Dropdown null异常ASP.NET

Question

我使用表单中的数据在数据库中保存产品。如果任何所需输入为空，则验证如此 - ＆gt;它返回带有错误语句的相同View。当我调试代码时，一切都很好，除了从下拉列表中选择的类别。它抛出一个null异常。

当我填写所有内容时，该类别获得了价值，我可以毫无问题地提交产品。如果其他一些输入为null，则ModelState设置为false，类别不会获得其值。

ADD行动

  [HttpPost]
    [Authorize(Roles = "prodejce")]
    public ActionResult Add(Ski ski, HttpPostedFileBase picture, int categoryId)
    {

        try
        {
            if (ModelState.IsValid)
            {


                if (picture != null)
                {
                    if (picture.ContentType == "image/jpeg" || picture.ContentType == "image/png")
                    {
                        Image image = Image.FromStream(picture.InputStream);

                        if (image.Height > 200 || image.Width > 200)
                        {
                            Image smallImage = ImageHelper.ScaleImage(image, 100, 100);
                            Bitmap b = new Bitmap(smallImage);

                            Guid guid = Guid.NewGuid();
                            string imageName = guid.ToString() + ".jpeg";

                            b.Save(Server.MapPath("~/uploads/Ski/" + imageName), ImageFormat.Jpeg);

                            smallImage.Dispose();
                            b.Dispose();

                            ski.ImageName = imageName;
                        }
                        else
                        {
                            picture.SaveAs(Server.MapPath("~/uploads/Ski/" + picture.FileName));
                        }
                    }
                }


                SkiCategoryDao skiCategoryDao = new SkiCategoryDao();
                SkiCategory skiCategory = skiCategoryDao.GetById(categoryId);

                ski.Category = skiCategory;

                SkiDao skiDao = new SkiDao();


                    skiDao.Create(ski);
                    TempData["message-success"] = "Lyže byly úspěšně zadané";

            }

            else
            {
                return View("Create", ski); // this throws a new view 
            }

        }

该部分视图效果正常

    @model DataAccess.Model.Ski
@{
    ViewBag.Title = "Create";
}

<h2>Nové lyže</h2>


@using (Html.BeginForm("Add", "Skies", FormMethod.Post, new { @class = "form-horizontal", enctype="multipart/form-data" }))
{

    <div class="form-group">
        <label class="col-sm-1 control-label">Délka</label>
        <div class="col-sm-10">
            @Html.TextBoxFor(x => x.Length, new { @class = "form-control" })
        @Html.ValidationMessageFor(x => x.Length)
        </div>
    </div>
    <div class="form-group">
        <label class="col-sm-1 control-label">Značka</label>
        <div class="col-sm-10">
            @Html.TextBoxFor(x => x.Brand, new { @class = "form-control" })
            @Html.ValidationMessageFor(x => x.Brand)
        </div>
    </div>
    <div class="form-group">
        <label class="col-sm-1 control-label">Popis</label>
        <div class="col-sm-10">
            @Html.TextAreaFor(x => x.Description, new { @class = "form-control formatedtext" })
            @Html.ValidationMessageFor(x => x.Brand)
        </div>
    </div>

以下是空例外

 <div class="form-group">
            <label class="col-sm-1 control-label">Kategorie</label>
            <div class="col-sm-10">
                @Html.DropDownList("categoryId", new SelectList(ViewBag.Categories, "Id", "Name"), new { @class = "form-control" })
            </div>
        </div>