COM in plain C

Content

• A COM object and its VTable
• A GUID
• QueryInterface(), AddRef(), and Release()
• An IClassFactory object
• Packaging into a DLL
• Our C++/C include file
• The Definition (DEF) file
• Install the DLL, and register the object
• An example C program
• An example C++ program
• Modifying the code
• What's next?

Introduction

There are numerous examples that demonstrate how to use/create COM/OLE/ActiveX components. But these examples typically use Microsoft Foundation Classes (MFC), .NET, C#, WTL, or at least ATL, because those frameworks have pre-fabricated "wrappers" to give you some boilerplate code. Unfortunately, these frameworks tend to hide all of the low level details from a programmer, so you never really do learn how to use COM components per se. Rather, you learn how to use a particular framework riding on top of COM.

If you're trying to use plain C, without MFC, WTL, .NET, ATL, C#, or even any C++ code at all, then there is a dearth of examples and information on how to deal with COM objects. This is the first in a series of articles that will examine how to utilize COM in plain C, without any frameworks.

With standard Win32 controls such as a Static, Edit, Listbox, Combobox, etc., you obtain a handle to the control (i.e., an HWND) and pass messages (via SendMessage) to it in order to manipulate it. Also, the control passes messages back to you (i.e., by putting them in your own message queue, and you fetch them with GetMessage) when it wants to inform you of something or give you some data.

Not so with an OLE/COM object. You don't pass messages back and forth. Instead, the COM object gives you some pointers to certain functions that you can call to manipulate the object. For example, one of Internet Explorer's objects will give you a pointer to a function you can call to cause the browser to load and display a web page in one of your windows. One of Office's objects will give you a pointer to a function you can call to load a document. And if the COM object needs to notify you of something or pass data to you, then you will be required to write certain functions in your program, and provide (to the COM object) pointers to those functions so the object can call those functions when needed. In other words, you need to create your own COM object(s) inside your program. Most of the real hassle in C will involve defining your own COM object. To do this, you'll need to know the minute details about a COM object -- stuff that most of the pre-fabricated frameworks hide from you, but which we'll examine in this series.

In conclusion, you call functions in the COM object to manipulate it, and it calls functions in your program to notify you of things or pass you data or interact with your program in some way. This scheme is analogous to calling functions in a DLL, but as if the DLL is also able to call functions inside your C program -- sort of like with a "callback". But unlike with a DLL, you don't use LoadLibrary() and GetProcAddress() to obtain the pointers to the COM object's functions. As we'll soon discover, you instead use a different operating system function to get a pointer to an object, and then use that object to obtain pointers to its functions.

Read more: Codeproject

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