Question

I’m implementing MVVM for a class which represents recursively nested types. For example:

class NestedType
{
    // Other properties here

    public Collection<NestedType> SubElements {get; set;}
}

class NestedTypeViewModel
{
    ObservableCollection<NestedType> ModelCollection { ??? }
}

How do I expose the items in the Model’s collection in an observable way? (i.e., the View will be adding, creating, and modifying the subelements) I assume I need the collection in the ViewModel to be ObservableCollection<T> but what about in the Model? I could also make that an ObservableCollection and just directly expose it…

Any suggestions?

Answer 1

You can directly return the ObservableList from your model. WPF will take the reference of the Collection and registers to the CollectionChanged Event of the model collection.

class MyViewModel
{
  public ObservableCollection<NestedType> MyItems { return Model.Items; }
} 

BUT, you will break what you are trying to achieve by using MVVM. A ViewModel should not expose a model, single or collection, otherwise your view could bind to a model which you should not do in MVVM.

Well MVVM is no religion and everyone implements certain parts differently. If this is no problem for you … Go ahead and do it directly.

If you want a clean solution, you have multiple options to go from here. Just allowing a Manager to modify the collection of the model and create a corresponding view model or My favorit approach is to have an object that syncs model and viewmodel collection, by automatically wrap newly added models in a new view model and add this vm into the parent viewmodel list.