The StartBlockUndoRedo method starts recording the UI operations as a block on undo/redo operations. The method has effect only if the AllowUndoRedo property is True. The EndBlockUndoRedo method collects all undo/redo operations since StartBlockUndoRedo method was called and add them to the undo/redo queue as a block. This way the next call on a Undo operation, the entire block is restored, so all UI operations are restored. The EndBlockUndoRedo method must be called the same number of times as the StartBlockUndoRedo method was called. For instance, if you have called the StartBlockUndoRedo twice the EndBlockUndoRedo method must be called twice too, and the collected operations are added to the control's queue of undo/redo operations at the end.