Control-clicking on a part will select it if it wasn't already, and will deselect if it had been selected. Shift-clicking on a part will add it to the selection (if it isn't already). Otherwise, clicking on a part will select it (if it isn't already).

Note that there are restrictions on selection. For example, a part cannot be selected in this manner if Northwoods.GoXam.Part.Selectable is false, or if Northwoods.GoXam.Diagram.MaximumSelectionCount would be exceeded.

A left click in the background of the diagram with no modifier keys clears the selection.

Reference

DraggingTool Class
DraggingTool Members