SelectList中的选定项未显示在视图下拉列表中

问题描述

我有一个asp.net MVC应用程序，其中有一个用于创建新事件的Razor视图.表单上是一个季节下拉列表，供用户选择.我希望在本列表中默认选择当前季节.

I have an asp.net MVC application in which I have a Razor view for creating a new event. On the form is a drop-down list of seasons for the user to select from. I would like the current season to be pre-selected in this list as a default.

我已经设置了两个季节:

I have set up two seasons:

第1季适用于2020-2021年，其IsCurrent属性设置为False；第2季适用于2021-2022，并且其IsCurrent属性设置为True

Season 1 is for 2020-2021 and has its IsCurrent property set to False;Season 2 is for 2021-2022 and has its IsCurrent property set to True

控制器代码...

ViewData["Seasons"] = new SelectList(_context.Seasons, "ID", "SeasonName",_context.Seasons.Where(s => s.IsCurrent==true));

查看...

<select asp-for="SeasonID" class="form-control" asp-items="ViewBag.Seasons"></select>

运行此命令时，我希望可以在下拉菜单中预选2021-2022赛季，但它仅显示2020-2021年(在列表中排在第一位).

When I run this, I am expecting to see the season 2021-2022 pre-selected in the drop-down, but it just shows 2020-2021 (being the first in the list).

这似乎应该很简单-有人可以告诉我我要去哪里了吗?

This seems like it should be so simple - can someone please tell me where I'm going wrong?

推荐答案

这个问题过去曾咬过我几次，一直追溯到MVC 3左右.

This one has bitten me a few times in the past, all the way back to MVC 3 or so.

基本上，如果要将模型传递给视图(即使用 @model MyTypeOfViewModel 指令)并将 select 绑定到该模型的属性，则选择列表的选定值是要在视图模型上绑定到的属性的值.直接在选择列表中设置选择的项目将被忽略.

Basically, if you're passing a model to your view (i.e. using a @model MyTypeOfViewModel directive) and binding a select to a property on that model, then the selected value of the select list is the value of the property you are binding to on your view model. Setting the selected item in a select list directly will be ignored.

在您的情况下，它是 SeasonID 的来源:

In your case, it's SeasonID from this:

<select asp-for="SeasonID" ...>

因此，要选择 Season 2 ，您需要将视图模型上的 SeasonID 属性的值设置为 ID 第2季中的>:

So for you to select Season 2, you'd need to set the value of the SeasonID property on your view model to the ID of Season 2:

var viewModel = new SeasonViewModel
{
    SeasonID = // Season 2's SeasonID
}

但是，如果您不直接绑定 select 的值，则像这样创建 select :

However, if you're not binding the value of the select directly, creating the select like this:

// Notice I've not specified `asp-for`.
<select class="form-control" asp-items="ViewBag.Seasons"></select>

将使用创建 SelectList 时设置的选定值.就是说，您目前执行此操作的代码略有错误.应该是:

will use the selected value you set when creating the SelectList. That said, your code to do that is currently slightly wrong. It should be:

var selected = _context.Seasons
    .Where(s => s.IsCurrent == true)
    .FirstOrDefault();
ViewData["Seasons"] = new SelectList(
                _context.Seasons,
                "ID",
                "SeasonName",
                // It needs to be the actual value of the field, not the season's object.
                selected?.ID.ToString());

---

简而言之，如果您想绑定到您的 SeasonID 属性，请创建您的 SelectList ，而不选择一个值，如下所示:

---

In short, if you want to bind to your SeasonID property, create your SelectList, without selecting a value, like so:

ViewData["Seasons"] = new SelectList(
            _context.Seasons,
            "ID",
            "SeasonName");

然后在视图模型本身上设置选定的值，其余的 asp-for 属性将完成其余工作.

Then set the selected value on the view model itself, and the asp-for attribute on the select will do the rest.

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