MVC ViewModel Error - No parameterless constructor

问题:

I would like to know how can I use my ViewModel on the Create Action? I tried several examples I found here in the forum, but none solved my problem. I've been racking my brain for a few days, but can't figure out what is wrong.

Whenever I click the Create button I get the following error: No parameterless constructor defined for this object.

@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 - Snippet

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; }
    }

回答1:

If I had a nickel for every time I've seen this problem. It's typically related to the naming of your model properties and how you use them in a DropDownList. 99.999% of the time it's because people are using Html.DropDownList() and naming it the same as their SelectList. This is one reason you should use the strongly typed DropDownListFor.

In this case, your problem is that you have SelectLists named Genres and Artists, then in your view you have:

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

See, same name.

What you should do is change your Model to make the SelectLists be named GenreList and ArtistList. Then, change your view to use strongly typed model.

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

The reason this happens is that you are posting a value called Genres to the controller. The default model binder dutifully looks in the model to find something called Genres and instantiate it. But, rather than an ID or string, it finds a SelectList named Genres, and when it tries to instantiate it, it finds there is no default constructor.

Thus your error. SO is filled with questions asking about this same thing.

回答2:

Similar to Erik Funkenbusch's answer I'd added a DropDownList to my form, however in my case it wasn't (and wasn't intended to be) submitted with the form as it was outside of the <form></form> tags:

@Html.DropDownList("myField", Model.MyField)

As the Model contained the field for display only, this also caused the No parameterless constructor defined for this object error because the field wasn't submitted at all.

In this case I fixed it by adding an exclude binding:

public ActionResult Foo(int id, int? page, [Bind(Exclude = "MyField")]MyModel model)

回答3:

For me the problem was in the BeginForm() method itself. It looked like this:

@using (Html.BeginForm("MyAccount", "MyController", Model))

Copied and pasted in from another project's Login page, which doesn't have a dropdown.

Anyway, remove the Model from the parameter list and it all works just fine :)