ASP 核心：@Html.ValidationMessage 迁移到 asp-validation-for 或类似的标签助手

Question

如何使用字符串值而不是表达式设置 asp-validation-for？

我要迁移多选列表：

@Html.ListBox("Privileges", ViewBag.PrivilegesMultiSelectList as MultiSelectList)
@Html.ValidationMessage("Privileges", "")

到

<select multiple="multiple" name="Privileges" asp-items="@ViewBag.PrivilegesMultiSelectList"></select>
<span asp-validation-for="Privileges" class="text-danger"></span>

但是最后一行无效：

Error CS1061 '...Model' does not contain a definition for 'Privileges' and no accessible extension method 'Privileges' accepting a first argument of type '..Model' could be found (are you missing a using directive or an assembly reference?)

为了保持一致性，我想继续使用标签助手。

Answer 1

此 asp-validation-for="Privileges" 试图在您的模型（不是 ViewBag）中寻找 Privileges 属性。如果它不存在，它会给你那个错误。该行相当于 ValidationMessageFor()，据我所知，ValidationMessage().

在 .net 核心中没有等价物

看看 asp-validation-for tag helper，如前所述，它应该与另一个 taghelper 的名称一致。

How to setup the asp-validation-for with string value, not with expression?

同样，在 TagHelpers 中没有 ValidationMessage() 它的等价物。所以你可以只使用 @Html.ValidationMessage().

https://docs.microsoft.com/en-us/aspnet/core/mvc/views/tag-helpers/intro?view=aspnetcore-2.2

it's important to recognize that Tag Helpers don't replace HTML Helpers and there's not a Tag Helper for each HTML Helper.

您也可以使用 ValidationMessage HtmlHelper

编写自己的标签助手

文档中关于 ViewBags 的一些建议：

We don't recommend using ViewBag or ViewData with the Select Tag Helper. A view model is more robust at providing MVC metadata and generally less problematic.

更好的方法：

您需要将 selected 权限添加到您希望 return 的模型中。

public class CustomViewModel {
    [Required]
    public string Privilege { get; set; } // update if you want to return multiple privileges

    public List<SelectListItem> PrivilegesMultiSelectList { get; set; }
}

然后在您的视图中使用它 @model CustomViewModel。

在你的 select 上使用 asp-for="Privilege"，它变成 m => m.Privilege。

https://docs.microsoft.com/en-us/aspnet/core/mvc/views/working-with-forms?view=aspnetcore-2.2

The asp-for attribute value is a special case and doesn't require a Model prefix, the other Tag Helper attributes do (such as asp-items)

然后你可以这样写：

<select asp-for="Privilege" asp-items="@Model.PrivilegesMultiSelectList"></select> 
<span asp-validation-for="Privilege" class="text-danger"></span>

希望对您有所帮助。