COM是一種介面技術，它提供了一種方法用於客戶程式進行呼叫。COM原理和技術的文章已經很多了，就不過多講解了，這裡只注重實現。下面分幾步對程序內COM元件進行編碼實現以達到對實現一個COM的清晰的認識，目的是通過COM介面方法呼叫DLL中的函式。

首先，建立一個標準的win32 DLL專案mycom，dllmain入口實現如下：

DWORD g_dwAttachedProcesses = 0L; // # of attached processes DWORD g_dwPageSize = 0L; // System page size HINSTANCE g_hInstance = 0L; // Instance Handle LONG g_cLock = 0L; // # of outstanding objects BOOL APIENTRY DllMain( HMODULE hModule, DWORD ul_reason_for_call, LPVOID lpReserved ) { BOOL fRetVal = FALSE; SYSTEM_INFO SystemInformation; switch(ul_reason_for_call) { case DLL_PROCESS_ATTACH: // Assume successfully initialized fRetVal = TRUE; // Do one-time initialization when first process attaches if(!g_dwAttachedProcesses) { g_hInstance = (HINSTANCE)hModule; // Get the system page size if(!g_dwPageSize) { GetSystemInfo( &SystemInformation ); // can't fail g_dwPageSize = SystemInformation.dwPageSize; } } // Do per-process initialization here... // Remember that another process successfully attached g_dwAttachedProcesses++; break; case DLL_THREAD_ATTACH: // Clean up when the last process is going away if(g_dwAttachedProcesses == 1) { } // Do per-process clean up here... // Remember that a process has detached g_dwAttachedProcesses--; break; case DLL_THREAD_DETACH: case DLL_PROCESS_DETACH: break; } return fRetVal; }

在工程中新增沒有mycom.DEF檔案用於匯出COM介面函式，如下：

LIBRARY "mycom" EXPORTS ; EntryName [=InternalName] [@Ordinal] [NONAME] [CONSTANT] ; This is the entry point that DllMainCRTStartup calls DllMain @12 ; Standard OLE exports DllGetClassObject PRIVATE DllCanUnloadNow PRIVATE ; For self-registration. DllRegisterServer PRIVATE DllUnregisterServer PRIVATE

DllMain 函式可以不匯出。

其次，在mycom.cpp中實現匯出函式，如下：

STDAPI DllUnregisterServer(void) { return S_OK; } STDAPI DllRegisterServer(void) { return S_OK; } STDAPI DllCanUnloadNow(void) { if(!g_cLock) return S_OK; return S_FALSE; } STDAPI DllGetClassObject(REFCLSID rclsid ,REFIID riid,void **ppv) { return CLASS_E_CLASSNOTAVAILABLE; }

通過以上步驟，一個基本的DLL建立完成，這還不能稱之為COM，僅僅是一個DLL匯出了幾個空函式而已。