Add fallback for when an interface is kept alive but its Vftbl type w…

…as linked away since no members on the interface were called.

WinRT.Runtime/IWinRTObject.net5.cs

@@ -15,7 +15,6 @@ bool IDynamicInterfaceCastable.IsInterfaceImplemented(RuntimeTypeHandle interfac
  {
  return false;
  }
- var vftblType = helperType.GetNestedType("Vftbl");
  int hr = NativeObject.TryAs<IUnknownVftbl>(GuidGenerator.GetIID(helperType), out var objRef);
  if (hr < 0)
  {
@@ -25,6 +24,20 @@ bool IDynamicInterfaceCastable.IsInterfaceImplemented(RuntimeTypeHandle interfac
  }
  return false;
  }
+ var vftblType = helperType.GetNestedType("Vftbl");
+ if (vftblType is null)
+ {
+ // The helper type might not have a vftbl type if it was linked away.
+ // The only time the Vftbl type would be linked away is when we don't actually use
+ // any of the methods on the interface (it was just a type cast/"is Type" check).
+ // In that case, we can use the IUnknownVftbl-typed ObjectReference since
+ // it has all of the information we'll need.
+ if (!QueryInterfaceCache.TryAdd(interfaceType, objRef))
+ {
+ objRef.Dispose();
+ }
+ return true;
+ }
  using (objRef)
  {
  IObjectReference typedObjRef = (IObjectReference)typeof(IObjectReference).GetMethod("As", Type.EmptyTypes).MakeGenericMethod(vftblType).Invoke(objRef, null);