The COM methods are already __stdcall.
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dlls/riched20/txthost.c | 128 ++++++++++++++--------------------------------
1 files changed, 39 insertions(+), 89 deletions(-)
COM methods are supposed to use the stdcall calling convention, but the native riched20 and the native headers just defines ITextHost and ITextServices to be a c++ class (i.e. using the thiscall calling convention). Just look at the native TextServ.h header.