asp.net mvc multiselect remember state after postback

Question

Hi,

I am using the DataAnnotations for error checking on my asp.net mvc app, I am also using strongly typed ViewModels too.

My error checking is working fine and is posting back to my view with error messages if a field is blank. However i have a MultiSelect / Listbox on my form which i need to remember it's state after an error.

At the moment my ViewModel looks like this (i have only included relevant properties):

public class ProfilePageViewModel : PageViewModel
{
    public IList<FavouriteGenreViewModel> FavGenres { get; set; }

    [Required(ErrorMessage = "*")]
    public string SelectedGenres { get; set; }


    public IDictionary<int, string> GenresList { get; set; }
}

This is my Action in my controller:

public ActionResult Profile(ProfilePageViewModel viewModel)
    {
        if(!ModelState.IsValid)
        {
            viewModel.CountriesList = dropDownTasks.GetCountries();
            viewModel.GendersList = dropDownTasks.GetGenders();
            viewModel.GenresList = dropDownTasks.GetGenres();
            viewModel.TimezonesList = dropDownTasks.GetTimezones();
            viewModel.FavGenres = 
            return View(viewModel); 
        }

        . . .

My MultiSelect takes a list of FavouriteGenreViewModel's to select the options in the GenresList, it does this using AutoMapper in GET action, but obviouslly i can't use AutoMapper on the post because it will forget my posted values.

I have thought about using a comma delimmated string of ID's instead of a list of FavouriteGenreViewModel's, that way i can re-use the value once posted back...however i am hoping someone has a more elegant way of dealing with this problem.

Thank you!

Paul

Answer 1

I think i can answer my own question after some poking around.

In my ViewModel i use a string array as the Data Type like so:

public class ProfilePageViewModel : PageViewModel 
{ 
[Required(ErrorMessage = "*")] 
public string[] FavGenres { get; set; } 

public IDictionary<int, string> GenresList { get; set; } 
} 

In my view i can set the selected values to FavGenres, then i have a custom model binder to convert the comma seperated string into valid objects again...in case you are wondering here is my custom model binder in full...

public class AccountCustomModelBinder : DefaultModelBinder
{
    private readonly IGenreRepository genreRepository;
    private readonly ITimezoneRepository timeZoneRepository;
    private readonly ICountryRepository countryRepository;

    public AccountCustomModelBinder() : this(
        ServiceLocator.Current.GetInstance<IGenreRepository>(),
        ServiceLocator.Current.GetInstance<ITimezoneRepository>(),
        ServiceLocator.Current.GetInstance<ICountryRepository>())
    {
    }

    public AccountCustomModelBinder(IGenreRepository genreRepository, ITimezoneRepository timeZoneRepository,
        ICountryRepository countryRepository)
    {
        Check.Require(genreRepository != null, "genreRepository is null");
        Check.Require(timeZoneRepository != null, "timeZoneRepository is null");
        Check.Require(countryRepository != null, "countryRepository is null");

        this.genreRepository = genreRepository;
        this.timeZoneRepository = timeZoneRepository;
        this.countryRepository = countryRepository;
    }

    protected override void BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor)
    {
        Account account = bindingContext.Model as Account;

        if (account != null)
        {

            // gender
            if (propertyDescriptor.Name == "Gender")
            {
                if (bindingContext.ValueProvider.ContainsKey("Gender"))
                {
                    account.Gender = bindingContext.ValueProvider["Gender"].AttemptedValue.ToString();
                    return;
                }
            }

            // TimezoneId
            if (propertyDescriptor.Name == "TimezoneId")
            {
                if (bindingContext.ValueProvider.ContainsKey("TimezoneId")) {
                    account.Timezone = timeZoneRepository.FindOne(Convert.ToInt32(bindingContext.ValueProvider["TimezoneId"].AttemptedValue));
                    return;
                }
            }

            // CountryId
            if (propertyDescriptor.Name == "CountryId")
            {
                if (bindingContext.ValueProvider.ContainsKey("CountryId")) {
                    account.Country = countryRepository.FindOne(Convert.ToInt32(bindingContext.ValueProvider["CountryId"].AttemptedValue));
                    return;
                }
            }

            // FavGenres
            if (propertyDescriptor.Name == "FavGenres")
            {
                if (bindingContext.ValueProvider.ContainsKey("FavGenres")) {
                    // remove all existing entries so we can add our newly selected ones
                    account.ClearFavGenres();
                    string favIds = bindingContext.ValueProvider["FavGenres"].AttemptedValue;
                    foreach (string gId in favIds.Split(',')) {
                        account.AddFavGenre(genreRepository.Get(Convert.ToInt32(gId)));
                    }
                    return;
                }
            }
        }

        base.BindProperty(controllerContext, bindingContext, propertyDescriptor);
    }

}