Why there is no SelectorT?

Question

I am trying to implement a custom control (I call it DataGrid) , which I choose to derive from Selector. In cppwinrt , I write some like this

struct DataGrid : SelectorT<DataGrid>
{
}

This code does not compile. In fact , there is no such a struct called SelectorT in any of the shipped Windows.UI.Xaml.Controls.Primitives.h files. I've checked the winmd file shipped with the SDK (19041) , found that Selector is Composable (has the ComposableAttribute attribute), so, SelectorT should have been generated , what I've missed ? Many thanks!

Answer 1

There is no SelectorT because Selector can not be subclassed. The TypenameT class templates in the consuming projection are not provided for convenience, they exist to provide user-defined subclasses of Xaml types that support it. This allows the user to define their own implementations of overrideable methods. (Think of it as similar to C++ virtual functions, but in a complicated COM aggregation way)

The docs don't appear to spell out in clear words which classes can be subclassed, and which ones can't. You can deduce this, though, from the presence of ComposableAttribute on the type, as in FrameworkElement.