The custom places pane of the file dialogs is not selectable itself. This means that it doesn't track a selected item. Instead, when you click an item from the custom places, the dialog is navigated to the corresponding directory. This said, you can get the currently selected directory and match it with the ones in the custom places collection. If the collection contains it, so a custom place is selected. The same is valid for the opposite direction - get a path from the custom places and open it in the dialog.
To achieve this, you can set the InitialDirectory property of the file dialog just before the ShowDialog() call. I hope this helps.
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