Question

I’m currently working on creating a new container in my WinRT project. This container will behave similar to a ListView, so I’ve decided to use similar names in its properties.

I created a DependencyProperty for ItemsSource to mimic that of the built-in ItemsControl (abstract parent to other lists). ItemsControl defines ItemsSource as a Platform::Object^, so I did the same. This works fine.

Later on, it’s time to create items to go in the list, populate them with instances of the specified ItemsTemplate, and set their data contexts. This means I need to actually cast ItemsSource from Platform::Object^ into something iterable. Unfortunately, to cast it to something iterable, I need to specify the templated type in the cast (e.g. IIterable<UIElement^>^ or IIterable<Object^>^). At this stage, I don’t particularly care what type of object it is because I’m just using it as the DataContext for the new list item, so casting to IIterable<Object^>^ would be fine. Unfortunately, safe_cast doesn’t let me do this unless the thing originally set to ItemsSource was also templated as a IIterable<Object^>^ or some child such as IVectorView<Object^>^.

With existing containers such as ListView, you can set the ItemsSource to a Vector<MyCustomViewModel^>^ without first converting it to a Vector<Object^>^ and it works just fine. So how do they do this? Are they not using safe_cast? I’m pretty sure using a less-safe cast would have adverse effects here. Have any other ideas?

TL;DR:

I have some Platform::Object^ reference that I know points to an IIterable of other references. Is it possible to cast it to IIterable<Platform::Object^>^ somehow, even if the iterable was originally created with some other template (such as IIterable<MyCustomViewModel^>^)?

Answer 1

Because C++/CX does not support type co/contravariance (Platform::Vector<T^> does not implement IVector<Object^>), the Xaml team added a special non-generic interface Windows.UI.Xaml.Interop.IBindableObservableVector, which gives you an object-based collection to iterate. Platform::Vector implements this interface specifically to handle the issue you’re struggling with. So in your case you should be able to cast to and make use of this special interface.

I am not certain if .NET collections support this interface, so for your custom control to support C#/VB, you may want to do a dynamic_cast to IBindableObservableVector, and if the result is nullptr, then try casting to other collection types (such as IVector<Object^>).