在MVC 5中填充DropDownList

Question

这是我的AddNewProductViewModel的代码

using AccessorizeForLess.Data;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web;

namespace AccessorizeForLess.ViewModels
{
    public class AddNewProductViewModel
    {
        public string Name { get; set; }

        [DataType(DataType.MultilineText)]
        public string Description { get; set; }

        public decimal Price { get; set; }

        public string AltText { get; set; }

        public int Quantity { get; set; }

        public string ImagePath { get; set; }

        public HttpPostedFileBase Image { get; set; }

        public List<ProductCategory> Category { get; set; }
    }
}

这是我的控制器中的Create方法

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Create(AddNewProductViewModel model)
        {
            ProductImageViewModel image = new ProductImageViewModel() { ImagePath = "/Content/ProductImages/" + model.Image.FileName, AltText = model.AltText };
            ProductImage newImage = new ProductImage() { ImagePath = image.ImagePath, AltText = image.AltText };
            entities.ProductImages.Add(newImage);
            await entities.SaveChangesAsync();
            int ImageId = newImage.ProductImageId;

            Product product = new Product()
            {
                ProductImageId = ImageId,
                ProductName = model.Name,
                ProductDescription = model.Description,
                ProductPrice = model.Price,
                Quantity = model.Quantity
                CategoryID = model.
            };

            string file = model.Image.FileName;
            string path = Server.MapPath(@"~/Content/ProductImages");
            string fullPath = path + @"\" + file;
            try
            {
                model.Image.SaveAs(path + @"\" + file);

                if (ModelState.IsValid)
                {
                    entities.Products.Add(product);
                    await entities.SaveChangesAsync();
                    return RedirectToAction("Create");
                }
            }
            catch (Exception ex)
            {
                ViewBag.Message = ex.ToString();
            }
           //ViewBag.ProductImageId = new SelectList(entities.ProductImages, "ProductImageId", "ImagePath", product.ProductImageId);
            return View("Create");
        }

现在我想在我的视图中填充我的DropDownList:

<div class="form-group">
            @Html.LabelFor(model => model.Category, new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.Category.Id, new SelectList(Model.Category,"Value","Text"), "- Please Select -")
                @Html.ValidationMessageFor(model => model.Category)
            </div>
        </div>

这是我得到的错误:

CS0411:方法'System.Web.Mvc.Html.SelectExtensions.DropDownListFor(System.Web.Mvc.HtmlHelper,System.Linq.Expressions.Expression>,System.Collections.Generic.IEnumerable,object)'的类型参数不能是从用法推断.尝试显式指定类型参数.

编辑

这是新的AddNewProductViewModel

using AccessorizeForLess.Data;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web;

namespace AccessorizeForLess.ViewModels
{
    public class AddNewProductViewModel
    {
        public string Name { get; set; }

        [DataType(DataType.MultilineText)]
        public string Description { get; set; }

        public decimal Price { get; set; }

        public string AltText { get; set; }

        public int Quantity { get; set; }

        public string ImagePath { get; set; }

        public HttpPostedFileBase Image { get; set; }

        public ProductCategory Category { get; set; }

        public int SelectedCategoryId { get; set; }

        public List<ProductCategory> Categories { get; set; }
}
}

我在下拉列表中的更新尝试

<div class="form-group">
            @Html.LabelFor(model => model.Category, new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DropDownListFor(model => model.SelectedCategoryId,
                 new SelectList(Model.Categories, "CategoryId", "CategoryName"), "- Please Select -")
                @Html.ValidationMessageFor(model => model.Category)
            </div>
        </div>

现在我在这一行上得到一个NullReferenceException

@Html.DropDownListFor(model => model.SelectedCategoryId,

鉴于我的代码可以有人请告诉我我在这里做错了什么,自从昨晚以来我一直在努力.

Answer 1

我解决了这个问题(感谢大家的提示).这适用于任何可能遇到类似问题的人.

我改变了我的Create方法,如下所示:

// GET: /Products/Create
public ActionResult Create()
{
    var p = new AddNewProductViewModel();
    p.Categories = entities.ProductCategories.ToList();
    return View(p);
}

我的AddNewProductViewModel看起来像这样:

using AccessorizeForLess.Data;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Web;

namespace AccessorizeForLess.ViewModels
{
    public class AddNewProductViewModel
    {
        public string Name { get; set; }

        [DataType(DataType.MultilineText)]
        public string Description { get; set; }

        public decimal Price { get; set; }

        public string AltText { get; set; }

        public int Quantity { get; set; }

        public HttpPostedFileBase Image { get; set; }

        public int SelectedCategoryId {get;set;}
        public List<ProductCategory> Categories { get; set; }
        public ProductCategory Category { get; set; }
    }
}

在我看来:

<div class="form-group">
    @Html.LabelFor(model => model.SelectedCategoryId, "Category",new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownListFor(model => model.SelectedCategoryId, new SelectList(Model.Categories, "CategoryId", "CategoryName"), "- Please Select -")
        @Html.ValidationMessageFor(model => model.SelectedCategoryId)
    </div>
</div>

感谢大家的帮助:)