Populate DropDownList in MVC 5

问题:

Here is my code for my 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 List<ProductCategory> Category { get; set; }
    }
}

Here's my Create method in my controller

[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");
        }

And now where I'm trying to populate my DropDownList in my view:

<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>

And this is the error I'm getting:

CS0411: The type arguments for method 'System.Web.Mvc.Html.SelectExtensions.DropDownListFor(System.Web.Mvc.HtmlHelper, System.Linq.Expressions.Expression>, System.Collections.Generic.IEnumerable, object)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

EDIT

Here's is the new 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; }
}
}

And my updated attempt at a 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.SelectedCategoryId,
                 new SelectList(Model.Categories, "CategoryId", "CategoryName"), "- Please Select -")
                @Html.ValidationMessageFor(model => model.Category)
            </div>
        </div>

Now I'm getting a NullReferenceException on this line

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

Given my code can someone please show me what I'm doing wrong here, I've been struggling with this since last night.

回答1:

I solved the issue (thanks to everyone for the tips). This is for anyone who may be having issues like I was.

I changed my Create method to look like so:

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

My AddNewProductViewModel looks like so:

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; }
    }
}

The in my view:

<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>

Thanks for the help everyone :)

回答2:

First I would change your ViewModel to include a SelectedCategoryId and I would change your options to be Categories.

Without seeing the code for your get I am assumbing ProductCategory is something like the following:

public class ProductCategory {
  public int Id {get;set;}
  public string Name { get;set;}
}

Your razor mark-up would them become:

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

The first parameter is the selected catgegory and your options are populated from Categories.

WorkingFiddle

回答3:

Use your controller to build the list, then call the list from the view.

Controller:

public static List<SelectListItem> GetDropDown()
    {
        List<SelectListItem> ls = new List<SelectListItem>();
        lm = (call database);
        foreach (var temp in lm)
        {
            ls.Add(new SelectListItem() { Text = temp.name, Value = temp.id });
        }
        return ls;
    }

Call the Dropdown:

@Html.DropDownListFor(x => x.Field, PathToController.GetDropDown())