How to do COM dll reference counting the right way? Official Microsoft samples are inconsistent

Question

• Some implementations calls DllAddRef() and DllRelease() in the CClassFactory constructor, destructor, and LockeServer member function:
  https://github.com/microsoft/workbooks/blob/master/Clients/Xamarin.Interactive.Client.Windows.ShellExtension/ClassFactory.cpp
• Some do it only in LockeServer:
  https://github.com/microsoft/Windows-classic-samples/blob/main/Samples/CredentialProvider/cpp/Dll.cpp
• And in others, the CClassFactory constructor and destructor use a totally different reference count variable from that of LockeServer:
  https://github.com/microsoft/Windows-classic-samples/blob/main/Samples/NetworkAccessProtectionExtensions/cpp/SampleShvUi/shvuicf.cpp

Which one is the right way? Also the use of global reference count variables seems like a rather outdated pattern. Could static class member variables be used instead?

Answer 1

COM makes a distinction between DLL reference counting and object server reference counting.

Sometimes the object server is exactly one DLL and no more and it makes sense to combine them, sometimes the object server needs to maintain additional state (open files, network connections) and therefore requires different lifetime.

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There's no disadvantage to using global variables for the DllAddRef and DllRelease reference counts -- those functions have to be global anyway so there's no possibility of having a class to store per-instance data.

And the globals will tend to be declared static in a single compilation unit -- that means no linkage and less work for the linker to do. Since these functions are called via LoadProcAddress there's no lost opportunity for inlining them (inlining would require linkage so the variables can be found at all call sites).

In contrast, CClassFactory reference counts will tend to be stored in the CClassFactory class instance, which typically will be a singleton but there's no prohibition on advanced COM scenarios like tearoffs.