為什么在調用視圖時，視圖中的嵌套對象為null。 ASP.Net MVC

Question

我是MVC的新手，我正在努力解決這個問題：

當我想使用Edit方法（POST）進行保存時，我進行了一些驗證，當發生錯誤時，我再次調用視圖並發送相同的對象。 但是在視圖中，所有內部對象都為null。

這是一個例子：

public class Product 
{
  public int Id { get; set; }
  public string Name { get; set; }
  public DateTime ExpirationDate { get; set; }
  public int CategoryId { get; set; }
  public virtual Category Category { get; set; }
}

public class Category 
{
  public int CategoryId { get; set; }
  public string Name { get; set; }
}

我的控制器是這樣的：

[HttpPost]
public ActionResult Edit(Product product, string submitButton)
{
  if(submitButton == "Save")
  {
    if (ModelState.IsValid)
    {
        db.Entry(product).State = EntityState.Modified;
        db.SaveChanges();
        return RedirectToAction("Index");
    }
  }
  else // Validate
  {
    if(product.Expiration <= DateTime.Now)
    {
      ViewBag.Message "Product is expired";
      return View(product); // In the view the object property 'Category' is null. Why?
    }
  }
}

更新。 這是觀點。

@model sgt.Models.Product

@{
    ViewBag.Title = "Edit Product";
}

<h2>Edit</h2>

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>Product</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        @Html.HiddenFor(model => model.Id)

        <div class="form-group">
            @Html.LabelFor(model => model.Name, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Name, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Name, "", new { @class = "text-danger" })
                <input type="submit" value="Validate" class="btn btn-default" />
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.ExpirationDate, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.ExpirationDate, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.ExpirationDate, "", new { @class = "text-danger" })
                <input type="submit" value="Validate" class="btn btn-default" />
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.Category, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.DisplayFor(model => model.Category.Name, new { htmlAttributes = new { @class = "form-control" } })
            </div>
        </div>

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Save" class="btn btn-default" />
            </div>
        </div>
    </div>
}

<div>
    @Html.ActionLink("Return", "Index")
</div>

@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
}

提前致謝

Answer 1

你的代碼中有一個邏輯錯誤。

如果(ModelState.IsValid)失敗，則不返回View

[HttpPost]
public ActionResult Edit(Product product, string submitButton)
{
    if(submitButton == "Save")
    {
        if (ModelState.IsValid)
        {
            db.Entry(product).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        // Here RETURN VIEW and MODEL

        return View(product);
    }
    else // Validate
    {
        if(product.Expiration <= DateTime.Now)
        {
        ViewBag.Message "Product is expired";
        return View(product); // In the view the object property 'Category' is null. Why?
        }

        // ALSO HERE YOU ARE NOT RETURNING A VIEW AND MODEL

        return View(product);
    }
}

只是想知道，但我想你想檢查Model is Valid product.Expiration <= DateTime.Now

[HttpPost]
public ActionResult Edit(Product product, string submitButton)
{
      if(product.Expiration <= DateTime.Now && submitButton != "Save"){
         ModelState.AddModelError("Expiration", "Product is expired");
       }


       if (ModelState.IsValid)
       {
           db.Entry(product).State = EntityState.Modified;
           db.SaveChanges();
           return RedirectToAction("Index");
        }
        return View(product);
}

編輯：嘗試這個：

   [HttpPost]
   public ActionResult Edit(Product product, string submitButton)
   {
    if(submitButton == "Save")
    {
            if (ModelState.IsValid)
            {
                db.Entry(product).State = EntityState.Modified;
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            return View(product);
    }
       if(product.Expiration <= DateTime.Now)
       {
           ViewBag.Message "Product is expired";
       }
       return View(product);
   }

編輯你的觀點：

<div class="form-group">
    @Html.HiddenFor(m => m.Category.CategoryId)
    @Html.HiddenFor(m => m.Category.Name )

    @Html.LabelFor(model => model.Category, htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DisplayFor(model => model.Category.Name, new { htmlAttributes = new { @class = "form-control" } })
    </div>
</div>