无法强制列表< int>被要求 [英] Can't force List<int> to be required

问题描述

我正在尝试使用数据注释来添加验证到我的模型中的列表，不能为空。我已经尝试了一些自定义属性的几个实现，包括此处和< a href =https://stackoverflow.com/questions/5146732/viewmodel-validation-for-a-list> here 。

I'm trying to use Data Annotations to add validation to a List in my model that cannot be empty. I've tried several implementations of a custom attribute, including ones here and here.

我的观点：

<div class="form-group">
    @* Model has a list of ints, LocationIDs *@
    @Html.LabelFor(model => model.LocationIDs, htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        <select class="select2 form-control" multiple id="LocationIDs" name="LocationIDs">
            @* Adds every possible option to select box *@
            @foreach (LocationModel loc in db.Locations)
            {
                <option value="@loc.ID">@loc.Name</option>
            }
        </select>
        @Html.ValidationMessageFor(model => model.LocationIDs, "", new { @class = "text-danger" })
    </div>
</div>

型号：

public class ClientModel
{
    public int ID { get; set; }
    [Required] // Does nothing
    public List<int> LocationIDs { get; set; }
}

控制器：

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create([Bind(Include = "ID,LocationIDs")] ClientModel clientModel)
{
    if (ModelState.IsValid)
    {
        db.Clients.Add(clientModel);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(clientModel);
}

我尝试过的（功能完全相同）属性之一：

One of the (functionally identical) attributes I've tried:

[AttributeUsage(AttributeTargets.Property)]
public sealed class CannotBeEmptyAttribute : RequiredAttribute
{
    public override bool IsValid(object value)
    {
        var list = value as IEnumerable;
        return list != null && list.GetEnumerator().MoveNext();
    }
}

目前，检查空或空列表通过验证，即使没有选择。在这种情况下，包含第一个选项的长度为1的列表将被绑定。

Currently, checking for a null or empty list passes validation, even if nothing is chosen. In this case, a list of length one, containing the first option, is bound.

我已经确认控制器实际上是发送一个长度为1的列表。但是，我不知道如何改变这种行为。我仍然认为这可能是下面的块引用所描述的。

I've confirmed that the controller is actually sent a List of length one. I'm not sure how to change this behavior, however. I still think this may be what was described in the below block quote.

我 / em> 我的问题可能会在此答案的编辑中描述，但我不确定如何解决。

I think my problem may be described in this answer's edit, but I'm not sure how to solve it.

以下摘录：

您还必须小心，在您的视图中绑定列表。
例如，如果您将列表绑定到如下视图：

You'll also have to be careful how you bind your list in your view. For example, if you bind a List to a view like this:

<input name="ListName[0]" type="text" />
<input name="ListName[1]" type="text" />
<input name="ListName[2]" type="text" />
<input name="ListName[3]" type="text" />
<input name="ListName[4]" type="text" /> 

MVC模型binder将始终将5个元素放在
列表，全部为String.Empty。如果这是您的View的工作原理，您的属性
将需要更复杂一些，例如使用Reflection来拉
通用类型参数，并将每个列表元素与
default（T ）或某事。

The MVC model binder will always put 5 elements in your list, all String.Empty. If this is how your View works, your attribute would need to get a bit more complex, such as using Reflection to pull the generic type parameter and comparing each list element with default(T) or something.

推荐答案

我可以获得零选择的唯一方法来实际绑定因为 null 是通过使用 Html.ListBoxFor （我最初无法弄清楚–它应该已经完成​​了这样从一开始）：

The only way I could get a zero-selection to actually bind as null was by using Html.ListBoxFor (which I couldn't figure out initially – it should have been done this way from the start):

@Html.ListBoxFor(model => model.SelectedLocations, Model.AllLocations, new { @class = "select2 form-control" })

我没有得到客户端验证工作，但我会把这个作为另一个问题。

I haven't gotten client-side validation to work, but I'll post that as another question.

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