How to bind Group Names and Items separately in different Comboboxes

Question

I've an ICollectionView named MyObjects with group description. In code I can access group Name and Items this way:

foreach (var gr in MyObjects.Groups) 
{ 
    var group = gr as CollectionViewGroup; 
    var name = group.Name; 
    var item = group.Items.OfType<TestObject>(); 
    ... 
} 

I want to display the Name in one ComboBox and Items related to the selected group of that ComboBox in another ComboBox in a Grid like this:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <ComboBox ItemsSource="{Binding MyObjects/Names}"
              DisplayMemberPath="Name"/>
    <ComboBox Grid.Column="1" 
              ItemsSource="{Binding MyObjects/Items}"
              DisplayMemberPath="Name"/>
</Grid>

obviously the above xaml doesn't work. Is there a way to bind those group Name and Items in different ComboBox?

Answer 1

Hi,
the easiest way is include additional property for binding second combobox. try following demo:

XAML:

<Window x:Class="WpfApp1.Window85"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp85"
        mc:Ignorable="d"
        Title="Window85" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <Window.Resources>
    <Style TargetType="ComboBox">
      <Setter Property="VerticalAlignment" Value="Top"/>
      <Setter Property="Margin" Value="5"/>
    </Style>
  </Window.Resources>
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <ComboBox ItemsSource="{Binding MyObjects.Groups}"
              SelectedItem="{Binding CurrentGroup}"
              DisplayMemberPath="Name"/>
    <ComboBox Grid.Column="1" 
              ItemsSource="{Binding GroupItems}"
              DisplayMemberPath="Info"/>
  </Grid>
</Window>

And classes:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Data;

namespace WpfApp85
{
  public class ViewModel : INotifyPropertyChanged
  {
    public ViewModel() => LoadData();

    ObservableCollection<TestObject> col = new ObservableCollection<TestObject>();
    CollectionViewSource cvs = new CollectionViewSource();
    public ICollectionView MyObjects { get => cvs.View; }

    private object _currentGroup = null;
    public object CurrentGroup
    {
      get => this._currentGroup;
      set
      {
        this._currentGroup = value;
        OnPropertyChanged(nameof(GroupItems));
      }
    }
    public IEnumerable<object> GroupItems
    {
      get
      {
        var group = this._currentGroup as CollectionViewGroup;
        return (group == null) ? null : group.Items.OfType<TestObject>();
      }
    }

    private void LoadData()
    {
      Random rnd = new Random();
      for (int i = 1; i < 100; i++)
        col.Add(new TestObject() { Group = $"Group{rnd.Next(1, 10)}", Info = $"Row {i:00}" });
      cvs.Source = col;
      cvs.GroupDescriptions.Add(new PropertyGroupDescription("Group"));
      cvs.SortDescriptions.Add(new SortDescription("Group", ListSortDirection.Ascending));
      cvs.SortDescriptions.Add(new SortDescription("Info", ListSortDirection.Ascending));
    }
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged([CallerMemberName] string propName = "") =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
  }

  public class TestObject
  {
    public string Group { get; set; }
    public string Info { get; set; }
  }
}