\nLet’s say we have the a simple COM local server called COM3264Server.exe. There is just one interface called ICoCOM3264Server. Here is the IDL file:<\/p>\n
\r\n[\r\n\tobject,\r\n\tuuid(733C70A7-F7EC-4C15-85D2-5CDB14F4110B),\r\n\tdual,\r\n\tnonextensible,\r\n\tpointer_default(unique)\r\n]\r\ninterface ICoCOM3264Server : IDispatch{\r\n [id(1), helpstring(\"Says hello\")] HRESULT SayHello(void);\r\n};\r\n[\r\n\tuuid(2F25FC66-2380-42FD-8476-8B5917FB1BF1),\r\n\tversion(1.0),\r\n]\r\nlibrary COM3264ServerLib\r\n{\r\n\timportlib(\"stdole2.tlb\");\r\n\t[\r\n\t\tuuid(9268A299-E817-4C5D-8627-C2582B66F16D)\t\t\r\n\t]\r\n\tcoclass CoCOM3264Server\r\n\t{\r\n\t\t[default] interface ICoCOM3264Server;\r\n\t};\r\n};\r\n<\/pre>\nThe implementation of the method SayHello() is straight forward; it just displays a message box with a text that varies between the two architectures, x64 and x86.<\/p>\n
\r\nSTDMETHODIMP CCoCOM3264Server::SayHello(void)\r\n{\r\n#ifdef _WIN64\r\n ::MessageBox(NULL, _T(\"Hello from 64-bit COM server!\"), _T(\"COM3264Server\"), MB_OK);\r\n#else\r\n ::MessageBox(NULL, _T(\"Hello from 32-bit COM server!\"), _T(\"COM3264Server\"), MB_OK);\r\n#endif\r\n\r\n return S_OK;\r\n}\r\n<\/pre>\nRegistry<\/strong>
\nWhen you register the COM server, the 32-bit and 64-bit versions are registered in different parts of the registry. On 64-bit machine, the registry has two views (or modes):<\/p>\n\n- a native view, for 64-bit application; (e.g. registry path for CLSIDs is HKEY_CLASSES_ROOT\\CLSID\\)<\/li>\n
- a WOW64 view, which enables redirections for 32-bit applications, a process transparent to the user (e.g. registry path for CLSIDs is HKEY_CLASSES_ROOT\\Wow6432Node\\CLSID\\)<\/li>\n<\/ul>\n
Here is the Registry registration of the (native) 64-bit COM server (notice the registry key in the status bar of the editor, and the path to the server executable).
\n![\"\"](\"\/blog\/wp-content\/uploads\/2010\/11\/com3264server_reg64.png\" "\\"Native")
<\/p>\n
On, the other hand, the 32-bit COM server is registered under the Wow6432 node.
\n![\"\"](\"\/blog\/wp-content\/uploads\/2010\/11\/com3264server_reg32.png\" "\\"Wow64")
<\/p>\n
So if both versions are registered in Windows Registry, which one is picked? Well, both the server and the client can specify which architecture to use:<\/p>\n
\n- the COM server can do this via the PreferredServerBitness<\/strong> Registry value<\/li>\n
- the client can do this using one of the flags CLSCTX_ACTIVATE_32_BIT_SERVER<\/strong> and CLSCTX_ACTIVATE_64_BIT_SERVER<\/strong>, which overrides the server preference<\/li>\n<\/ul>\n
If neither the client nor the server specifies a preference, then:<\/p>\n
\n- If the computer that hosts the server is running Windows Server 2003 with Service Pack 1 (SP1) or a later system, then COM will try to match the server architecture to the client architecture. In other words, for a 32-bit client, COM will activate a 32-bit server if available; otherwise it will activate a 64-bit version of the server. For a 64-bit client, COM will activate a 64-bit server if available; otherwise it will activate a 32-bit server.<\/li>\n
- If the computer that hosts the server is running Windows XP or Windows Server 2003 without SP1 or later installed, then COM will prefer a 64-bit version of the server if available; otherwise it will activate a 32-bit version of the server.<\/li>\n<\/ul>\n
Server Preference<\/strong>
\nThe server can specify its preferred architecture in the PreferredServerBitness<\/a> value under AppId (available only on 64-bit Windows). This integer value can be:<\/p>\n\n- 1<\/strong>: Match the server architecture to the client architecture. For example, if the client is 32-bit, use a 32-bit version of the server, if it is available. If not, the client’s activation request will fail.<\/li>\n
- 2<\/strong>: Use a 32-bit version of the server. If one does not exist, the client’s activation request will fail.<\/li>\n
- 3<\/strong>: Use a 64-bit version of the server. If one does not exist, the client’s activation request will fail.<\/li>\n<\/ul>\n
Here is the value set in Registry to specify the 64-bit architecture.
