Binding "SelectedItems" of a Listbox

You are right: ListBox.SelectedItems cannot be directly data-bound in WPF because it is not a dependency property. This is a known limitation.

So your reading is correct
The solution is to use an intermediate mechanism, most commonly a CheckBox inside the ListBox item or code-behind behavior.

Below is the cleanest and recommended approach for your scenario.

Recommended solution: Bind selection using a CheckBox (MVVM-friendly)

Instead of binding SelectedItems, store selection state on each item.

1. Update Item class

Add a boolean property to track selection.

public class Item : INotifyPropertyChanged
{
    public string _type { get; set; }
    public string _name { get; set; }

    private bool _isSelected;
    public bool IsSelected
    {
        get => _isSelected;
        set
        {
            _isSelected = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsSelected)));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

2. Use a CheckBox in ListBox2

Bind the checkbox to IsSelected.

Now selection is fully bindable and editable.

3. Sync group selection ? items selection

When a group is selected, update items manually.

private void ListBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (ListBox1.SelectedItem is Group group)
    {
        foreach (Item item in il)
        {
            item.IsSelected = group._members.Contains(item._name);
        }
    }
}

Attach it in XAML:

Why this works (important)

• SelectedItems cannot be bound

• IsChecked can be bound

• You gain two-way editing

• Group ? items sync becomes simple

• This pattern is widely used in WPF

Summary

• You cannot directly bind SelectedItems

• Use a CheckBox inside the ListBox

• Track selection via a boolean property

• Update selection when group changes

This is the correct and stable WPF solution for your requirement.