Methods/properties that return WinRT objects should be nullable

[halildurmus]

For example, NetworkInformation.GetInternetConnectionProfile Method states:

The profile for the connection currently used to connect the machine to the Internet, or null if there is no connection profile with a suitable connection.

Here is an example that calls this method (make sure you are disconnected from the internet):

import 'package:win32/winrt.dart';

class ConnectionProfile extends IInspectable {
  ConnectionProfile.fromRawPointer(super.ptr);
}

ConnectionProfile getInternetConnectionProfile() {
  final activationFactoryPtr = CreateActivationFactory(
      'Windows.Networking.Connectivity.NetworkInformation',
      IID_INetworkInformationStatics);
  final object =
      INetworkInformationStatics.fromRawPointer(activationFactoryPtr);

  try {
    return ConnectionProfile.fromRawPointer(
        object.getInternetConnectionProfile());
  } finally {
    object.release();
  }
}

void main() {
  final profile = getInternetConnectionProfile();
  print(profile.runtimeClassName); // The program will exit with an Access Violation error code
}

There is no information on WinMD files on whether a method/property can return null or not. That's why we need to mark methods/properties that return WinRT objects (except async delegates e.g. IAsyncAction, collection interfaces e.g. IVector, IMap etc., and methods from XXFactory interfaces) as nullable (see microsoft/windows-rs#331).

I actually did a similar thing in #465. FileOpenPicker's pickSingleFileAsync() returns IAsyncOperation<StorageFile>. Since the type argument is a WinRT object, we consider it nullable and expose it as Future<StorageFile?>. As pickMultipleFilesAsync() returns IAsyncOperation<IVectorView<StorageFile>> and the type argument is a collection interface we don't consider it nullable and expose it as Future<List<StorageFile>>.

[timsneath]

Yuck. I don't love it, but I also see that it's necessary! Thanks for catching this.

[halildurmus]

Yep, I hate it too :(.