Question

Sorry for the confusing title of the question. I am uncertain about how should I implement ViewModels and Models which derive from a base class.

My ViewModel classes are based on a ViewModelBase, the ViewModelBase holds a ModelBase which serves as a base class for all other models.

Each ViewModel holds a “Model” Property, however, because a Model property was defined by the base ViewModel class as the ModelBase class, I always have to create another property which casts the Model – from ModelBase to the relevant Model class.

My question is – is there not a simpler solution? Is there a design pattern which is relevant for these issues?

Here’s a code sample:

public abstract class BasicViewModel : ViewModelBase
{
    public BasicViewModel()
    {
    }
    public ModelBase Model { get; set; }
}

public class ModelBase
{
}

public class ContainableViewModel : BasicViewModel
{
    public ContainableViewModel(ContainableModel model)
    {
        this.Model = model;
    }

    public ContainableModel MyModel { get { return (ContainableModel)Model; } }

    public int Children { get { return MyModel.Children; } set { MyModel.Children = value; } }
}

public class ContainableModel : ModelBase
{
    public ContainableModel()
    {
        Children = 2;
    }

    public int Children { get; set; }
}

As you can see, the “MyModel” property is the one which bugs me.

Thank you very much for your help and time!

Answer 1

If each of your view model contains a Model property, you could use generics:

public abstract class BasicViewModel<TModelType> : ViewModelBase
    where TModelType : ModelBase
{
    public BasicViewModel(TModelType model)
    {
        Model = model;
    }

    public TModelType Model { get; set; }
}

public class ModelBase
{
}

Your containable view model is now defined as follows:

public class ContainableViewModel : BasicViewModel<ContainableModel>
{
    public ContainableViewModel(ContainableModel model)
        : base(model)
    {
    }

    // you can now omit this method, it is defined on the abstract superclass
    //public ContainableModel Model { get { return ()Model; } }

    public int Children { get { return MyModel.Children; } set { MyModel.Children = value; } }
}

public class ContainableModel : ModelBase
{
    public ContainableModel()
    {
        Children = 2;
    }

    public int Children { get; set; }
}