public void CreateSomeCommandsWithSameKey()
        {
            ICommand command1 = ReactiveCommandEx.CreateWithHistoryFromObservable <Unit, Unit>(CommandKey,
                                                                                               (parameter, result) => Observables.Unit,
                                                                                               (parameter, result) => Observables.Unit,
                                                                                               _canExecuteSubject,
                                                                                               _scheduler);

            Assert.Throws <ArgumentException>(() =>
            {
                ICommand command2 = ReactiveCommandEx.CreateWithHistoryFromObservable <Unit, Unit>(CommandKey,
                                                                                                   (parameter, result) => Observables.Unit,
                                                                                                   (parameter, result) => Observables.Unit,
                                                                                                   _canExecuteSubject,
                                                                                                   _scheduler);
            });
        }