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How to make MIDL3 arrays with "weak" ownership? #770

Closed forderud closed 1 year ago

forderud commented 2 years ago

I'm working on a code base that is using "classical" COM interfaces with SAFEARRAY for exchanging image data efficiently both in-proc and across process boundaries, regardless if the client is C++, C# or Python (using comtypes). This is achieved by tagging the SAFEARRAY object with FADF_AUTO to make the ownership weak and implementing IMarshal for custom marshaling, where the image data buffer is stored in a shared memory segment with lifetime controlled by the ref-count of the parent object providing the image data.

One nice property of this is that the client only sees a "regular" SAFEARRAY buffer. It don't need to know that it's using shared memory behind the scenes for zero-copy data exchange across process boundaries. This is just an implementation detail.

I've recently started to "modernize" the interfaces to WinRT/MIDL3. I then want to achieve something similar with UInt8[] or a similar array type. However, I've so far been unable to discover any flags for making ownership "weak" when creating a winrt::com_array<uint8_t> in the C++ server implementation.

Would it be possible to get some guidance on how to create a MIDL3 interface with arrays with "weak" ownership that is out-of-proc compatible?

JaiganeshKumaran commented 1 year ago

Would it be possible to get some guidance on how to create a MIDL3 interface with arrays with "weak" ownership that is out-of-proc compatible?

There's no such feature. A MIDL 3 array simply represents a pair of size and data.

forderud commented 1 year ago

There's no such feature. A MIDL 3 array simply represents a pair of size and data.

Thanks for confirming my assumption. It then seems like I'm stuck with SAFEARRAY and "classical" COM for the time being.

github-actions[bot] commented 1 year ago

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forderud commented 1 year ago

Is there somewhere where I can submit a MIDL3 enhancement request for supporting "SAFEARRAY" or similar memory-buffers that can optionally utilize externally managed buffers (like with FADF_AUTO)?

kennykerr commented 1 year ago

MIDL3 is constrained by the WinRT type system so this is unlikely to happen, but you can raise the question here: https://docs.microsoft.com/en-us/answers/topics/windows-api.html