All calls to AddUndoStep(ReversibleChange) that appeare inside the same context are considered to belong to the same action and override the previous call from the same context. So only the last call will be saved in the undo stack. Use ContextFrame with the C# "using" schema or call IDisposable.Dispose() to close the frame.

public IDisposable ContextFrame(
	Object context
)

Public Function ContextFrame ( _
	context As Object _
) As IDisposable

public:
IDisposable^ ContextFrame(
	Object^ context
)