最近在做Windows shell extension 的开发工作，对shell extension handler的注册机制有点疑问，以下摘自MSDN：http://msdn.microsoft.com/en-us/library/windows/desktop/cc144067(v=vs.85).aspx

Registering Shell Extension Handlers

A Shell extension handler object must be registered before the Shell can use it. This section is a general discussion of how to register a Shell extension handler.

Any time you create or change a Shell extension handler, it is important to notify the system that you have made a change with SHChangeNotify, specifying the SHCNE_ASSOCCHANGED event. If you do not call SHChangeNotify, the change might not be recognized until the system is rebooted.

As with all COM objects, you must create a GUID for the handler using a tool such as UUIDGEN.exe. Create a key under HKEY_CLASSES_ROOT\CLSID whose name is the string form of the GUID. Because Shell extension handlers are in-process servers, you must create an InProcServer32 key under the GUID key with the default value set to the path of the handler's DLL. Use the Apartment threading model.

Any time the Shell takes an action that can involve a Shell extension handler, it checks the appropriate registry key. The key under which an extension handler is registered thus controls when it will be called. For instance, it is a common practice to have a shortcut menu handler called when the Shell displays a shortcut menu for a member of a file type. In this case, the handler must be registered under the file type's ProgID key.

Handler Names

To enable a Shell extension handler, create a subkey with the handler subkey name (see below) under the ShellEx subkey of either the ProgID (for file types) or the Shell object type name (for Predefined Shell Objects).

For example, if you wanted to register a shortcut menu extension handler for MyProgram.1, you would begin by creating the following subkey:

HKEY_CLASSES_ROOT

   MyProgram.1

      ShellEx

         ContextMenuHandlers

For the following handlers, create a subkey underneath the "Handler Subkey name" key whose name is the string version of the CLSID of the Shell extension. Multiple extensions can be registered under the handler subkey name key by creating multiple subkeys.

Handler	Interface	Handler Subkey Name
Shortcut menu handler	IContextMenu	ContextMenuHandlers
Copyhook handler	ICopyHook	CopyHookHandlers
Drag-and-drop handler	IContextMenu	DragDropHandlers
Property sheet handler	IShellPropSheetExt	PropertySheetHandlers
Column provider handler (deprecated in Windows Vista)	IColumnProvider	ColumnHandlers

For the following handlers, the default value of the "Handler Subkey Name" key is the string version of the CLSID of the Shell extension. Only one extension can be registered for these handlers.

Handler	Interface	Handler Subkey Name
Data handler	IDataObject	DataHandler
Drop handler	IDropTarget	DropHandler
Icon handler	IExtractIconA/W	IconHandler
Image handler	IExtractImage	{BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}
Thumbnail image handler	IThumbnailProvider	{E357FCCD-A995-4576-B01F-234630154E96}
Infotip handler	IQueryInfo	{00021500-0000-0000-C000-000000000046}
Shell link (ANSI )	IShellLinkA	{000214EE-0000-0000-C000-000000000046}
Shell link (UNICODE)	IShellLinkW	{000214F9-0000-0000-C000-000000000046}
Structured storage	IStorage	{0000000B-0000-0000-C000-000000000046}
Metadata	IPropertyStore	PropertyHandler
Metadata	IPropertySetStorage (deprecated in Windows Vista)	PropertyHandler
Pin to Start Menu	IStartMenuPinnedList	{a2a9545d-a0c2-42b4-9708-a0b2badd77c8}
Pin to Taskbar		{90AA3A4E-1CBA-4233-B8BB-535773D48449}

The subkeys specified to add Pin to Start Menu and Pin to Taskbar to an item's shortcut menu are only required for file types that include the IsShortCut entry.

Support for column provider handlers was removed in Windows Vista. Also, as of Windows Vista,IPropertySetStorage has been deprecated in favor of IPropertyStore.

While IExtractImage remains supported, IThumbnailProvider is preferred for Windows Vista and later.

Predefined Shell Objects

The Shell defines additional objects under HKEY_CLASSES_ROOT which can be extended in the same way as file types. For example, to add a property sheet handler for all files, you can register under thePropertySheetHandlers key.

HKEY_CLASSES_ROOT

   *

      shellex

         PropertySheetHandlers

The following table gives the various subkeys of HKEY_CLASSES_ROOT under which extension handlers can be registered. Note that many extension handlers cannot be registered under all of the listed subkeys. For further details, see the specific handler's documentation.

Subkey	Description	Possible Handlers	Version
*	All files	Shortcut Menu, Property Sheet, Verbs (see below)	All
AllFileSystemObjects	All files and file folders	Shortcut Menu, Property Sheet, Verbs	4.71
Folder	All folders	Shortcut Menu, Property Sheet, Verbs	All
Directory	File folders	Shortcut Menu, Property Sheet, Verbs	All
Directory\Background	File folder background	Shortcut Menu only	4.71
Drive	All drives in MyComputer, such as "C:\"	Shortcut Menu, Property Sheet, Verbs	All
Network	Entire network (under My Network Places)	Shortcut Menu, Property Sheet, Verbs	All
Network\Type\#	All objects of type # (see below)	Shortcut menu, Property Sheet, Verbs	4.71
NetShare	All network shares	Shortcut menu, Property Sheet, Verbs	4.71
NetServer	All network servers	Shortcut menu, Property Sheet, Verbs	4.71
network_provider_name	All objects provided by network provider "network_provider_name"	Shortcut menu, Property Sheet, Verbs	All
Printers	All printers	Shortcut Menu, Property Sheet	All
AudioCD	Audio CD in CD drive	Verbs only	All
DVD	DVD drive (Windows 2000)	Shortcut Menu, Property Sheet, Verbs	4.71

Notes:

The file folder background shortcut menu is accessed by right-clicking within a file folder, but not over any of the folder's contents.
"Verbs" are special commands registered under HKEY_CLASSES_ROOT\Subkey\Shell\Verb .
For Network\Type\# , "#" is a network provider type code in decimal. The network provider type code is the high word of a network type. The list of network types is given in the Winnetwk.h header file (WNNC_NET_* values). For example, WNNC_NET_SHIVA is 0x00330000, so the corresponding type key would be HKEY_CLASSES_ROOT\Network\Type\51 .
"network_provider_name" is a network provider name as specified by WNetGetProviderName, with the spaces converted into underscores. For example, if the Microsoft Networking network provider is installed, its provider name is "Microsoft Windows Network", and the correspondingnetwork_provider_name is Microsoft_Windows_Network.

Example of an Extension Handler Registration

To enable a particular handler, create a subkey under the extension handler type key with the name of the handler. The Shell does not use the handler's name, but it must be different from all other names under that type subkey. Set the default value of the name subkey to the string form of the handler's GUID.

The following example illustrates registry entries that enable shortcut menu and property sheet extension handlers, using an example .myp file type:

HKEY_CLASSES_ROOT

   .myp

      (Default) = MyProgram.1

   CLSID

      {00000000-1111-2222-3333-444444444444}

         InProcServer32

            (Default) = C:\MyDir\MyCommand.dll

            ThreadingModel = Apartment

      {11111111-2222-3333-4444-555555555555}

         InProcServer32

            (Default) = C:\MyDir\MyPropSheet.dll

            ThreadingModel = Apartment

   MyProgram.1

      (Default) = MyProgram Application

      Shellex

         ContextMenuHandler

            MyCommand

               (Default) = {00000000-1111-2222-3333-444444444444}

         PropertySheetHandlers

            MyPropSheet

               (Default) = {11111111-2222-3333-4444-555555555555}

The registration procedure discussed in this section must be followed for all Windows systems.