Create different ViewModel objects for derived Model classes

Question

Implementing the MVVM pattern in C#: How can I create a ViewModel instance, given a Model object whose exact type is only known at runtime?

I have an abstract ModelBase class and an abstract ViewModelBase class as well as several derived classes, eg FirstModel : ModelBase , SecondModel : ModelBase and so on, as well as FirstViewModel : ViewModelBase , SecondViewModel : ViewModelBase etc.

Now I want to implement a function that creates the appropriate view model for a given model object. Something along these lines:

ViewModelBase CreateViewModel(ModelBase someObject)
{
    return new ViewModelBase(someObject);
}

The above code does not work of course, because ViewModelBase is abstract. I rather want to create a new FirstViewModel , SecondViewModel and so on depending on the type of someObject which is only known at runtime.

How would I achieve this?

Answer 1

static Dictionary<Type, Type> TypeMap = new Dictionary<Type, Type>
{
    {typeof(ModelA), typeof(ViewModelA)},
    {typeof(ModelB), typeof(ViewModelB)},
    {typeof(ModelC), typeof(ViewModelC)}
};

static ViewModelBase CreateViewModel(ModelBase someObject)
{
    return Activator.CreateInstance(TypeMap[someObject.GetType()]);
}

Answer 2

It wouldn't be wise to allow any model to be coupled with a view model, simply because of type safety. For example, if your view model expects a Movie , and instead, at run-time, you get a Genre , how can the view model be expected to behave using this model?

Maybe I'm completely missing your requirements, but you could instead use a generic view model base:

public class ViewModel<TModel> : ViewModelBase
    where TModel : ModelBase
{
    public TModel Model { get; protected set; }

    public ViewModel(TModel model)
    {
        Model = model;
    }

    ...
}

You can then create your view models based on their mapping . For example:

public class MovieViewModel : ViewModel<Movie>
{
    ...
}

public class GenreViewModel : ViewModel<Genre>
{
    ...
}