GeneratedComClass doesn't apply to inherited classes

[jtschuster]

Placing GeneratedComClass on a base class doesn't enable deriving types to work with source generated COM.

Should we check for base classes that have the IComExposedDetails as well here?
https://github.com/dotnet/runtime/blob/df6fdefa27068126794b253d4d822706221a92db/src/libraries/System.Runtime.InteropServices/src/System/Runtime/InteropServices/Marshalling/IComExposedDetails.cs#L21C49-L21C49

	[GeneratedComInterface]
	[Guid("c5356681-8f8c-423a-959c-2d91f17215d3")]
	internal partial interface IFoo
	{
		int get();
	}

	[GeneratedComClass]
	[Guid("9f826481-4d92-4499-ab26-c40d9f77435b")]
	internal partial class FooBase : IFoo
	{
		public int get() => 42;
	}

	internal class Foo<T> :FooBase
	{
	}
int Main()
{
                        var cw = new StrategyBasedComWrappers();
			var foo = new Foo<int>();
			nint iptr = cw.GetOrCreateComInterfaceForObject(foo, CreateComInterfaceFlags.None);
			IFoo f = (IFoo)cw.GetOrCreateObjectForComInstance(iptr, CreateObjectFlags.None); // Fails - iptr does not implement IFoo
}

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Issue Details

---

Placing GeneratedComClass on a base class doesn't enable deriving types to work with source generated COM.

Should we check for base classes that have the IComExposedDetails as well here?
https://github.com/dotnet/runtime/blob/df6fdefa27068126794b253d4d822706221a92db/src/libraries/System.Runtime.InteropServices/src/System/Runtime/InteropServices/Marshalling/IComExposedDetails.cs#L21C49-L21C49

	[GeneratedComInterface]
	[Guid("c5356681-8f8c-423a-959c-2d91f17215d3")]
	internal partial interface IFoo
	{
		int get();
	}

	[GeneratedComClass]
	[Guid("9f826481-4d92-4499-ab26-c40d9f77435b")]
	internal partial class FooBase : IFoo
	{
		public int get() => 42;
	}

	internal class Foo<T> :FooBase
	{
	}
int Main()
{
                        var cw = new StrategyBasedComWrappers();
			var foo = new Foo<int>();
			nint iptr = cw.GetOrCreateComInterfaceForObject(foo, CreateComInterfaceFlags.None);
			IFoo f = (IFoo)cw.GetOrCreateObjectForComInstance(iptr, CreateObjectFlags.None); // Fails - iptr does not implement IFoo
}

Author:	jtschuster
Assignees:	-
Labels:	area-System.Runtime.InteropServices
Milestone:	-

[AaronRobinsonMSFT]

I'd like to understand this scenario before we go after it. I have some questions about how overrides are expected to and what benefits we gain as opposed to just having people place it on the most derived type.

[jtschuster]

The more I think about it, I'm not sure there's a great way to do this without breaking trimming and AOT. Maybe we should just require it on all classes that are exposed to COM.

[jkoritzinsky]

There's also no efficient way to detect this scenario in a source generator. We should just require it on all classes that are exposed to COM.

[AaronRobinsonMSFT]

There's also no efficient way to detect this scenario in a source generator. We should just require it on all classes that are exposed to COM.

Nice. Does it make sense to detect that if the immediate base class has GeneratedComClass that we emit a warning? I can imagine people doing odd things with managed inheritance for COM class implementation but I'm not sure that is something we should encourage.

[jkoritzinsky]

Yes, we can write an analyzer that detects that case and emits a warning in an efficient way.