[API Proposal]: Add ability to safely get the managed object from a ComWrappers CCW

[JeremyKuhne]

Background and motivation

Using COM Interop it is a common pattern to cast incoming RCW's against internal instances. WinForms does this in a number of places. For example, we check incoming OLE control interfaces to see if they are our own hosting control and utilize fields/methods on the instance if it is.

Using ComWrappers this isn't possible to do safely without adding an additional interface to the outgoing CCW. It would be good to add functionality to ComWrappers to query for a wrapped managed object on any IUnknown.

Without a way for users to construct ComWrappers CCWs that we can attempt to unwrap when receiving from native methods they cannot enable trimming in their apps.

API Proposal

namespace System.Runtime.InteropServices;

public abstract class ComWrappers
{
+    public static bool TryGetObject(void* unknown, out object? obj);
}

API Usage

public static MyCallbackApi(IUnknown* unknown)
{
   if (ComWrappers.TryGetObject(unknown, out object? myObject)
       && myObject is MyObject)
   {
        // Do stuff
   }
}

Alternative Designs

We could potentially publish a new COM interface that we can have users implement in their ComWrappers CCWs that we can look for to know that it is ok to unwrap with ComInterfaceDispatch.GetInstance.

Risks

No response

[JeremyKuhne]

cc: @lonitra, @AaronRobinsonMSFT, @jkoritzinsky, @LakshanF

[AaronRobinsonMSFT]

This should be done via your own ComWrappers implementation as opposed to relying upon it "magically" working. If we did this the API would need to throw if there wasn't a globally registered ComWrappers instance. One should really call ComWrappers.GetOrCreateObjectForComInstance().

[AaronRobinsonMSFT]

Using ComWrappers this isn't possible to do safely without adding an additional interface to the outgoing CCW. It would be good to add functionality to ComWrappers to query for a wrapped managed object on any IUnknown.

Which wrapper do you want? The ComWrappers mechanism is designed around a 1:N model - there are N potential CCWs, one for each ComWrappers implementation. It isn't possible to know which one is desired and would be incredibly expensive to create an API that permits this. Your implementation of ComWrappers should attempt to unwrap and if it can, it will, otherwise it is an native COM object and will be wrapped.

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Issue Details

---

Background and motivation

Using COM Interop it is a common pattern to cast incoming RCW's against internal instances. WinForms does this in a number of places. For example, we check incoming OLE control interfaces to see if they are our own hosting control and utilize fields/methods on the instance if it is.

Using ComWrappers this isn't possible to do safely without adding an additional interface to the outgoing CCW. It would be good to add functionality to ComWrappers to query for a wrapped managed object on any IUnknown.

Without a way for users to construct ComWrappers CCWs that we can attempt to unwrap when receiving from native methods they cannot enable trimming in their apps.

API Proposal

namespace System.Runtime.InteropServices;

public abstract class ComWrappers
{
    public struct ComInterfaceDispatch
    {
        public static bool TryGetInstance<T>(IUnknown* unknown, out T? instance)  where T: class;
    }
}

API Usage

public static MyCallbackApi(IUnknown* unknown)
{
   if (ComWrappers.TryGetInstance(unknown, out MyObject? myObject)
   {
        // Do stuff
   }
}

Alternative Designs

We could potentially publish a new COM interface that we can have users implement in their ComWrappers CCWs that we can look for to know that it is ok to unwrap with ComInterfaceDispatch.GetInstance.

Risks

No response

Author:	JeremyKuhne
Assignees:	-
Labels:	api-suggestion, area-System.Runtime.InteropServices, untriaged
Milestone:	-

[AaronRobinsonMSFT]

Without a way for users to construct ComWrappers CCWs that we can attempt to unwrap when receiving from native methods they cannot enable trimming in their apps.

See CreateObjectFlags.Unwrap, which will perform the unwrap for you instead of trying to do all this manually.

[AaronRobinsonMSFT]

Spoke to @JeremyKuhne offline. I get this scenario now. The implementation here is a going to be more complicated than #79679, but is possible. In its simplest form, we can add a sentinel interface to the generated CCW and use that to return the associated object or perhaps the GCHandle that is embedded in the CCW.