\n![\"\"](\"\/blog\/wp-content\/uploads\/2010\/11\/com3264server_regappid.png\" "\\"Registry")
<\/p>\n
When you run the client, it launches the 64-bit version and in my example the following window pops-up:
\n![\"\"](\"\/blog\/wp-content\/uploads\/2010\/11\/com3264server_64bit.png\" "\\"64-bit")
<\/p>\n
If I change the value to indicate the 32-bit architecture, the other COM server is launched and the displayed message is:
\n![\"\"](\"\/blog\/wp-content\/uploads\/2010\/11\/com3264server_32bit.png\" "\\"32-bit")
<\/p>\n
Client Preference<\/strong>
\nThe client code I used so far looked like this:<\/p>\n\r\n ICoCOM3264Server* pServer;\r\n\r\n HRESULT hr = ::CoCreateInstance(\r\n CLSID_CoCOM3264Server, \r\n NULL, \r\n CLSCTX_LOCAL_SERVER|CLSCTX_REMOTE_SERVER, \r\n IID_ICoCOM3264Server, \r\n (void**)&pServer);\r\n\r\n if(SUCCEEDED(hr))\r\n {\r\n pServer->SayHello();\r\n\r\n pServer->Release();\r\n }\r\n<\/pre>\nHowever, the 64-bit version of Windows added new flags to the CLSCTX<\/a> enumeration.<\/p>\n\n- CLSCTX_ACTIVATE_32_BIT_SERVER<\/strong>: used to activate or connect to a 32-bit version of the server; fail if one is not registered.<\/li>\n
- CLSCTX_ACTIVATE_64_BIT_SERVER<\/strong>: used to activate or connect to a 64 bit version of the server; fail if one is not registered.<\/li>\n<\/ul>\n
As I mentioned earlier, if the client uses one of these flags, it overrides the server preference (specified via the PreferredServerBitness Registry value). <\/p>\n
In the following example, the client requests the 64-bit COM server:<\/p>\n
\r\n HRESULT hr = ::CoCreateInstance(\r\n CLSID_CoCOM3264Server, \r\n NULL, \r\n CLSCTX_LOCAL_SERVER|CLSCTX_REMOTE_SERVER|CLSCTX_ACTIVATE_64_BIT_SERVER, \r\n IID_ICoCOM3264Server, \r\n (void**)&pServer);\r\n<\/pre>\nAnd if you run it, no matter what the server specified, the 64-bit COM server is launched.<\/p>\n
To read more about the subject see the MSDN links above.<\/p>\n","protected":false},"excerpt":{"rendered":"
It is possible to register both 32-bit and 64-bit versions of the same COM server on 64-bit machine. This leads to several questions such as how are they registered and which one of the two is used. I will try to answer them below. But first, let’s start with an example. Example Let’s say we … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","advgb_blocks_editor_width":"","advgb_blocks_columns_visual_guide":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[237,7,211,38,146],"tags":[451,461,212,238],"class_list":["post-775","post","type-post","status-publish","format-standard","hentry","category-64bit","category-c","category-com","category-tutorials","category-windows","tag-c","tag-com","tag-registry","tag-x64"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t