Question

My Model is a generic class that contains a (for example) Value property which can be int, float, string, bool, etc. So naturally this class is represented something like Model<T>. For the sake of collections Model<T> implements the interface IModel, although IModel is itself empty of any content.

My ViewModel contains and instance of Model<T> and it is passed in through ViewModel‘s constructor. I still want to know what T is in ViewModel, so when I expose Model to the View I know the datatype of Model‘s buried Value property. The class for ViewModel ends up looking like the following:

class ViewModel<T>
{
   private Model<T> _model;

   public ViewModel(Model<T> model) { ....blah.... }

   public T ModelsValue {get; set; }

}

This works fine, but is limited. So now I need to expose a collection of IModels with varying Ts to my View, so I’m trying to set up an ObservableCollection of new ViewModel<T>s to a changing list of IModels. The problem is, I can’t figure out how to get T from Model<T> from IModel to construct ViewModel<T>(Model<T>) at runtime.

In the VS2010 debugger I can mouseover any IModel object and see its full Model<int> for example at runtime so I know the data is in there.

Any ideas?

Answer 1

Here’s what I’m using for view model collections:

Preface:

Your view model objects can be weakly typed. Give IModel a property object Value {get;} and expose that in a ModelViewModel : ViewModel<IModel> that you use for all IModel objects (see my ViewModel<T> implementation below). If you have various combinations of ObservableCollection<IModel>, ICollection<Model<T>>, etc., the framework shown here is a lifesaver. If you still need generic view model, you can derive a ModelViewModel<T> : ModelViewModel that takes a Model<T> in its constructor. The logic to create the appropriate type would go in the converter passed to ViewModelCollection.Create below. Do be warned that this design will impose a performance penalty.

ModelViewModel CreateModelViewModel(IModel model)
{
    Type viewModelType = typeof(ModelViewModel<>).MakeGenericType(model.Type);
    ModelViewModel viewModel = Activator.CreateInstance(viewModelType, model);
    return viewModel;
}

Example usage:

public class CatalogViewModel : ViewModel<ICatalog>
{
    public CatalogViewModel(ICatalog catalog)
        : base(catalog)
    {
        Func<ICatalogProduct, ProductViewModel> viewModelFactory = CreateProductViewModel;

        this.Products = ViewModelCollection.Create(catalog.Products, viewModelFactory);
    }

    public ICollection<ProductViewModel> Products
    {
        get;
        private set;
    }

    private ProductViewModel CreateProductViewModel(ICatalogProduct product)
    {
        return new ProductViewModel(product, this);
    }
}

Benefits:

• Uses lazy implementations to allow for efficient and even recursive bindings in trees.
• The view model collections only implement INotifyCollectionChanged if the underlying model collection implements INotifyCollectionChanged.

Overview of the classes (full implementations linked to github):

• ViewModel<TModel>: Base class for my view model classes. Exposes a Model property that I use in the view model’s backing code.

• ObservableViewModelCollection<TViewModel, TModel>: Lazy (actually not currently, but definitely should be), observable mapping from a model to a view model. Implements INotifyCollectionChanged.

• ViewModelCollection<TViewModel, TModel>: Lazy mapping from a collection of TModel to a collection of TViewModel.

• ViewModelCollection: Static helper – returns an ICollection<TViewModel>, using ObservableViewModelCollection<TViewModel, TModel> when the source collection implements INotifyCollectionChanged, otherwise using ViewModelCollection<TViewModel, TModel>.

A few extra types that might be useful for your view model collections:

ConcatCollection: Like ViewModelCollection, this includes a static helper to automatically choose an appropriate implementation. The ConcatCollection concatenates collections by binding directly to the source collection(s).

• ConcatCollection
• ConcatCollection<T>
• ObservableConcatCollection<T>

Here is an example of how I used this type to expose a Children property to the view while maintaining my observable collections all the way to back to the original source.

public class ProductViewModel : ViewModel<IProduct>
{
    public ProductViewModel(IProduct product)
        : base(product)
    {
        Func<IProduct, ProductViewModel> productViewModelFactory = CreateProductViewModel;
        Func<IRelease, ReleaseViewModel> releaseViewModelFactory = CreateReleaseViewModel;

        this.Products = ViewModelCollection.Create(product.Products, productViewModelFactory);
        this.Releases = ViewModelCollection.Create(product.Releases, releaseViewModelFactory);
        this.Children = ConcatCollection.Create<object>((ICollection)this.Products, (ICollection)this.Releases);
    }

    public IList<ProductViewModel> Products
    {
        get;
        private set;
    }

    public IList<ReleaseViewModel> Releases
    {
        get;
        private set;
    }

    public IEnumerable<object> Children
    {
        get;
        private set;
    }

    private ProductViewModel CreateProductViewModel(IProduct product)
    {
        return new ProductViewModel(product);
    }

    private ReleaseViewModel CreateReleaseViewModel(IRelease release)
    {
        return new ReleaseViewModel(release);
    }
}