MVC DropDownList lagging

Question

I am posting the id of a dropdownlist back to the index (index2 view). but is lagging behind. After a second time pressing Select it shows me the correct list.

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Answer 1

You have added a route value using new { @id = Request.Form["SelectedContinent"] } in your BeginForm() method.

Assuming the initial value is 0, then it generates action = "/CountriesWorld/Index2/0". Lets assume you select the option with value="1" and you now post the form. The id attribute is bound to 0 and you filter the Countries based on .Where(e => e.Continent == 0) - no where have you ever used the value of the selected option which is bound to a non-existent property named SelectedContinent.

Now you return the view and the forms action attribute is now action = "/CountriesWorld/Index2/1" (because Request.Form["SelectedContinent"] is 1). If you select the option with value="2", the same thing occurs - you ignore the value of the selected option and the filter the Countries based on .Where(e => e.Continent == 1) because the id parameter is 1.

Always bind to a model, which in your case will be

public class CountriesVM
{
    public int? SelectedContinent { get; set }
    public IEnumerable ContinentsList { get; set; }
    public IEnumerable Countries { get; set; }
}

and in the view, strongly bind to your model (note the FormMethod.Get and the 3rd parameter in DropDownListFor())

@model CountriesVM
@using (Html.BeginForm("Index", "CountriesWorld", FormMethod.Get))
{
    @Html.DropDownListFor(m => m.SelectedContinent, Model.ContinentsList, "All")
    Select
}

    ....
    @foreach(var country in Model.Countries)
    {
        ....
    }
 

and you need only one method

public ActionResult Index(int? selectedContinent)
{
    var countries = _context.Countries.OrderBy(e => e.Country);
    if (selectedContinent.HasValue)
    {
        countries = countries.Where(e => e.Continent == selectedContinent.Value);
    }
    continentsList = _context.Continents.Select(x => new SelectListItem
    {
        Value = x.Continent.ToString(),
        Text = x.Continentomschrijving
    });
    var model = new CountriesVM
    {
        SelectedContinent = selectedContinent,
        ContinentsList = continentsList,
        Countries = countries
    };
    return View(model);
}

Note you might also want to consider caching the Continents to avoid repeated database calls assuming they do not change often (and invalidate the cache if their values are updated)