☔ COM interop on Windows

[grynspan]

Tracking research and work to support COM interfaces in Swift with better ergonomics.

[grynspan]

Tracked as rdar://152309114.

[grynspan]

A few possible ways to improve things:

• Import COM interfaces as Swift classes with foreign refcounting
• Automatically conform imported types to a COMInterface protocol:
  • The protocol would have a static interfaceID: IID { get } property
  • swift_dynamicCast() would know to look for this protocol conformance and call QueryInterface() when appropriate
• Implement ARC atop IUnknown::AddRef() and IUnknown::Release() for COM objects
  • Disallow calling IUnknown::AddRef() and IUnknown::Release() directly from Swift
  • Direct developers to use Unmanaged when they really need to manually refcount
• Teach the importer to map functions of the form HRESULT GetFoo([out] IWhatever**whatever) to func GetFoo() throws -> IWhatever
  • Decide if we want to map GetFoo() to foo() or leave it as-is, while we're in there

[grynspan]

Related:

#83914
swiftlang/swift-installer-scripts#458

[egorzhdan]

Exposing WinRT headers to Swift:

#84669
swiftlang/swift-installer-scripts#465

[stevenbrix]

Hey all, I love that Swift is starting to think of how to better support COM!

How are you thinking this would work with WinRT (which is built on COM)?

Import COM interfaces as Swift classes with foreign refcounting

this sounds like it would make it impossible to derive from two COM interfaces?

swift_dynamicCast() would know to look for this protocol conformance and call QueryInterface() when appropriate

I've created a related (and more generic pitch) here.

[grynspan]

Related/helpful change to the WinSDK overlay: #84792

[grynspan]

Hey all, I love that Swift is starting to think of how to better support COM!

How are you thinking this would work with WinRT (which is built on COM)?

I don't have any opinions here. My hope would be that with C++ interop enabled, WinRT works "for free" but I suspect that won't be the case.

Import COM interfaces as Swift classes with foreign refcounting

this sounds like it would make it impossible to derive from two COM interfaces?

In discussions we've had outside GitHub, I don't think we thought this would be true for COM types in the Windows SDK. I don't know if anyone has dug very deeply into what sort of COM object you could declare in Swift.