Feature request: Fix CO_CLASS not following default interface

[thorntonryan]

Observed Behavior

We ran into an issue invoking a method on a COM object provided by ActiveQt, because PyWin32 decided to pass the object as a VT_UNKNOWN instead of a VT_DISPATCH.

ActiveQt generated a type library of the form:

[
  helpstring("IFoo Interface")
]
dispinterface IIFoo {
    properties:
    methods:
        SAFEARRAY(BSTR) getBar([in] IFilter* p_filter);
};

[
  helpstring("IFilter Class")
]
coclass IFilter {
!    [default] dispinterface IIFilter;
};

[
  helpstring("IFilter Interface")
]
dispinterface IIFilter {
    properties:
    methods:
};

But the following failed:

filter = win32com.client.Dispatch("MyComServer.IFilter")        
Foo = win32com.client.Dispatch("MyComServer.IFoo")
bar = Foo.getBar(filter)

The call to getBar failed because pywin32 decided to pass filter as an IUnknown instead of recognizing it was an IDispatch.

Upcasting is safe to do, but ran into a bug in the ActiveQt implementation that didn't unwrap our VT_UNKOWN object the way it would if VT_DISPATCH had been chosen.

Expected Behavior

If the coclass has a default interface, and that interface is a dispinterface, then pywin32 should use VT_DISPATCH instead of VT_UNKOWN.

In other words, we agree with the noted FIXME:

should probably get default interface for CO_CLASS???

https://github.com/mhammond/pywin32/blob/70ddf693927fa1635f15e9ef41eb1aea37fdf32a/com/win32com/client/build.py#L603-L607

So probably something of the form:

elif typeKind == pythoncom.TKIND_COCLASS:
    coclassdefaultinterface = findCoClassDefaultInterface(...) # walk the type object looking for IMPLTYPEFLAG_FDEFAULT
    return _ResolveType(coclassdefaultinterface, resultTypeInfo)

I can bug a coworker to provide a better pseudo code sketch if needed, and we actually have a reference C++ implementation of findCoClassDefaultInterface I might be able to share if I get approval

Then pywin32 would recognize that our filter should be passed as VT_DISPATCH and avoid the Qt bug.

Workaround

We can work around the problem by forcing pywin32 to use VT_DISPATCH:

filter = win32com.client.Dispatch("MyComServer.IFilter")        
-Foo = win32com.client.Dispatch("MyComServer.IFoo")
+Foo = win32com.client.dynamic.DumbDispatch("MyComServer.IFoo")
bar = Foo.getBar(filter)

Additional Details

Python 3.10 pywin32 304

[mhammond]

This sounds reasonable to me, but something I'd probably want to do in conjunction changes to one of our tests to ensure the right thing is happening.