Hi 3ib,

I do one sample about populating combobox based on another combobox using xaml, please take a look:

xaml:

 <StackPanel>

        <ComboBox
            Width="300"
            Margin="0,20,0,20"
            DisplayMemberPath="Id"
            ItemsSource="{Binding parents}"
            SelectedItem="{Binding selectedparent}" />
        <ComboBox
            Width="100"
            DisplayMemberPath="name"
            ItemsSource="{Binding childs}"
            SelectedItem="{Binding selectedchild}" />

    </StackPanel>

xaml.cs:

 /// </summary>
    public partial class Window2 : Window
    {

        public Window2()
        {
            InitializeComponent();
            this.DataContext = new viewmodel();

        }
    }
    public class viewmodel:ViewModelBase
    {
        private ObservableCollection<parent> _parents;
        public ObservableCollection<parent> parents
        {
            get { return _parents; }
            set
            {
                _parents = value;
                RaisePropertyChanged("parents");
            }
        }
        private ObservableCollection<child> _childs;
        public ObservableCollection<child> childs
        {
            get { return _childs; }
            set
            {
                _childs = value;
                RaisePropertyChanged("childs");
            }
        }
        private parent _selectedparent;
        public parent selectedparent
        {
            get { return _selectedparent; }
            set
            {
                _selectedparent = value;
                RaisePropertyChanged("selectedparent");
                childs = _selectedparent.childs;
                RaisePropertyChanged("childs");
            }
        }
        private child _selectedchild;
        public child selectedchild
        {
            get { return _selectedchild; }
            set
            {
                _selectedchild = value;
                RaisePropertyChanged("selectedchild");
              
            }
        }

        public viewmodel()
        {
            parents = new ObservableCollection<parent>()
            {
                new parent(){Id=1,FirstChar="a", childs=new ObservableCollection<child>()
                {
                    new child(){Id=1,name="a1"},
                    new child(){Id=2,name="a2"},
                    new child(){Id=3,name="a3"},
                    new child(){Id=4,name="a4"},

                }
                },
                  new parent(){Id=2,FirstChar="b", childs=new ObservableCollection<child>()
                {
                    new child(){Id=1,name="b1"},
                    new child(){Id=2,name="b2"},
                    new child(){Id=3,name="b3"},
                    new child(){Id=4,name="b4"},

                }
                  },
                    new parent(){Id=3,FirstChar="c", childs=new ObservableCollection<child>()
                {
                    new child(){Id=1,name="c1"},
                    new child(){Id=2,name="c2"},
                    new child(){Id=3,name="c3"},
                    new child(){Id=4,name="c4"},

                }
                  }

        };
            selectedparent = parents[0];

        }
    }
    public class parent
    {
        public Int32 Id { get; set; }
        public string FirstChar { get; set; }
        public ObservableCollection<child> childs { get; set; }
    }
    public class child
    {
        public Int32 Id { get; set; }
        public string name { get; set; }
    }

Here is also a sample, you can take a look:

https://social.technet.microsoft.com/wiki/contents/articles/17946.cascading-comboboxes-in-wpf-using-mvvm.aspx

Best Regards,

Cherry