为什么我的对象列表呈现为复选框返回一个空列表？ [ASP.NET 核心 - 3.1]

Question

I'm trying to render a List<PlayerCheckbox> as Checkboxes.

PlayerCheckbox looks like this: (added empty constructor as per @jcwmoore 's answer, sadly nothing changed with this addition alone)

 public class PlayerCheckbox
    {
        public PlayerCheckbox(string discordName, ulong discordId, bool selected = false)
        {
            DiscordName = discordName;
            DiscordId = discordId;
            Selected = selected;
        }

        public PlayerCheckbox()
        {

        }

        public string DiscordName { get; set; }

        public ulong DiscordId { get; set; }

        public bool Selected { get; set; }
    }

The CreateBingoTaskModel looks like this: (ViewModel I am working with on the Create page, added a default constructor here aswell per @jcwmoore 's input, just in case)

 public class CreateBingoTaskModel
    {
        public CreateBingoTaskModel()
        {

        }

        /// <summary>
        /// Discord Id of the creator of this BingoTask
        /// </summary>
        public BingoPlayer Creator { get; set; }

        [Range(0, 100)]
        public int Difficulty { get; set; }

        /// <summary>
        /// List of IDs that are not allowed to receive this bingo task
        /// </summary>
        public List<BingoPlayer> BlackListIds { get; set; } = new List<BingoPlayer>();

        /// <summary>
        /// Title of this bingo task
        /// </summary>
        [Required]
        public string BingoTitle { get; set; }

        /// <summary>
        /// Description of this bingo task
        /// </summary>
        [Required]
        public string BingoDescription { get; set; }

        public DateTime RaffledOn { get; set; }

        public List<PlayerCheckbox> BlackList;
    }

The setup GET-Create looks like this:

public async Task<IActionResult> Create()
        {
            CreateBingoTaskModel createBingoModel = new CreateBingoTaskModel();
            createBingoModel.BlackList = new List<PlayerCheckbox>();

            var user = await appDb.BingoPlayers.SingleOrDefaultAsync(player =>
                player.LoginName == HttpContext.Session.GetString("user"));

            foreach (BingoPlayer bp in appDb.BingoPlayers)
            {
                PlayerCheckbox playerCheckbox =
                    new PlayerCheckbox(bp.Name, bp.DiscordId);
                if (playerCheckbox.DiscordId != user.DiscordId)
                {
                    createBingoModel.BlackList.Add(playerCheckbox);
                }
            }

            return View(createBingoModel);
        }

The POST-Create looks like this:

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create(CreateBingoTaskModel createBingoTaskModel)
        {
            if (ModelState.IsValid)
            {
                var user = await appDb.BingoPlayers.SingleOrDefaultAsync(player =>
                    player.LoginName == HttpContext.Session.GetString("user"));

                foreach (PlayerCheckbox checkbox in createBingoTaskModel.BlackList)
                {
                    if (checkbox.Selected)
                    {
                        var player = appDb.BingoPlayers.Single(bingoPlayer => bingoPlayer.DiscordId == checkbox.DiscordId);
                        createBingoTaskModel.BlackListIds.Add(player);
                    }
                }

                createBingoTaskModel.BlackListIds.Add(user);

                BingoTask newBingoTask = new BingoTask
                {
                    Difficulty = createBingoTaskModel.Difficulty,
                    BingoTitle = createBingoTaskModel.BingoTitle,
                    BingoDescription = createBingoTaskModel.BingoDescription,
                    BlackListIds = createBingoTaskModel.BlackListIds
                };

                appDb.Add(newBingoTask);
                await appDb.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(createBingoTaskModel);
        }

Looking at the Model in the POST-Create Header: shows that the BlackList is now null .

I Do not understand why this is null, I was expecting all of the players except for the current logged in user, even if the logic was incorrect, .BlackList should at least be initialized.

The Checkbox List Code I am using looks like this:

@using App.API.Models
@model App.API.Models.CreateBingoTaskModel

@{
    ViewData["Title"] = "Create";
}

<h1>Create</h1>

<h4>BingoTask</h4>
<hr />
<div class="row">
    <div class="col-md-4">
        <form asp-action="Create">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="Difficulty" class="control-label"></label>
                <input asp-for="Difficulty" class="form-control" />
                <span asp-validation-for="Difficulty" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="BingoTitle" class="control-label"></label>
                <input asp-for="BingoTitle" class="form-control" />
                <span asp-validation-for="BingoTitle" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="BingoDescription" class="control-label"></label>
                <input asp-for="BingoDescription" class="form-control" />
                <span asp-validation-for="BingoDescription" class="text-danger"></span>
            </div>

            <div class="form-group">
                <span>BlackList</span>

               @for (int i = 0; i < Model.BlackList.Count; i++)
                {
                    <input hidden asp-for="BlackList[i].DiscordId" />
                    <input hidden asp-for="BlackList[i].DiscordName" />
                    <input asp-for="BlackList[i].Selected" type="checkbox" />
                    @Model.BlackList[i].DiscordName
                    <br />
                }
            </div>

            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-primary" />
            </div>
        </form>
    </div>
</div>

<div>
    <a asp-action="Index">Back to List</a>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

and is taken from this answer here: https://forums.asp.net/t/2164304.aspx?Asp+net+MVC+Core+giving+null+checkbox+data+instead+of+all+selected+checkboxes+on+HTTPPost

Many implementations of CheckboxLists have extremely similar frontend code with the only major change being a replacement of for with foreach(playerCheckbox in BlackList) which I have tried too.

My FormData looks... fine? (Except the extra values at the bottom)

I do NOT understand at all why the binding doesn't work here. Am I so tired that I am overlooking somethingy extremely simple?

If more information / code is needed, I will gladly provide more context where needed.

Answer 1

To my knowledge, the model binder requires a default (parameterless) constructor in the model class. The model class is fine, but the PlayerCheckbox does not have a parameterless constructor. Try either removing your constructor, or adding a parameterless one, to the PlayerCheckbox class.

public class PlayerCheckbox
{
    // default parameterless constructor to support deserialization
    public PlayerCheckbox()
    {

    }

    public PlayerCheckbox(string discordName, ulong discordId, bool selected = false)
    {
        DiscordName = discordName;
        DiscordId = discordId;
        Selected = selected;
    }

    public string DiscordName { get; set; }

    public ulong DiscordId { get; set; }

    public bool Selected { get; set; }
}