ArgumentNullException for 'items' when using SelectList with ViewBag in ASP.NET Core MVC

Question

ArgumentNullException for 'items' when using SelectList with ViewBag in ASP.NET Core MVC

MrBarszcz 20

I'm developing an e-commerce project in ASP.NET Core MVC 9, running on a Debian server. I have a form to register products, where I need to allow the user to select a gender for the product from a list of options loaded from the database.

In my RegisterProduct.cshtml View, I'm using radio buttons to display the gender options, inside a form that uses the asp-for Helper Tag to link the fields to the properties of my ProductModel. The list of genders is passed to the View through ViewBag.Genders, which contains a List<GenderModel>.

RegisterProduct.cshtml:

@model ProductModel

@{
    Layout = "_Layout";
}

<div class="text-center">
    <h1 class="display-4">Cadastrar Produto</h1>

    <form asp-controller="Product" asp-action="Register" method="post" enctype="multipart/form-data">
        <div class="form-group">
            <label class="control-label">Imagens do Produto</label>
            <input type="file" name="ProductImages" id="productImagesInput" class="form-control" multiple accept="image/*"/>
            <div id="imagePreviewContainer" class="row mt-3">
                @if (ViewBag.ImagePreviews != null) {
                    for (var i = 0; i < ((List<string>)ViewBag.ImagePreviews).Count; i++) {
                        <div class="image-preview-container" draggable="true" data-index="@i">
                            <img alt="" class="image-preview" src="@((List<string>)ViewBag.ImagePreviews)[i]"/>
                        </div>
                    }
                }
            </div>
        </div>


        <div class="form-group p-2">
            <label for="name" class="control-label">Nome do Produto</label>
            <input asp-for="ProductName" class="form-control" id="name" placeholder="Camisa Regular Em Sarja"/>
        </div>

        <div class="form-group p-2">
            <label for="description" class="control-label">Descrição Curta</label>
            <textarea asp-for="ProductDescription" class="form-control" id="description"
                      placeholder="Essa camisa regular em sarja oferece um visual clássico e confortável, ideal para diversas ocasiões. O tecido de sarja proporciona um toque macio e durabilidade, enquanto a modelagem regular garante um caimento solto e elegante."></textarea>
        </div>

        <div class="form-group p-2">
            <label for="price" class="control-label">Valor</label>
            <input type="number" min="0" step=".01" asp-for="ProductPrice" class="form-control" id="price"/>
        </div>

        <div class="form-group">
            <div class="dropdown p-2">
                <button class="btn btn-light dropdown-toggle" type="button" id="dropdownGender"
                        data-bs-toggle="dropdown" aria-expanded="false">
                    Selecionar Gênero
                </button>

                <ul class="dropdown-menu" aria-labelledby="dropdownGender">
                    @if (ViewBag.Genders != null) {
                        foreach (var gender in (List<GenderModel>)ViewBag.Genders) {
                            <li>
                                <div class="form-check ms-2">
                                    <input type="radio" class="form-check-input" name="GenderId"
                                           value="@gender.GenderId"
                                           asp-for="GenderId"/>
                                    <label class="form-check-label" asp-for="GenderId">@gender.Gender</label>
                                </div>
                            </li>
                        }
                    } else {
                        <li>
                            <span class="dropdown-item">Nenhum gênero cadastrado</span>
                        </li>
                    }
                </ul>
            </div>
        </div>

        <div class="form-group">
            <div class="dropdown p-2">
                <button class="btn btn-light dropdown-toggle" type="button" id="dropdownCategories"
                        data-bs-toggle="dropdown" aria-expanded="false">
                    Selecionar Categorias
                </button>

                <ul class="dropdown-menu" aria-labelledby="dropdownCategories">
                    @if (ViewBag.Categories != null) {
                        foreach (var category in (List<CategoryModel>)ViewBag.Categories) {
                            <li>
                                <div class="form-check ms-2">
                                    <input type="checkbox" class="form-check-input" name="SelectedCategories"
                                           value="@category.CategoryId"/>
                                    <label class="form-check-label">@category.Category</label>
                                </div>
                            </li>
                        }
                    } else {
                        <li>
                            <span class="dropdown-item">Nenhuma categoria cadastrada</span>
                        </li>
                    }
                    <li>
                        <a role="button" class="btn" asp-controller="Category" asp-action="Register">cadastrar</a>
                    </li>
                </ul>
            </div>
        </div>

