使用 Select TagHelper 设置默认值/空值

Question

In asp.net mvc you can use:

@Html.DropDownListFor(model => model.Category, ViewBag.Category as IEnumerable<SelectListItem>, "-- SELECT --", new { @class = "form-control" })

Using asp.net 5, how do I include the default or null value (-- SELECT --) in a taghelper:

<select asp-for="Category" asp-items="@ViewBag.Category"  class="form-control"></select>

Answer 1

You can just insert an option item inside the select:

<select asp-for="Category" asp-items="@ViewBag.Category"  class="form-control">
    <option disabled selected>--- SELECT ---</option>
</select>

The disabled keyword in the code above means that the "--- SELECT ---" row cannot be picked again once a choice has been selected in the list. If you want the user to be able to blank the selection again (ie if it's bound to a nullable field) then just omit disabled.

Answer 2

如果要将值 null 存储到数据库，请使用<option selected value="">Full Access</option>

Answer 3

If you use asp-items for your model , It uses value 0 for selected item , If you have tag <option> with your default string without value ,

You must use a loop for <option> and put your default option out of loop. This is an example for .Net Core using asp-items:

<select asp-for="DepartmentUnit" asp-items="@Model.DepartmentUnits" class="form-control"></select>

and Now this is another example for using loop:

<select>
<option val="">--Select--</option>
foreach(item in Model.DepartmentUnits)
{<option val="@item.val">@item.title</option>}
</select>

Answer 4

This answer by Matt:

    <select asp-for="Category" asp-items="@ViewBag.Category"  class="form-control">
        <option disabled selected>--- SELECT ---</option>
    </select>

also works with validation. Probably due to the option being disabled it seems not to be a valid selection.

btw.: this also works with MVC Core 3 ;-)