Preselect Items in Multiselect-Listbox (MVC3 Razor

Question

I have a problem with the preselection of Items in a listbox. I am using razor view engine with mvc 3. I know there are a few posts with the same issue but they don't work for me.

Code in Class:

public class Foo{
    private int _id;
    private string _name;

    public string Name{
       get{
           return _name;
       }

    public int Id {
       get{
           return _id;
       }

}

Code in Model:

public class FooModel{

    private readonly IList<Foo> _selectedFoos;
    private readonly IList<Foo> _allFoos;

    public IList<Foo> SelectedFoos{
         get{ return _selectedFoos;}
    }

    public IList<Foo> AllFoos{
         get{ return _allFoos;}
    }

}

Code in cshtml:

 @Html.ListBoxFor(model => model.Flatschels, 
        Model.AllFlatschels.Select(fl => new SelectListItem {
             Text = fl.Name,
             Value = fl.Id.ToString(),
             Selected = Model.Flatschels.Any(y => y.Id == fl.Id)
   }), new {Multiple = "multiple"}) 

I tried lots of other things but nothing worked. Hope someone can help.

Answer 1

I can't really explain why, but I managed to get it working. Either of these worked:

@Html.ListBoxFor(m => m.SelectedFoos,
            new MultiSelectList(Model.AllFoos, "ID", "Name"), new {Multiple = "multiple"}) 

@Html.ListBoxFor(m => m.SelectedFoos, Model.AllFoos
            .Select(f => new SelectListItem { Text = f.Name, Value = f.ID }),
                new {Multiple = "multiple"}) 

The problem seems to be that the Selected property on SelectListItem is ignored, and instead the ToString()(!) method is being called, so if you need to add this to your Foo class:

public override string ToString()
{
    return this.ID;
}

I'm guessing it has something to do with being able to persist across requests (which will be flattened to strings to be passed over the wire), but it's a bit confusing!

Answer 2

In MVC5 you can directly use ListBoxFor with multiselect. Make sure while loading the view your selectedItem should have list of items.

@Html.ListBoxFor(m => m.SelectedItem, new MultiSelectList(Model.Item.ToList(), "Value", "Text"), new { @class = "form-control" })