        <div class="form-group">
            <div class="dropdown p-2">
                <button class="btn btn-light dropdown-toggle" type="button" id="dropdownColors"
                        data-bs-toggle="dropdown" aria-expanded="false">
                    Selecionar Cores
                </button>
                <ul class="dropdown-menu" aria-labelledby="dropdownColors">
                    @if (ViewBag.Colors != null) {
                        foreach (var color in (List<ColorModel>)ViewBag.Colors) {
                            <li>
                                <div class="form-check ms-2">
                                    <input type="checkbox" class="form-check-input" name="SelectedColors"
                                           value="@color.ColorId"/>
                                    <label class="form-check-label">
                                        <div class="color-box color-s"
                                             style="background-color: @color.HexColor"></div>
                                        @color.Color
                                    </label>
                                </div>
                            </li>
                        }
                    } else {
                        <li>
                            <span class="dropdown-item">Nenhuma cor cadastrada</span>
                        </li>
                    }
                    <li>
                        <a role="button" class="btn" asp-controller="Color" asp-action="Register">cadastrar</a>
                    </li>
                </ul>
            </div>
        </div>

        <div class="form-group">
            <div class="dropdown p-2">
                <button class="btn btn-light dropdown-toggle" type="button" id="dropdownSizes"
                        data-bs-toggle="dropdown" aria-expanded="false">
                    Selecionar Tamanhos
                </button>
                <ul class="dropdown-menu" aria-labelledby="dropdownSizes">
                    @if (ViewBag.Sizes != null) {
                        foreach (var size in (List<SizeModel>)ViewBag.Sizes) {
                            <li>
                                <div class="form-check ms-2">
                                    <input type="checkbox" class="form-check-input" name="SelectedSizes"
                                           value="@size.SizeId"/>
                                    <label class="form-check-label">@size.Size</label>
                                </div>
                            </li>
                        }
                    } else {
                        <li>
                            <span class="dropdown-item">Nenhum tamanho cadastrado</span>
                        </li>
                    }
                    <li>
                        <a role="button" class="btn" asp-controller="Size" asp-action="Register">cadastrar</a>
                    </li>
                </ul>
            </div>
        </div>

        <div class="form-group">
            <div id="stockInputsContainer">
            </div>
        </div>

        <div class="form-group">
            <a role="button" class="btn btn-secondary" asp-controller="Product" asp-action="Index">Cancelar</a>
            <button type="submit" class="btn btn-primary">Enviar</button>
        </div>
    </form>
</div>

ProductController.cs:

using System.Net.Http.Headers;
using System.Text.Json;
using DLyah_Boutique_System.Models;
using DLyah_Boutique_System.Repository;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Hosting;

namespace DLyah_Boutique_System.Controllers;

public class ProductController : Controller {
    private readonly IProductRepository _productRepository;
    private readonly IGenderRepository _genderRepository;
    private readonly ICategoryRepository _categoryRepository;
    private readonly IColorRepository _colorRepository;
    private readonly ISizeRepository _sizeRepository;
    private readonly IWebHostEnvironment _environment;
    private readonly ILogger<ProductController> _logger;

    private readonly string _pathImage;

    public ProductController(
        IProductRepository productRepository, IGenderRepository genderRepository,
        ICategoryRepository categoryRepository, IColorRepository colorRepository, ISizeRepository sizeRepository,
        IWebHostEnvironment environment, IFileUploadService fileUploadService, ILogger<ProductController> logger
    ) {
        _productRepository = productRepository;
        _genderRepository = genderRepository;
        _categoryRepository = categoryRepository;
        _colorRepository = colorRepository;
        _sizeRepository = sizeRepository;
        _environment = environment;
        _logger = logger;
        _pathImage = Path.Combine(_environment.WebRootPath, "images", "products");
    }

    // GET
    public IActionResult Index() {
        return View();
    }

    public IActionResult Register() {
        List<GenderModel> genders = _genderRepository.FindAll();
        List<CategoryModel> categories = _categoryRepository.FindAll();
        List<ColorModel> colors = _colorRepository.FindAll();
        List<SizeModel> sizes = _sizeRepository.FindAll();

        ViewBag.Genders = genders;
        ViewBag.Categories = categories;
        ViewBag.Colors = colors;
        ViewBag.Sizes = sizes;

        return View();
    }

    [HttpPost]
    public async Task<IActionResult> Register(
        List<IFormFile> pi, ProductModel product, int genderId, List<int> ct, List<int> c, List<int> s,  List<StockProductModel> stck
    ) {
        Console.WriteLine("Register método iniciado");
        
        if (ModelState.IsValid) {
            Console.WriteLine("ModelState é válido");
            
