Проверка нулевой модели, но все еще получение ссылки на нулевой объект ASP.NET MVC

Question

контроллер

public ActionResult EditProduct(int id)
{
     ProductViewModel ViewModel = new ProductViewModel();         
     ViewModel.SingleProduct = DB.Prouducts.Where(x => x.ProductID == id).FirstOrDefault();
     ViewModel.ImageList = DB.ImageGalleries.Where(x => x.ProductIdFk == id).ToList();
     return View(ViewModel);
}


[HttpPost]
public ActionResult EditProduct(Prouduct product, IEnumerable<HttpPostedFileBase> thumb, ImageGallery images)
{
        CategoryDropdown();
        BrandDropdown();
        if (ModelState.IsValid)
        {       
            HttpPostedFileBase Image1 = thumb.FirstOrDefault();        
                product.ProductSlug = slug;
                var userID = Convert.ToInt32(Session["UserID"]);
                product.UserIdFk = userID;
                DB.Entry(product).State = System.Data.Entity.EntityState.Modified;
                DB.SaveChanges();
                int LastInsertedID = product.ProductID;
                foreach (var tmb in thumb)
                {
                    if (tmb != null)
                    {
                        string FileName = tmb.FileName;
                        string Extenstion = Path.GetExtension(FileName);
                        if (Extenstion.ToLower() == ".jpeg" | Extenstion.ToLower() == ".jpg" | Extenstion.ToLower() == ".png" | Extenstion.ToLower() == ".webp")
                        {
                            FileName = FileName + DateTime.Now.ToString("yyyyMMddHHmmssfff") + Extenstion;
                            string ImageSavePath = Server.MapPath("/Content/Assets/Photos/");
                            tmb.SaveAs(Path.Combine(ImageSavePath + FileName));
                            string ThumbSavePath = Server.MapPath("/Content/Assets/Photos/Thumbs/");
                            ThumbGenration.ResizeStream(522, tmb.InputStream, Path.Combine(ThumbSavePath + FileName));
                            images.ImageName = FileName;
                            images.ImageThumb = FileName;
                            images.ProductIdFk = LastInsertedID;
                            //var userID = Convert.ToInt32(Session["UserID"]);
                            images.UserIdFk = userID;
                            DB.ImageGalleries.Add(images);
                            DB.SaveChanges();
                            TempData["Success"] = "Data Added Successfully!";
                        }
                    }
                }
        }
       return View();
    }

View

@model  RentalServices.Models.ProductViewModel


 @using (Html.BeginForm("EditProduct", "Product", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
@Html.AntiForgeryToken()
@Html.ValidationSummary(true, "", new { @class = "text-danger" })
@Html.HiddenFor(model => model.SingleProduct.ProductID);
<div class="add-item-wrapper">
    <h4>Listing Details</h4>
    <hr class="noPadMar" />
    <div class="add-item">
        <div class="row">
            <div class="col-sm-10 col-sm-offset-1">
                <div class="col-md-12 col-sm-12 form-group">
                    @*<label class="col-sm-3 col-md-3 control-label">Title</label>*@
                    @Html.LabelFor(model => model.SingleProduct.Title, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @*<input type="text" class="form-control" placeholder="TITLE" />*@
                        @Html.EditorFor(model => model.SingleProduct.Title, new { htmlAttributes = new { @class = "form-control" } })
                        @Html.ValidationMessageFor(model => model.SingleProduct.Title, "", new { @class = "text-danger" })
                    </div>
                </div>
                <div class="col-md-12 col-sm-12 form-group">
                    @Html.LabelFor(model => model.SingleProduct.Price, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @Html.EditorFor(model => model.SingleProduct.Price, new { htmlAttributes = new { @class = "form-control" } })
                        @Html.ValidationMessageFor(model => model.SingleProduct.Price, "", new { @class = "text-danger" })
                    </div>
                </div>
                <div class="col-md-12 col-sm-12 form-group">
                    @Html.Label("CATEGORY", htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @Html.DropDownListFor(model => model.SingleProduct.CategoryIdFk, ViewBag.CategoryDropdown as SelectList, "CHOOSE CATEGORY", new { @class = "form-control", id = "CategoryID" })
                        @Html.ValidationMessageFor(model => model.SingleProduct.CategoryIdFk, "", new { @class = "text-danger" })
                    </div>
                </div>
                <div id="hide">
                    <div class="col-md-12 col-sm-12 form-group">
                        @Html.Label("BRAND", htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                        <div class="col-sm-9">
                            @Html.DropDownListFor(model => model.SingleProduct.BrandIdFk, ViewBag.BrandDropdown as SelectList, "CHOOSE BRAND", new { @class = "form-control", id = "BrandID" })
                            @Html.ValidationMessageFor(model => model.SingleProduct.BrandIdFk, "", new { @class = "text-danger" })
                        </div>
                    </div>


