Question

I’m using ASP.NET MVC 3 and I post a form in my view, containing a @Html.ListBoxFor

When I receive the posted form as a FormCollection, how can I check if an item was selected in the ListBox?

In my controller there seems to be no item named collection["Company.RepresentingCountries"] since no <select> option was selected.. This results in a “Object reference not set to an instance of an object.” error message when I try to check for it! What’s the protocol here?

Thanks!

Answer 1

You haven’t shown how your ListBoxFor helper is defined, so I can only be guessing here. This being said you talked about FormCollection which usage I wouldn’t recommend. What I recommend is using view models. So let’s take an example:

Model:

public class MyViewModel
{
    [Required(ErrorMessage = "Please select at least one item")]
    public string[] SelectedItemIds { get; set; }

    public SelectListItem[] Items
    {
        get
        {
            return new[]
            {
                new SelectListItem { Value = "1", Text = "Item 1" },
                new SelectListItem { Value = "2", Text = "Item 2" },
                new SelectListItem { Value = "3", Text = "Item 3" },
            };
        }
    }
}

Controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new MyViewModel
        {
            SelectedItemIds = new[] { "1", "3" }
        };
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        if (ModelState.IsValid)
        { 
            // The model is valid we know that the user selected
            // at least one item => model.SelectedItemIds won't be null
            // Do some processing ...
        }
        return View(model);
    }
}

View:

@model MyViewModel

@using (Html.BeginForm())
{
    @Html.ListBoxFor(
        x => x.SelectedItemIds, 
        new SelectList(Model.Items, "Value", "Text")
    )
    @Html.ValidationMessageFor(x => x.SelectedItemIds)
    <input type="submit" value="OK" />
}