InvalidOperationException：沒有具有鍵“marka”的“IEnumerable<SelectListItem>”型別的ViewData項

在 MVC 中系結下拉串列時，我收到此錯誤：

InvalidOperationException：沒有具有鍵“marka”的“IEnumerable”型別的 ViewData 項。

在另一個專案中，這種語法完美地作業

SO上有幾篇關于此的帖子，但沒有一個答案似乎可以解決我目前情況下的問題。

型號：Product.cs：

 public class Product
{
    [Key]

    public int Id { get; set; }

    [Required(ErrorMessage = "Prosz? podac nazw? produktu")]
    [Display(Name = "Nazwa produktu")]
    [MaxLength(50, ErrorMessage = "Max 50 char")]

    public string Name { get; set; }

    [Display(Name = "Marka produktu")]
    [Required]
    [MaxLength(5, ErrorMessage = "Max 5 char")]

    public string Brand { get; set; }

    [Display(Name = "Cena produktu")]
    [Required]


    public int ProductPrice { get; set; }

    [Display(Name = "Dzia?")]
    [Required]
    [MaxLength(1, ErrorMessage = "Max 1 char")]

    public string Section { get; set; }

    [Display(Name = "Materia?")]
    [Required]
    [MaxLength(5, ErrorMessage = "Max 5 char")]

    public string Material { get; set; }

    [Display(Name = "Typ")]
    [Required]
    [MaxLength(5, ErrorMessage = "Max 5 char")]

    public string Type { get; set; }
}

品牌.cs：

public class Brand
{
    [Key]
    [Display(Name = "Identyfikator")]
    [MaxLength(5, ErrorMessage = "Max 5 chars")]
    public string Id { get; set; }

    [Display(Name = "Nazwa")]
    [MaxLength(50, ErrorMessage = "Max 50 chars")]
    public string Name { get; set; }
}

看法：

<div class="form-group">
   <label asp-for="Brand" class="control-label"></label>
   <div class="col-md-5">
      @Html.DropDownList("marka", "Select brand")
   </div>
   <span asp-validation-for="Brand" class="text-danger"></span>
</div>

控制器：

// GET: Products/Edit/5
    public async Task<IActionResult> Edit(int? id)
    {
        if (id == null)
        {
            return NotFound();
        }

        var product = await _context.Product.FindAsync(id);
        if (product == null)
        {
            return NotFound();
        }

        IEnumerable<SelectListItem> items = _context.Brand.Select(c => new SelectListItem
        {
            Value = c.Id,
            Text = c.Name,
            Selected = c.Id == product.Brand
        }); ;

        if (items != null)
        {
            ViewBag.marka = items;
        }

        return View(product);
    }
 // POST: Products/Edit/5
    // To protect from overposting attacks, enable the specific properties you want to bind to.
    // For more details, see http://go.microsoft.com/fwlink/?LinkId=317598.
    [HttpPost]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Edit(int id, [Bind("Id,Name,Brand,ProductPrice,Section,Material,Type")] Product product)
    {
        if (id != product.Id)
        {
            return NotFound();
        }

        if (ModelState.IsValid)
        {
            
            try
            {
                
                string selBra = Request.Form["marka"].ToString();
                product.Brand = selBra;
                _context.Update(product);
                await _context.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!ProductExists(product.Id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }
            return RedirectToAction(nameof(Index));
        }
        return View(product);
    }

uj5u.com熱心網友回復：

修復視圖我建議你使用選擇而不是下拉串列，它會自動選擇一個專案

 <select class="form-control"  asp-for="brand" asp-items="@ViewBag.marka"></select>

并通過洗掉 Selected 并添加到 List() 來修復 get 操作

var items = _context.Brand.Select(c => new SelectListItem
        {
            Value = c.Id,
            Text = c.Name
          }).ToList(); 

也修復 post 操作，洗掉 Bind，

 public async Task<IActionResult> Edit(int id, Product product)

并從代碼中洗掉

string selBra = Request.Form["marka"].ToString();
product.Brand = selBra;

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