ASP.NET MVC3:控制器中DropdownList中的对象为Null

本文关键字：对象 Null DropdownList ASP MVC3 控制器 NET | 更新日期: 2023-09-27 18:24:58

我为BookShop创建了一个示例MVC3应用程序。我有一个创造动作。在创建动作中，用户输入书籍名称、作者名称并选择流派名称。它工作时没有出错。但在控制器中，我没有得到所选的流派名称（-流派对象为null）。我们如何纠正它？

注意：当我编写@Html.DropDownList("GenreId", String.Empty)时，ASP.NET MVC中内置的模型绑定功能将尝试使用表单输入填充该类型的对象。例如，它将尝试设置图书对象的GenreId值。但是Book不包含属性GenreId。但是，下拉列表可以通过从ViewBag中读取所需的值来显示值。

代码

@model MyBookShopApp.Book
@{
ViewBag.Title = "Create";
}
<h2>Create</h2>
@using (Html.BeginForm()) {
@Html.ValidationSummary(true)
<fieldset>
    <legend>BOOK</legend>

    <div >
        @Html.LabelFor(model => model.Title) :~: @Html.EditorFor(model => model.Title)
    </div>

    <div >
        @Html.LabelFor(model => model.Artist.Name )  :*: @Html.EditorFor(model => model.Artist.Name)
    </div>

    <div >
        @Html.LabelFor(model => model.GenreEntity.Name, "The Genre") ::   @Html.DropDownList("GenreId", String.Empty)
    </div>

    <p>
        <input type="submit" value="Create" />
    </p>
</fieldset>
}
<div>
 @Html.ActionLink("Back to List", "Index")
</div>

控制器

namespace MyBookShopApp.Controllers
{
public class StoreManagerController : Controller
{
    List<GenreEntity> listOfGenres = new List<GenreEntity>()
                                    {
                                       new GenreEntity { Name = "Fiction", GenreId = 1 },
                                       new GenreEntity { Name = "Science", GenreId = 2 },
                                       new GenreEntity { Name = "Religious", GenreId = 3 },
                                    };
    List<Book> bookList = new List<Book>()
                          {
                            new Book
                            {
                                BookId = 1,Title = "Book1",
                                GenreEntity = new GenreEntity { Name = "Fiction", GenreId = 1 },
                                Artist = new Artist { ArtistId = 1, Name = "Dinesh" }
                            },
                            new Book
                            {
                                BookId = 2,Title = "Book2",
                                GenreEntity = new GenreEntity { Name = "Science", GenreId = 2 },
                                Artist = new Artist { ArtistId = 1, Name = "Lijo" }
                            },
                            new Book
                            {
                                BookId = 3,Title = "Book3",
                                GenreEntity = new GenreEntity { Name = "Religious", GenreId = 3 },
                                Artist = new Artist { ArtistId = 1, Name = "Santhosh" }
                            }
                          };


    #region CREATE

    // GET: 
    public ActionResult Create()
    {
                    ViewBag.GenreId = new SelectList(listOfGenres, "GenreId", "Name");
        return View();
    }
    // POST:
    [HttpPost]
    public ActionResult Create(Book theBook)
    {
        if (ModelState.IsValid)
        {
            //Save the book in DB first and then redirectToAction.
            return RedirectToAction("Index");
        }
        return View(theBook);
    }
    #endregion

}
}

图书类

public class Book
{
    public int BookId { get; set; } 
    public string Title { get; set; }
    public GenreEntity GenreEntity { get; set; }
    public Artist Artist { get; set; }
}

GenreEntity

  public class GenreEntity
{
    public string Name { get; set; }
    public int GenreId { get; set; }
}

艺术家级

 public class Artist
{
    public int ArtistId { get; set; }
    public string Name { get; set; }
}

参考：

  1. mvc3编辑表单中的下拉列表

  2. 如何在包含可为null枚举属性的模型的强类型视图中使用Html.DropDownList？

  3. MVC3 HTML帮助程序没有'；t在提交表单时更新DropdownListFor

  4. 关于选择更改事件-Html.DropDownListFor

  5. MVC3@Html.DropDownListFor not populating selected item

ASP.NET MVC3:控制器中DropdownList中的对象为Null

@Html.DropDownList("GenreId", String.Empty)创建一个名为GenreId的SELECT。但是，MVC3需要名称GenreEntity.name绑定到Book。

简单的解决方案是将public ActionResult Create(Book theBook) 修改为public ActionResult Create(Book theBook, string genreId)，以接收张贴回的genreId

或者

@Html.LabelFor(model => model.GenreEntity.GenreId, "The Genre")
@Html.DropDownListFor(model => model.GenreEntity.GenreId, (SelectList)ViewBag.GenreId, String.Empty)

还记得在HttpPost Create方法中设置ViewBag。

编辑将属性名称从BookId更正为GenreId

您的DropDown正试图将其值分配给一个名为Book.GenreId的属性。GenreEntity类具有GenreId属性，而不是Book（基于上面粘贴的代码）。

您可以通过将下拉菜单修改为以下内容来更正此问题：

@Html.DropDownList("GenreEntity.GenreId", String.Empty)

这将告诉MVC将下拉列表的值分配给Book.GenreEntity.GenreId。

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