public void OnActionExecuting(ActionExecutingContext filterContext) { if (!filterContext.ModelState.IsValid) { if (filterContext.HttpContext.Request.Method == "GET") { var result = new BadRequestResult(); filterContext.Result = result; } else { var result = new ContentResult() { Content = ApiHelpers.JsonSerialize(new ApiError(filterContext.ModelState)), ContentType = "application/json" }; filterContext.HttpContext.Response.StatusCode = 400; filterContext.Result = result; } } }