public CalculatorViewModel() { CalculateCommand = new RelayCommand <string>(param => OutputText = IsValidInput(InputText) ? _model.Calculate(InputText) : "Invalid Input" ); InputCommand = new RelayCommand <string>(input => { InputText += input; InputTextBackground = IsValidInput(InputText) ? Brushes.White : Brushes.Crimson; }); EarseCommand = new RelayCommand <string>(param => InputText = ""); BackspaceCommand = new RelayCommand <string>(param => { if (InputText.Length > 0) { InputText = InputText.Remove(InputText.Length - 1); } InputTextBackground = IsValidInput(InputText) ? Brushes.White : Brushes.Crimson; }); _model = new CalculatorModel(); WindowCloseCommand = new RelayCommand <object>(o => _model.CloseService()); }