public void BillingView_UserLogsOut_ShowsLogoutTimestamp()
        {
            var billingService = new BillingAppService(_eventAggregator, _messagesMock.Object);

            _authService.Logout(_username, _timestamp);
            _messagesMock.Verify(x => x.Add($"{_username} logged out at {_timestampString}"));
        }
        public void BillingView_UserSendsMessage_ShowsMessageCountForUser()
        {
            var billingService = new BillingAppService(_eventAggregator, _messagesMock.Object);

            _messageService.Send("Hello World", _username, "bob");
            _messageService.Send("Hello World", _username, "bob");
            _messageService.Send("Hello World", _username, "bob");

            Assert.Equal(new[]
            {
                $"{_username} has sent 1 message(s)",
                $"{_username} has sent 2 message(s)",
                $"{_username} has sent 3 message(s)"
            }, _messageArgs);
        }