What is COM?

Question

I searched hard, but was unable to grasp the whole idea. Can anyone tell me:

• What COM actually is?
• How do GUIDs work, and how are they used by COM?
• How does COM resolve the issues of different DLL versions.

Or at least, point me to a good article somewhere that explains these concepts? Thanks!

Answer 1

COM is "Component Object Model". It is one of the first technologies designed to allow "binary reuse" of components... Originally, it was the rewrite of what was, in Microsoft Office circa 1988-1992 time frame, referred to as Dynamic Data Exchange (DDE), a technology designed to allow the various Office applications to talk to one another. The first attempt to rewrite it was called OLE-Automation (Object Linking and Embedding). But when they got done they renamed it to COM.

How it works:

Essentially, before COM, when a client component wanted to use a component (written as a C++ library), it had to be compiled WITH the library, so it could know exactly how many bytes into the compiled binary file to find each method or function call.

With COM, there is a defined mechanism as to how these methods will be structured, and then the compiler produces a separate file (called a type library or an Interface Definition Language (IDL) file, that contains all this function offset data.

Then, as a user of the component, you have to "register" it, which writes all this information (Keyed off of GUIDs) into the OS Registry, where any client app can access it, and by reading the data from the registry, it can know where in the binary file to find each method or class entry point.

Answer 2

COM is Microsoft's Component Object Model, a binary-compatible interface for programs written in various languages to interoperate with each other. It is the "evolutionary step" between the OLE and .NET technologies.

If you want to learn about COM from the C++ perspective, take a look at Don Box's Essential COM, or ATL Internals by Rector and Sells.

The group microsoft.public.vc.atl is probably the best place to ask questions you can't get answers for here. It's primarily an ATL newsgroup, but it seems to be the newsgroup with the most traffic for general COM questions as well. (just be prepared for the usual newsgroup curtness & impatience)

Answer 3

10,000 foot view:

COM is the communication mechanism for software components. Example, you can interact with COM interfaces (COM interop in .NET) to use functionality not exposed through a common interface (.NET assembly).

GUIDs are explained fairly decent on Wikipedia http://en.wikipedia.org/wiki/Globally_Unique_Identifier

I always understood LIB files to be object files for the C++ linker. They contain the code for all objects in a cpp file. The compiler optimizes when it links disregarding portions of the object file that it doesn't need.

Someone please clarify as I am sure I butchered some of this.

Answer 4

Your question is a little large for a full explanation here. A quick high-level introduction to COM can be found in the book Understanding ActiveX and OLE. A more detailed but still introductory introduction is Inside COM. The best book on the subject is Don Box's Essential COM.

A couple of quick answers:

• COM is a binary interface standard for objects. It allows various programs to write to interfaces without all having to have been written in the same langauge with the same compiler. There are also related services available.
• GUIDs are globally unique numbers that COM uses to identify interfaces.
• COM doesn't resolve different DLL version problems. It only allows a single DLL to be registered for each GUID.

Answer 5

COM enables reusable software. Like building blocks, you can create COM objects (or now Assemblies in .NET) to provide functionality to a larger piece of software. I have used COM to provide DB integration for Excel and MS BizTalk. Software like MS BizTalk use COM/Assemblies to extend the processing capabilities of a standard process; you can insert a COM into the message workflow to do more processing than is implemented by Microsoft. COM also allows use of Component Services providing built in object pooling, security, and control interface.

Wikipedia has a good definition of GUID. Note that Microsoft has a formatting that is not necessarly used in the rest of development community.

COM by itself does not resolve DLL version issues. It enables you to extend software incrementally if you use the COM versioning capability. So if you have an application that uses a COM to convert XML to Text (for example) and you want to enhance, you can create a new version (2.0) which you can roll-out slowly as you update the source application to use the new COM. This way you could (if need be) have a switch statement that can still use the old COM if required by system limitations, or use the new one (they would be different DLLs).

Answer 6

COM is a method to develop software components, small binary exe, that provides services for applications, OS and other components. Developing custom COM comnponent is like developing Object oriented API. GUID is a Global unique ID and used to identify a COM component uniquely.

You can refer a very good book by Dale Rogerson for more details. Inside COM