            // Adicionar o produto e salvar para obter o ProductId
            product.GenderId = genderId;
            
            _productRepository.Create(product);
            await _productRepository.SaveChanges();
            
            Console.WriteLine($"Produto adicionado com ID: {product.ProductId}");

            if (pi != null && pi.Any()) {
                Console.WriteLine($"{pi.Count} imagens recebidas.");
                
                string uploadDir = Path.Combine(_environment.WebRootPath, "images", "products");
                
                Console.WriteLine($"Diretório de upload: {uploadDir}");
                if (!Directory.Exists(uploadDir)) {
                    Console.WriteLine($"Diretório não existe, criando: {uploadDir}");

                    Directory.CreateDirectory(uploadDir);
                }

                int order = 1;
                foreach (var imageFile in pi) {
                    if (imageFile.Length > 0) {
                        Console.WriteLine($"Processando imagem: {imageFile.FileName}, Tamanho: {imageFile.Length}");

                        try {
                            string imageName = $"{product.ProductName.Replace(" ", "-")}-{Guid.NewGuid().ToString().Substring(0, 8)}{Path.GetExtension(imageFile.FileName)}";
                            string filePath = Path.Combine(uploadDir, imageName);

                            using (var stream = new FileStream(filePath, FileMode.Create)) {
                                await imageFile.CopyToAsync(stream);
                                
                                Console.WriteLine($"[DEBUG] Imagem salva em: {filePath}");
                            }

                            _productRepository.CreateProductImage(new ProductImageModel {
                                ProductId = product.ProductId,
                                ProductImagePath = $"/images/products/{imageName}",
                                ImageOrder = order++
                            });
                        } catch (Exception ex) {
                            Console.WriteLine($"[ERROR] Erro ao salvar a imagem {imageFile.FileName}: {ex.Message}");

                            ModelState.AddModelError("pi", $"Ocorreu um erro ao salvar a imagem {imageFile.FileName}.");
                        }
                    }
                }

                await _productRepository.SaveChanges();
                Console.WriteLine("Imagens salvas.");
            }


            if (ct != null && ct.Any()) {
                foreach (var categoryId in ct) {
                    _productRepository.CreateProductCategory(product.ProductId, categoryId);
                }
            }

            if (c != null && c.Any()) {
                foreach (var colorId in c) {
                    _productRepository.CreateProductColor(product.ProductId, colorId);
                }
            }

            if (s != null && s.Any()) {
                foreach (var sizeId in s) {
                    _productRepository.CreateProductSize(product.ProductId, sizeId);
                }
            }

            if (stck != null && stck.Any()) {
                foreach (var stockItem in stck) {
                    stockItem.ProductId = product.ProductId; // Garante que o ProductId esteja definido
                    _productRepository.CreateStockProduct(stockItem);
                }
                await _productRepository.SaveChanges();
            }

            await _productRepository.SaveChanges();

            return RedirectToAction("Index");
        } else {
            Console.WriteLine("ModelState não é válido");
            foreach (var error in ModelState.Values.SelectMany(v => v.Errors)) {
                Console.WriteLine($"Erro: {error.ErrorMessage}");
            }
        }

        // Se o modelo não for válido, retornar à view com os dados do produto
        List<GenderModel> genders = _genderRepository.FindAll();
        List<CategoryModel> categories = _categoryRepository.FindAll();
        List<ColorModel> colors = _colorRepository.FindAll();
        List<SizeModel> sizes = _sizeRepository.FindAll();

        ViewBag.Genders = genders;
        ViewBag.Categories = categories;
        ViewBag.Colors = colors;
        ViewBag.Sizes = sizes;

        return View(product);
    }
}

ProductRepository.cs:

using DLyah_Boutique_System.Data;
using DLyah_Boutique_System.Models;
using Microsoft.EntityFrameworkCore;

namespace DLyah_Boutique_System.Repository;

public class ProductRepository : IProductRepository {
    private readonly BankContext _context;

    public ProductRepository(BankContext context) {
        _context = context;
    }

    public List<ProductModel> FindAll() {
        return _context.Products
            .Include(pg => pg.Gender)
            .Include(p => p.ProductCategories)
            .ThenInclude(pc => pc.Category)
            .Include(p => p.ProductColors)
            .ThenInclude(pc => pc.Color)
            .Include(p => p.ProductSizes)
            .ThenInclude(ps => ps.Size)
            .Include(p => p.ProductImages.OrderBy(pi => pi.ImageOrder))
            .Include(pstck => pstck.StockProducts)
            .ToList();
    }

