In AvaloniaUI, there are two ways to handle selected items in a ListBox, each corresponding to a different property on the control: SelectedItems and Selection. The AvaloniaUI team recommends against the use of SelectedItems for large collection, so I started using the Selection property instead.

I needed to support multi-selection, so I set the SelectionMode property to Multiple, created a SelectionModel in my ViewModel, added the Items, and started the application. Unfortunately, the ListBox did not allow multiple selection.

I found a boolean property on the SelectionModel named SingleSelect. Once I set this property to false, I was able to select multiple items in the ListBox.