COM Objects

Component Object Model (COM) is a binary interface standard for software components. It was developed by Microsoft, and it is a precursor to the Microsoft .NET Framework's Common Language Runtime.

COM enables different applications to interact with each other and to reuse functionality without needing to know the implementation details. It is a cornerstone of Windows development, powering many core system features and providing a flexible way to extend application capabilities.

Key Concepts in COM

Understanding these fundamental concepts is crucial for working with COM objects:

• Interfaces: A contract that defines a set of related functions. COM objects implement one or more interfaces. Clients interact with COM objects solely through interfaces.
• CLSID (Class Identifier): A globally unique identifier (GUID) that uniquely identifies a COM class.
• IID (Interface Identifier): A GUID that uniquely identifies a COM interface.
• Reference Counting: COM uses reference counting to manage the lifetime of objects. Objects are created with a reference count of one and are destroyed when the reference count drops to zero.
• Registry: COM class information, including CLSIDs and their associated DLLs or EXEs, is stored in the Windows Registry.

Working with COM Objects

You can interact with COM objects from various programming languages, including C++, C#, and Visual Basic. The primary mechanism for interacting with a COM object is to query it for specific interfaces using the QueryInterface method.

Example: Creating and Using a COM Object (C++)

This example demonstrates how to initialize the COM library, create an instance of a COM object (e.g., the Scripting.FileSystemObject), and use one of its methods.

#include <iostream>
#include <windows.h>
#include <objbase.h>
#include <comdef.h>

// Include type library for Scripting.FileSystemObject if available,
// otherwise use dynamic dispatch or IDispatch::Invoke.
// For simplicity, we'll use IUnknown and dynamic calls here.

int main() {
    HRESULT hr = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED);
    if (FAILED(hr)) {
        std::cerr << "COM library initialization failed." << std::endl;
        return 1;
    }

    IUnknown* pUnknown = NULL;
    CLSID clsid;

    // Get the CLSID for Scripting.FileSystemObject
    if (CLSIDFromProgID(L"Scripting.FileSystemObject", &clsid) == S_OK) {
        // Create an instance of the COM object
        hr = CoCreateInstance(clsid, NULL, CLSCTX_INPROC_SERVER, IID_IUnknown, (void**)&pUnknown);

        if (SUCCEEDED(hr)) {
            std::cout << "COM object created successfully." << std::endl;

            // You would typically query for specific interfaces here,
            // e.g., IID_FileSystemObject if you had its definition.
            // For demonstration, we'll just show basic interaction.

            // To use methods, you'd need the specific interface pointer.
            // Example for a hypothetical IFilesystemObject interface:
            // IFileSystemObject* pFileSystem = NULL;
            // hr = pUnknown->QueryInterface(IID_FileSystemObject, (void**)&pFileSystem);
            // if (SUCCEEDED(hr)) {
            //     // Use pFileSystem->CreateTextFile(...) or other methods
            //     pFileSystem->Release();
            // }

            pUnknown->Release(); // Release the object
        } else {
            std::cerr << "Failed to create COM object. HRESULT: " << hr << std::endl;
        }
    } else {
        std::cerr << "Failed to get CLSID for Scripting.FileSystemObject." << std::endl;
    }

    CoUninitialize(); // Uninitialize the COM library
    return 0;
}

COM Interfaces in Windows

Resources

• Microsoft Learn: Component Object Model
• Programming with COM