How to access the native GlobalCoordinateSystem pointer with OpenXR

[Audenrith]

Before Unity 2019.3, one has been able to access the native pointer to the world SpatialCoordinateSystem via UnityEngine.XR.WSA.WorldManager.GetNativeISpatialCoordinateSystemPtr().
Switching to Unity 2019 one is able to access the same pointer via UnityEngine.XR.WindowsMR.WindowsMREnvironment.OriginSpatialCoordinateSystem using the Windows Mixed Reality XR Plugin.
Can you describe the way of accessing the native SpatialCoordinateSystem using OpenXR?

There are some issues which describe how to access a SpatialCoordinateSystem as a Unity/C# Object (e.g. in the Unity forum or in this repository) with OpenXR, but the solutions do not describe how to get the native pointer (IntPtr) (Allocating a GCHandle from the C# object obviously can not be used as a Windows::Perception::Spatial::ISpatialCoordinateSystem^ in c++).

Is there a way on accessing the native pointer of the origin SpatialCoordinateSystem ?

In c++ we are currently casting the pointer via:

reinterpret_cast<Windows::Perception::Spatial::ISpatialCoordinateSystem ^>(unitySpatialCoordinateSystemIntPtr);

and use it for getting the transform to a specific frame:

unitySpatialCoordinateSystem->TryGetTransformTo(frameCoordinateSystem);

[Audenrith]

According to this comment one can access the "sceneCoordinateSystem" via:

using namespace Microsoft.MixedReality.OpenXR;
SpatialCoordinateSystem origin = PerceptionInterop.GetSceneCoordinateSystem(UnityEngine.Pose.identity) as SpatialCoordinateSystem;

But this does not answer the question on how to get the underlying native pointer. I tried several approaches which all resulted in some error:

The following errors occur:

var handle = GCHandle.Alloc(origin , GCHandleType.Pinned); // <-- Error
IntPtr coordinateSystemNative = (IntPtr)(handle);

ArgumentException: Object contains non-primitive or non-blittable data.

var handle = GCHandle.Alloc(origin);
IntPtr coordinateSystemNative = (IntPtr)(handle);
// Error in assigning the variable in c++

Access violation reading location

__pUnknown->__abi_AddRef(); in __abi_winrt_ptr_assign(void** __ppTargetArg, const volatile ::Platform::Object^ __objArg)

var handle = GCHandle.Alloc(origin);
IntPtr coordinateSystemNative = handle.AddrOfPinnedObject(); // <-- Error

InvalidOperationException: Handle is not pinned.

Can you tell me, how is the correct way to get this object to a Windows::Perception::Spatial::ISpatialCoordinateSystem^ in c++?