The difference between traditional DLL and COM DLL

Question

I am currently studying COM. I found that COM DLL is kind of built upon the traditional DLL infrastructure. When we build COM DLLs, we still rely on the traditional DLL export m

Answer 1

DLL has many uses in windows. Many types of library code are stored in DLL. One of these, for example, is .net assemblies, which is a different beast.

COM DLL is not better than "plain binary PE DLL" because COM DLL is also a plain DLL. What makes DLL a COM DLL is exposing, possibly in addition to other things, certain exports that match a certain contract (signature) [lookup entry IUnknown], or even several types of canonized interfaces [lookup entry 'dual interface'] that allow not only instantiation of specific objects inside the DLL but also automated discovery of services, including function names and parameter types.

The dual interfaces make it very handy to link to scripting languages (used in web, shell scripts, educational programs, etc) because the script programmers do not care about strict typing. A dual interface exposed by COM DLL allows the scripting runtime to query exactly what types it expects and do the appropriate casts, seamlessly to the user.

This flexibility allowed a whole gigantic COM infrastructure to be built upon, including Component Registration, DCOM (invocation over network), etc. It made it rather convenient to provide COM interfaces into windows components (WMI, for example) and office components. Many other popular interfaces were implemented above COM, such as ADO.

All this is good, but COM does not "prevail" in any sense. COM DLLs are minority of DLLs. Plain DLLs and .NET DLLs are very popular. Microsoft considers .net interfaces superior to COM. Many unix freaks and others consider DLLs to be a bad idea altogether, as it does not provide run-time linking services in the same sense that unix shared objects always had. Despite being a windows developer, I also think SOs provide a superior alternative, and I hope to have them once.