    public ProductModel? FindById(int id) {
        return _context.Products
            .Include(pg => pg.Gender)
            .Include(p => p.ProductCategories)
            .ThenInclude(pc => pc.Category)
            .Include(p => p.ProductColors)
            .ThenInclude(pc => pc.Color)
            .Include(p => p.ProductSizes)
            .ThenInclude(ps => ps.Size)
            .Include(p => p.ProductImages.OrderBy(pi => pi.ImageOrder))
            .Include(p => p.StockProducts) // Inclui os itens de estoque
            .FirstOrDefault(p => p.ProductId == id);
    }

    public ProductModel Update(ProductModel product) {
        ProductModel productDb = FindById(product.ProductId);

        if (productDb == null) throw new Exception("Houve um erro ao atualizar o produto");

        productDb.ProductName = product.ProductName;
        productDb.ProductDescription = product.ProductDescription;
        productDb.ProductPrice = product.ProductPrice;
        productDb.GenderId = product.GenderId;

        productDb.ProductColors.Clear();
        if (product.ProductColors != null) {
            foreach (var pc in product.ProductColors) {
                productDb.ProductColors.Add(new ProductColorModel
                    { ProductId = product.ProductId, ColorId = pc.ColorId });
            }
        }

        // Atualizar ProductSizes
        productDb.ProductSizes.Clear();
        if (product.ProductSizes != null) {
            foreach (var ps in product.ProductSizes) {
                productDb.ProductSizes.Add(new ProductSizeModel { ProductId = product.ProductId, SizeId = ps.SizeId });
            }
        }

        // Atualizar ProductCategories
        productDb.ProductCategories.Clear();
        if (product.ProductCategories != null) {
            foreach (var pc in product.ProductCategories) {
                productDb.ProductCategories.Add(new ProductCategoryModel
                    { ProductId = product.ProductId, CategoryId = pc.CategoryId });
            }
        }

        // Atualizar ProductImages (requer mais lógica dependendo de como você gerencia as imagens - adicionar, remover, etc.)
        // Este é um exemplo básico que remove todas as existentes e adiciona as novas
        productDb.ProductImages.Clear();
        if (product.ProductImages != null) {
            foreach (var pi in product.ProductImages) {
                productDb.ProductImages.Add(new ProductImageModel {
                    ProductId = product.ProductId, ProductImagePath = pi.ProductImagePath, ImageOrder = pi.ImageOrder
                });
            }
        }

        _context.Products.Update(productDb);
        _context.SaveChanges();

        return productDb;
    }

    public ProductModel Create(ProductModel product) {
        _context.Products.Add(product);
        _context.SaveChanges();

        return product;
    }

    public ProductCategoryModel CreateProductCategory(int productId, int categoryId) {
        ProductCategoryModel productCategory = new ProductCategoryModel {
            ProductId = productId,
            CategoryId = categoryId
        };

        _context.ProductCategories.Add(productCategory);

        return productCategory;
    }

    public ProductColorModel CreateProductColor(int productId, int colorId) {
        ProductColorModel productColor = new ProductColorModel {
            ProductId = productId,
            ColorId = colorId
        };

        _context.ProductColors.Add(productColor);

        return productColor;
    }

    public ProductSizeModel CreateProductSize(int productId, int sizeId) {
        ProductSizeModel productSize = new ProductSizeModel {
            ProductId = productId,
            SizeId = sizeId
        };

        _context.ProductSizes.Add(productSize);

        return productSize;
    }

    public ProductImageModel CreateProductImage(ProductImageModel productImage) {
        _context.ProductImages.Add(productImage);

        return productImage;
    }

    public StockProductModel CreateStockProduct(StockProductModel stock) {
        _context.StockProducts.Add(stock);

        return stock;
    }

    public ProductModel Kill(ProductModel product) {
        throw new NotImplementedException();
    }

    public Task<int> SaveChanges() {
        return _context.SaveChangesAsync();
    }
}

SurferOnWww 4,631 Reputation points

2025-05-17T00:57:29.8+00:00

ArgumentNullException for 'items' when using SelectList with ViewBag in ASP.NET Core MVC

Where is the "SelectList" in your code?

It is hard to find the cause as your code shown in your question is messy. Please provide a simple sample code which can only reproduce your issue by eliminating unnecessary portion to cause the "ArgumentNullException."

