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? <

Answer 1

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 2

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 3

COM is a lot of different things. I recommend Don Box's book, Essential COM as a good way to learn.

At a bare minimum, a COM object is an object that exposes a single interface, IUnknown. This interface has 3 methods, AddRef, Release, and QueryInterface. AddRef/Release enables the object to be reference counted, and automatically deleted when the last reference is released. QueryInterface allows you to interrogate the object for other interfaces it supports.

Most COM objects are discoverable. They are registered in the registry under HKEY_CLASSES_ROOT with an identifying GUID, called a CLSID (class ID). This enables you to call CoCreateInstance to create an instance of a registered object if you know a GUID. You can also query the registry via a COM API for the CLSID that backs a ProgId (program id), which is a string that identifies the object.

Many COM objects have typelibs that specify the interfaces and methods the object supports, as well as IDispatch which has a method, Invoke, that allows you to dynamically call methods on the object. This enables the object to be used from scripting languages that don't support strong typing.

Some objects support being run in a different process, on a different thread, or on a different machine. COM supports marshalling for these types of objects. If possible, a standard marshaller can use the object's typelib to marshal calls to the object, but custom marshallers can be provided as well.

And there's a whole lot more to COM objects, I'm barely scratching the surface.

Answer 4

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 5

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

Answer 6

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.