Sync items in comboboxes at runtime

Question

I’ve been trying to figure out how to synchronize a combobox with another one for some time. I’m looking to synchronize comboboxes at runtime. When the user selects a category in the combobox, depending on his choice, the combobox subcategory will show the subcategories of the selected category.

I am using WPF and C# with Entityframework 6.

In my application, I can read in my Viewmodel what was selected in the combobox in View. I wanted to know how to update, dynamically, a combobox depeded from what was selected in another combobox, using the MVVM standard. Follow excerpts from the code:

<ComboBox Name="cbCategoria" 
          ItemsSource="{Binding NomeCategorias}" 
          SelectedItem="{Binding NomeCategoria}"
          Width="175" />


<ComboBox Name="cbSubCtg" 
          ItemsSource="{Binding NomeSubCategorias}" 
          SelectedItem="{Binding NomeSubCategoria}"
          Width="175" />

As you can see, in my Viewmodel I keep what I read in Comobobox cbCategoria in the variable NomeCategoria.

From now on I appreciate any help.

Answer 1

I managed to solve my problem! I used the event SelectedChanged in the Combobox that "commands" in the second. Now I am able to dynamically update the second Combobox. Follow the code:

private void cbCategoria_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            var cbCategoria = sender as ComboBox;
            int ctgId = -1;
            string temp = string.Empty;
            ObservableCollection<string> listaCtg;

            nomeCtg = cbCategoria.SelectedItem as string;
            Debug.WriteLine($"Sub categoria lida no handler {nomeCtg}");

            using (DataContext db = new DataContext())
            {
                // Procura o nome da categoria selecionada na ComboBox
                var categoria = from x in db.Categorias
                                where x.CategoriaNome.Equals(nomeCtg)
                                select x;

                foreach (var p in categoria) { ctgId = p.CategoriaId; }

                // Procura o Id correspondente com a das categorias
                var subCtg = from x in db.SubCategorias
                             where x.Categoria.CategoriaId == ctgId
                             select x;

               listaCtg = new ObservableCollection<string>();

                foreach (var p in subCtg)
                {
                    temp = p.SubCtgNome;
                    listaCtg.Add(temp);
                    Debug.WriteLine($"Subcategoria aaaa {temp}");
                }
            }

            cbSubCtg.ItemsSource = listaCtg;                        
        }

I know it still needs some tweaking, but this example will give some idea to other members of this community. I hope someone will contribute something, can still improve.

Answer 2

I do it another way. I do a Binding of the second ComboBox in the first. Suppose the ItemsSource of the first combo is List<Categorias> and each item on that list has a List<SubCategorias>

Classes would be like this

public class Categoria 
{
   public int CodigoCategoria { get; set; }
   public string NomeCategoria { get; set; }
   public ICollection<SubCategoria> SubCategorias { get; set; }
}

public class SubCategoria 
{
   public int CodigoSubCategoria { get; set; }
   public int CodigoCategoria { get; set; }
   public string NomeSubCategoria { get; set; }
}

Binding would be like this:

<ComboBox x:Name="cboGrupo" 
          SelectedValue="{Binding CodigoCategoria, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          SelectedValuePath="CodigoCategoria" 
          DisplayMemberPath="NomeCategoria" />


<ComboBox x:Name="cboSubGrupo"
          ItemsSource="{Binding ElementName=cboGrupo, Path=SelectedItem.SubCategorias, Mode=TwoWay}"
          SelectedValue="{Binding CodigoSubGrupo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          SelectedValuePath="CodigoSubCategoria" 
          DisplayMemberPath="NomeSubCategoria" />

You need to load the first ComboBox. Follow an example with Entityframework.

public void Carregar(){
  // selecionar as categorias no banco incluindo as subcategorias
  List<Categorias> categorias = db.Categorias
                                  .Include(cat => cat.SubCategorias)
                                  .ToList();
  cboGrupo.ItemsSource = categorias;
}

Thus, by changing a category in the first ComboBox, automatically change the subcategories.