In Extended SelectionMode, the SelectedItems collection will give you the collection of all selected Viewmodels in databound scenario. If you mean CheckedItems, please note that there are some known bugs with this collection and we highly encourage you to avoid using it.
In the attached solution you can find a possible approach for checking all selected items in code behind.
In PreviewKeyDown event you can iterate the selected models and set their property:
private void tree_PreviewKeyDown(object sender, KeyEventArgs e)
if (e.Key == Key.Space)
foreach (var item in this.tree.SelectedItems)
(item as DataItem).IsChecked = true;
e.Handled = true;
The built-in logic for checking items with space will work only for not-databound trees and built-in checkboxes. Also it will only check the item which holds the keyboard focus.
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