                    <div class="col-md-12 col-sm-12 form-group">
                        @Html.LabelFor(model => model.SingleProduct.Ram, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                        <div class="col-sm-9">
                            @Html.EditorFor(model => model.SingleProduct.Ram, new { htmlAttributes = new { @class = "form-control" } })
                            @Html.ValidationMessageFor(model => model.SingleProduct.Ram, "", new { @class = "text-danger" })
                        </div>
                    </div>
                    <div class="col-md-12 col-sm-12 form-group">
                        @Html.LabelFor(model => model.SingleProduct.Processor, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                        <div class="col-sm-9">
                            @Html.EditorFor(model => model.SingleProduct.Processor, new { htmlAttributes = new { @class = "form-control" } })
                            @Html.ValidationMessageFor(model => model.SingleProduct.Processor, "", new { @class = "text-danger" })
                        </div>
                    </div>
                </div>
                <div class="col-md-12 col-sm-12 form-group">
                    @Html.Label("CONDITION", htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @Html.DropDownListFor(model => model.SingleProduct.Conditon, selectList, "CHOOSE CONDITION", new { @class = "form-control" })
                        @Html.ValidationMessageFor(model => model.SingleProduct.Conditon, "", new { @class = "text-danger" })
                    </div>
                </div>
                <div class="col-md-12 col-sm-12 form-group">
                    @Html.LabelFor(model => model.SingleProduct.Location, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @Html.EditorFor(model => model.SingleProduct.Location, new { htmlAttributes = new { @class = "form-control" } })
                        @Html.ValidationMessageFor(model => model.SingleProduct.Location, "", new { @class = "text-danger" })
                    </div>
                </div>
                <div class="col-md-12 col-sm-12 form-group">
                    @Html.LabelFor(model => model.SingleProduct.Description, htmlAttributes: new { @class = "col-sm-3 col-md-3 control-label" })
                    <div class="col-sm-9">
                        @Html.TextAreaFor(model => model.SingleProduct.Description, new { htmlAttributes = new { @class = "form-control" } })
                        @Html.ValidationMessageFor(model => model.SingleProduct.Description, "", new { @class = "text-danger" })
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>
<div class="image-gallery-wrapper">
    <div class="img-gallery">
        @if (Model.ImageList.Any())
        {
            foreach (var item in Model.ImageList)
            {
                <div class="img-wrapper">
                    <p>Image 1</p>
                    <div class="img-box">
                        <input type="file" name="thumb" value="" class="file-style" onchange="readURL(this)" ; />
                        <img src="/Content/Assets/Photos/Thumbs/@item.ImageName" alt="your image" id="imgName" value="@item.ImageName" />
                        <button id="RemoveImage">Remove Image</button>
                    </div>
                </div>

            }
        }

    </div>        
</div>
<div class="text-center">
    <button type="submit" class="roundSubmitBtn" style="background:#7048f0 !important;font-size:14px !important; margin-top:40px;">SUMBIT <i class="fa fa-arrow-right"></i></button>
</div>
}

Когда я добавил свой код, я получаю ошибку исключения null. Но у меня есть проверка нуля или нет, поэтому я получаю эту ошибку ссылки на нулевой объект. и я также попытался использовать Count () и! = null в операторе IF. Я получаю ошибку, когда я отправляю форму, а error is null исключения ошибки, поэтому скажите мне, где я ошибаюсь

Answer 1

При проверке метода действия POST, представленного в вопросе, кажется, что проблема возникает из оператора return View(), который возвращает ту же страницу представления, что и в методе действия GET, но без возврата экземпляра класса viewmodel, в результате чего ProductViewModel.ImageList содержит нулевое значение.

Краткий код ниже показывает проблему:

[HttpPost]
public ActionResult EditProduct(Prouduct product, IEnumerable<HttpPostedFileBase> thumb, ImageGallery images)
{
    CategoryDropdown();
    BrandDropdown();
    if (ModelState.IsValid)
    {       
       // image processing and saving to DB
    }

    // the view returned without viewmodel
    // this will trigger NullReferenceException because ProductViewModel.ImageList is not reassigned yet
    return View();
}

Следовательно, вам следует переназначить свойство ProductViewModel.ImageList после сохранения опубликованных данных в базу данных и вернуть то же представление вместе с новым экземпляром ProductViewModel (или, при необходимости, перенаправить на другое действие, следуя шаблону PRG с RedirectToAction):

[HttpPost]
public ActionResult EditProduct(Prouduct product, IEnumerable<HttpPostedFileBase> thumb, ImageGallery images)
{
    CategoryDropdown();
    BrandDropdown();
    if (ModelState.IsValid)
    {       
       // image processing and saving to DB
    }

    // create viewmodel instance
    var ViewModel = new ProductViewModel();         

    ViewModel.ImageList = ...; // reassign ImageList property here
    return View(ViewModel);
}