MVC ViewModel Error - 没有为此对象定义的无参数构造函数

Question

我想知道如何在Create Action上使用我的ViewModel？我尝试了几个我在论坛中找到的例子,但都没有解决我的问题.我已经绞尽脑汁待了几天,但无法弄清楚出了什么问题.

每当我单击"创建"按钮时,我都会收到以下错误:没有为此对象定义无参数构造函数.

@model MvcMusicStore.ViewModels.AlbumViewModel

@{
    ViewBag.Title = "Create";
}

<h2>Create</h2>

<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>

@using (Html.BeginForm()) {
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>Album</legend>

        <div class="editor-label">
            @Html.LabelFor(model => model.AlbumItem.GenreId, "Genre")
        </div>
        <div class="editor-field">
            @Html.DropDownList("Genres", String.Empty)
            @Html.ValidationMessageFor(model => model.AlbumItem.GenreId)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.AlbumItem.ArtistId, "Artist")
        </div>
        <div class="editor-field">
            @Html.DropDownList("Artists", String.Empty)
            @Html.ValidationMessageFor(model => model.AlbumItem.ArtistId)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.AlbumItem.Title)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.AlbumItem.Title)
            @Html.ValidationMessageFor(model => model.AlbumItem.Title)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.AlbumItem.Price)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.AlbumItem.Price)
            @Html.ValidationMessageFor(model => model.AlbumItem.Price)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.AlbumItem.AlbumArtUrl)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.AlbumItem.AlbumArtUrl)
            @Html.ValidationMessageFor(model => model.AlbumItem.AlbumArtUrl)
        </div>

        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>
}

<div>
    @Html.ActionLink("Back to List", "Index")
</div>

Create.cshtml

    public class StoreManagerController : Controller
        {
            private MusicStoreDB db = new MusicStoreDB();

            //
            // GET: /StoreManager/Create

            public ActionResult Create()
            {
                var viewModel = new AlbumViewModel()
                {
                    Genres = new SelectList(db.Genres, "GenreId", "Name"),
                    Artists = new SelectList(db.Artists, "ArtistId", "Name")
                };
                return View(viewModel);
            } 

            //
            // POST: /StoreManager/Create

            [HttpPost]
            public ActionResult Create(AlbumViewModel vm)
            {
                if (ModelState.IsValid)
                {
                    db.Albums.Add(vm.AlbumItem);
                    db.SaveChanges();
                    return RedirectToAction("Index");  
                }

                vm.Genres = new SelectList(db.Genres, "GenreId", "Name", vm.AlbumItem.GenreId);
                vm.Artists = new SelectList(db.Artists, "ArtistId", "Name", vm.AlbumItem.ArtistId);
                return View(vm);
            }
}

StoreManager.cs - 片段

public class AlbumViewModel
    {
        public AlbumViewModel()
        {
            //  nothing
        }

        public Album AlbumItem { get; set; }
        public SelectList Genres { get; set; }
        public SelectList Artists { get; set; }
    }

public class Album
    {
        public Album()
        {
            //  nothing
        }

        public virtual int AlbumId { get; set; }
        public virtual int GenreId { get; set; }
        public virtual int ArtistId { get; set; }
        public virtual string Title { get; set; }
        public virtual decimal Price { get; set; }
        public virtual string AlbumArtUrl { get; set; }
        public virtual Genre Genre { get; set; }
        public virtual Artist Artist { get; set; }
    }

public class Artist
    {
        public Artist()
        {
            // nothing
        }

        public virtual int ArtistId { get; set; }
        public virtual string Name { get; set; }
    }

public class Genre
    {
        public Genre()
        {
            // nothing
        }

        public virtual int GenreId { get; set; }
        public virtual string Name { get; set; }
        public virtual string Description { get; set; }
        public virtual List<Album> Albums { get; set; }
    }

Answer 1

如果我每次看到这个问题都有镍.它通常与模型属性的命名以及如何在a中使用它们有关DropDownList.99.999%的时间是因为人们正在使用Html.DropDownList()它并将其命名为与它们相同SelectList.这是你应该使用强类型的一个原因DropDownListFor.

在这种情况下,你的问题是,你必须SelectList的named Genres和Artists,然后在您的视图您有:

@Html.DropDownList("Genres", String.Empty)
@Html.DropDownList("Artists", String.Empty)

看,同名.

你应该做的是改变你的模型,使其SelectList命名GenreList和ArtistList.然后,更改视图以使用强类型模型.

@Html.DropDownListFor(m => m.AlbumItem.GenreID, Model.GenreList)
@Html.DropDownListFor(m => m.AlbumItem.ArtistID, Model.ArtistList)

发生这种情况的原因是您将一个名为Genres的值发布到控制器.默认模型绑定器在模型中尽职尽责地查找名为Genres的内容并实例化它.但是,它找到一个名为Genres的SelectList,而不是ID或字符串,当它尝试实例化它时,它发现没有默认构造函数.

因此你的错误.SO充满了询问同样事情的问题.