[release/10.0] __ComObject doesn't support dynamic interface map (redux)#123762
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steveisok merged 4 commits intorelease/10.0from Feb 2, 2026
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[release/10.0] __ComObject doesn't support dynamic interface map (redux)#123762steveisok merged 4 commits intorelease/10.0from
__ComObject doesn't support dynamic interface map (redux)#123762steveisok merged 4 commits intorelease/10.0from
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This was originally fixed naively which resulted in an incomplete fix. The activation path issues remained and this current fix addresses the underlying issue - adding IDIC to __ComObject.
AaronRobinsonMSFT
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Backport of #123725 to release/10.0
/cc @AaronRobinsonMSFT
Customer Impact
In #112375 a fix was checked in with insufficient validation. An issue was reported offline that uncovered a deficiency in that fix and the validation. This PR revisits that issue for .NET 10, adds the missing test cases, and adds comments on the utility of the dynamic interface map.
The regression is the casting of built-in COM interop objects to some managed (COM and non-COM) interfaces, depending on how the COM object enters the runtime. Occasionally there are workarounds, but in the most recent offline issue the workaround is very impactful.
Re fixes #112371
Regression
The PR that originally started this all off was #105965.
Testing
Added and verified tests for the original reported test and the one described offline - all now pass. Both the original OP reported issue and most recent issue were verifiied fixed with this change.
Risk
Low to Medium. The risk here isn't destabilizing issue, but rather introducing or not fixing a regression on some important niche scenario from the original change, #105965. This change is also considered low risk because this change represents a much more targeted change relative to the change that introduced the regression (that is, the additional of a new interface on the
__ComObjectwhen previously it had none).