//显示验证码
public ActionResult SecurityCode()
{
string oldcode = TempData["SecurityCode"] as string;
string code = JsHelper.CreateRandomCode(4); //验证码的字符为4个
TempData["SecurityCode"] = code; //验证码存放在TempData中
return(File(JsHelper.CreateValidateGraphic(code), "image/Jpeg"));
}