新建一个一般处理程序 Handler.ashx
public void ProcessRequest (HttpContext context) {

        context.Response.ContentType = "image/jpeg";
        using (System.Drawing.Bitmap map = new System.Drawing.Bitmap(100, 30))
        {
            using (System.Drawing.Graphics gr = System.Drawing.Graphics.FromImage(map))
            {
                string strArray = "1234567890ABCDEFGHIJKLMNUVWXYZ";
                Random rand = new Random();
                string str = "";
                for (int i = 0; i < 4; i++)
                {
                    int j = rand.Next(0, 35);
                    str += strArray[j];
                }
                HttpCookie cookie = new HttpCookie("ckkd");
                cookie.Value = str;
                cookie.Expires = DateTime.MaxValue;
                context.Response.Cookies.Add(cookie);
                gr.DrawString(str, new System.Drawing.Font("Courier New", 20, System.Drawing.FontStyle.Italic), System.Drawing.Brushes.Blue, new System.Drawing.PointF(2, 2));
                map.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
        }
                 
                 
    }
使用方法：
<asp:TextBox ID="txtCoder" runat="server"></asp:TextBox><img src="Handler.ashx" />