public virtual void Refresh() { var img = new ImageBuilder(); const string chars = "1234567890qwertyuioplkjhgfdsazxcvbnmQWERTYUIOPLKJHGFDSAZXCVBNM"; var code = string.Empty; var ramdon = new Random(); for (var i = 0; i < 4; i++) { code += chars.Substring(ramdon.Next(0, chars.Length - 1), 1); } var bmp = img.Generate(code); Session[Config.SessionKey] = code; bmp.Save(HttpContext.Response.OutputStream, ImageFormat.Jpeg); }