MVVM & business logic Layer

Question

I have a problem with MVVM pattern and binding collection. My ViewModel provides a collection to the View but to get this collection I use this:

public BindingList<Car> BindingListCars { get; set; }

public CarsVm()
{
    BindingListVoiture = carServices.ListCars;
}

When I bind my View on this List it's as if I bind directly my View on the Model because they use the same reference. So when I edit one property of a Car, the model is directly edited without using carServices validation method.

What is the best solution to correct this problem ?

Do I have to expose a copy of my Model to my View to not edit directly my Model from the View?

Do I have to use BindingList in my Model and subsribe to ListChanged in my carServices to validate each change?

Answer 1

You should either perform the validation directly in the Car class itself or expose wrapper objects instead of exposing the "real" Car objects to the view.

The following sample code should give you the idea about what I mean:

//the "pure" model class:
public class Car
{
    public string Model { get; set; }
}


public class CarService
{
    public List<CarWrapper> ListCar()
    {
        List<Car> cars = new List<Car>(); //get your Car objects...

        return cars.Select(c => new CarWrapper(c, this)).ToList();
    }

    public bool Validate()
    {
        //
        return true;
    }
}

public class CarWrapper
{
    private readonly Car _model;
    CarService _service;
    public CarWrapper(Car model, CarService service)
    {
        _model = model;
        _service = service;
    }

    //create a wrapper property for each property of the  Car model:
    public string Model
    {
        get { return _model.Model; }
        set
        {
            if(_service.Validate())
                _model.Model = value;
        }
    }
}

Obviously if you expose an IEnumerable<Car> from your view model for the view to bind, you are effectively bypassing any validation that is dedined outside of the Car class if the view is able to set any properties of the Car class.