During eliminating unnecessary portion, you will probably be able to find the cause and fix the issue by yourself. Try it.
AgaveJoe 30,126 Reputation points

2025-05-17T11:37:22.0066667+00:00

It looks like there are a few things to clarify regarding your ASP.NET MVC code.

First, based on the standard MVC conventions, if your ProductController has GET and POST actions named "Register," then MVC will look for a view file named Register.cshtml within the Product folder. The fact that your question mentions a RegisterProduct.cshtml view suggests a potential mismatch between your code structure and your description. When the code and the description don't align, it makes it difficult for forum members to understand the actual problem you're facing.

Secondly, I noticed a line in your POST action: product.GenderId = genderId;. Directly assigning a property from one submitted model (genderId) to another (product) this isn't the typical way model binding works in MVC.

As Bruce correctly pointed out, a better approach is to create a dedicated view model. This view model acts as a specific data transfer object between your controller and your Register.cshtml view. It belongs to the UI layer and doesn't need to know anything about your underlying data access logic (like your repository pattern).

Currently, it seems your repository and domain models are being directly used in your view, which is generally considered a code smell. It's best practice to have a clear separation by mapping data between your view model and your domain models.

On a broader note, it's a common issue we see here where developers focus heavily on implementing a repository pattern as the single solution for all architectural concerns. While the repository pattern is valuable for data access, it's just one piece of a larger puzzle. Over-reliance on a single pattern can sometimes lead to unnecessary complexity and limitations down the road.
Bruce (SqlWork.com) 77,686 Reputation points Volunteer Moderator

2025-05-17T18:37:37.7433333+00:00

Also the repository has no conflict resolution other than last one wins.

1 answer

Your answer

SurferOnWww 4,631 Reputation points

2025-05-17T00:57:29.8+00:00

ArgumentNullException for 'items' when using SelectList with ViewBag in ASP.NET Core MVC

Where is the "SelectList" in your code?

It is hard to find the cause as your code shown in your question is messy. Please provide a simple sample code which can only reproduce your issue by eliminating unnecessary portion to cause the "ArgumentNullException."

During eliminating unnecessary portion, you will probably be able to find the cause and fix the issue by yourself. Try it.
AgaveJoe 30,126 Reputation points

2025-05-17T11:37:22.0066667+00:00

It looks like there are a few things to clarify regarding your ASP.NET MVC code.

First, based on the standard MVC conventions, if your ProductController has GET and POST actions named "Register," then MVC will look for a view file named Register.cshtml within the Product folder. The fact that your question mentions a RegisterProduct.cshtml view suggests a potential mismatch between your code structure and your description. When the code and the description don't align, it makes it difficult for forum members to understand the actual problem you're facing.

Secondly, I noticed a line in your POST action: product.GenderId = genderId;. Directly assigning a property from one submitted model (genderId) to another (product) this isn't the typical way model binding works in MVC.

As Bruce correctly pointed out, a better approach is to create a dedicated view model. This view model acts as a specific data transfer object between your controller and your Register.cshtml view. It belongs to the UI layer and doesn't need to know anything about your underlying data access logic (like your repository pattern).

Currently, it seems your repository and domain models are being directly used in your view, which is generally considered a code smell. It's best practice to have a clear separation by mapping data between your view model and your domain models.

On a broader note, it's a common issue we see here where developers focus heavily on implementing a repository pattern as the single solution for all architectural concerns. While the repository pattern is valuable for data access, it's just one piece of a larger puzzle. Over-reliance on a single pattern can sometimes lead to unnecessary complexity and limitations down the road.
Bruce (SqlWork.com) 77,686 Reputation points Volunteer Moderator

2025-05-17T18:37:37.7433333+00:00

Also the repository has no conflict resolution other than last one wins.

Answer 1

your question is not clear. there is no <select> or SelectItemList in the sample code.

the action:

public async Task<IActionResult> Register(
   List<IFormFile> pi, 
   ProductModel product, 
   int genderId, 
   List<int> ct, 
   List<int> c, 
   List<int> s,  
   List<StockProductModel> stck
) {

has a bunch of invalid binding (pi, ct, c, s and stck) that have no matching form item. genderId is expected from the route or query string instead of the form, but you defined a form item.

why didn't you just define a postback view model, so there was just one parameter, and binding be the same for render and postback?

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ArgumentNullException for 'items' when using SelectList with ViewBag in ASP.NET Core MVC

1 answer

Your answer