Why is ListBoxFor not selecting items, but ListBox is?

user_247241 0
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I have the following code in my view:

<%= Html.ListBoxFor(c => c.Project.Categories,
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

<%= Html.ListBox("MultiSelectList", 
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

The only difference is that the first helper is strongly typed (ListBoxFor), and it fails to show the selected items (1,2), even though the items appear in the list, etc. The simpler ListBox is working as expected.

I'm obviously missing something here. I can use the second approach, but this is really bugging me and I'd like to figure it out.

For reference, my model is:

public class ProjectEditModel
{
    public Project Project { get; set; }
    public IEnumerable<Project> Projects { get; set; }
    public IEnumerable<Client> Clients { get; set; }
    public IEnumerable<Category> Categories { get; set; }
    public IEnumerable<Tag> Tags { get; set; }
    public ProjectSlide SelectedSlide { get; set; }
}

Update

I just changed the ListBox name to Project.Categories (matching my model) and it now FAILS to select the item.

<%= Html.ListBox("Project.Categories",
        new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

I'm obviously not understanding the magic that is happening here.

Update 2

Ok, this is purely naming, for example, this works...

<%= Html.ListBox("Project_Tags",
new MultiSelectList(Model.Tags, "Id", "Name", Model.Project.Tags.Select(t => t.Id)))%>

...because the field name is Project_Tags, not Project.Tags, in fact, anything other than Tags or Project.Tags will work. I don't get why this would cause a problem (other than that it matches the entity name), and I'm not good enough at this to be able to dig in and find out.

asp.net-mvc listbox
7 Answers

Although this isn't an answer to your main question, it is worth noting that when MVC generates names it will turn something like Project.Tags into Project_Tags, replacing periods with underscores.

The reason that it does this is because a period in an element ID would look like an element named Project with a class of Tags to CSS. Clearly a bad thing, hence the translation to underscores to keep behaviour predictable.

In your first example, 

<%= Html.ListBoxFor(c => c.Project.Categories,
    new MultiSelectList(Model.Categories, "Id", "Name", new List<int> { 1, 2 }))%>

the listbox is attempting to bind to Model.Project.Categories for your strongly typed Model which has been provided to the page (using the lambda notation). I'm not sure what the second parameter in the ListBoxFor is doing though.

What is the Model that is being passed to the page?

user_1771760
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