCode()
        {string code = CreateCode(4); 
            TempData["SecurityCode"] = code; //驗證碼存放在TempData中
            return File(CreateImage(code), "image/Jpeg");
        }