<select> tag helper not marking options as "selected" in multiple select

Question

Take the following model:

public class SomeModel
{
    public ICollection<Tag> Tags { get; set; }
}

public class Tag
{
    public int Id { get; set; }
    public string Value { get; set; }
}

The model is some kind of object (e.g., a blog entry), that features a list of tags or keywords.

I have a Razor view showing a form to edit a SomeModel object:

@model SomeModel
@{
    // List of available tags is passed by controller
    var tags = (List<Tag>)ViewData["Tags"];
}
<label asp-for="Tags">Tags</label>
<select asp-for="Tags"
        asp-items="@(new SelectList(tags, nameof(Tag.Id), nameof(Tag.Value)))">
</select>

As expected, this generates a multiple select showing the available tags:

<label for="Tags">Tags</label>
<select class="input-validation-error"
        data-val="true" data-val-required="The Tags field is required."
        id="Tags" multiple="multiple" name="Tags">
    <option value="1">tag1</option>
    <option value="2">tag2</option>
</select>

@Model.Tags correctly contains the currently assigned tags; however, the corresponding options are not marked as selected.

How do I fix this behavior?

Answer 1

You need to create a property to pass the selected datas and bind it to the asp-for attribute of select tag.

Index.cshtml.cs :

public class IndexModel : PageModel
{
    [BindProperty]
    public List<int> SelectedTags { get; set; } =  new List<int>{  2, 3 };
    [BindProperty]
    public List<Tag> Tags { get; set; }
    public void OnGet()
    {
        Tags = new List<Tag> {
    new Tag { Id = 1, Value="Mike" },
    new Tag { Id = 2, Value="Pete" },
    new Tag { Id = 3, Value="Katy" },
    new Tag { Id = 4, Value="Carl" } };

    }
}

Index.cshtml:

@page
@model WebApplication1_rzaor_page.IndexModel
@{
    ViewData["Title"] = "SelectTag";
    Layout = "~/Pages/Shared/_Layout.cshtml";
}

<h1>SelectTag</h1>

<label asp-for="Tags">Tags</label>
<select asp-for="SelectedTags"
        asp-items="@(new SelectList(Model.Tags, "Id","Value"))">
</select>

Result of this demo

You can refer to this link ：Setting Selected Item