Indexing Service Ifilters

What are Ifilters?

Ifilters are basically plug-in's for Indexing Service that tell Indexing Service how to Index (e.g.. read the content and properties) a type of file as determined by the files file extension. Indexing service calls the appropriate Ifilter for a file, when it comes across it during a sweep of a file system.

Ifilters are integrated into > Indexing Service using the Ifilter interface. They are normally written in C++ as they require rather low level programming, and C++ is designed for exactly that kind of work.

In reality an Ifilter is just a DLL file that contains the code necessary to tell Indexing Service how to handle a specific file type. This isn’t strictly true though as you could say that the Ifilter itself is responsible for extracting the content from the file in a round about way.

Do you really need a new Ifilter for that strange file type?

Indexing Service can still Index a file in most cases even if it does not have an Ifilter loaded for that file type.

If the file contains text content then there is a VERY good chance Indexing Service can still Index it. You do this by telling Indexing service to “Index files with unknown extensions”.

Right click on your “My Computer” Icon and click “Manage”
Expand out “Services and Applications”
Right click on “Indexing Service” then click on “Properties”
Tick the “Index files with unknown extensions” box
Click “Ok”, then restart Indexing Service

Now Indexing Service will try to index pretty much every file it comes across.

Finding out which Ifilter DLL is used to filter a file type.

The following example, for HTML files, shows how to find the filter DLL for a document. Determine the persistent handler registered for an extension. Check to see whether the extension for the type of files that the DLL filters has a persistent handler registered under the registry entry.

\ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes.
\HKEY_LOCAL_MACHINE\SOFTWARE\Classes.htm
PersistentHandler
{EEC97550-47A9-11CF-B952-00AA0051FE20}

If this entry exists, skip to step 4 and use there. Alternatively, determine the CLSID. If there is not a persistent handler registered for the extension, find the CLSID associated with the document type under the registry entry \ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes. Let this be .

\ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes
htmlfile
= Class for WWW HTML files
CLSID
= {25336920-03F9-11CF-8FD0-00AA00686F13}

Determine the persistent handler. Using determined in Step 2, find the PersistentHandler value for the \HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes \ CLSID \ entry. Let this be .

\ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes \ CLSID
{25336920-03F9-11CF-8FD0-00AA00686F13}
= WWW HTML files
PersistentHandler
= {EEC97550-47A9-11CF-B952-00AA0051FE20}

Determine the IFilter persistent handler GUID. Using determined in Step 1 or determined in Step 3, find the IFilter Persistent Handler GUID for the document type. The value under the registry entry \ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes \ CLSID \\ PersistentAddinsRegistered \ 89BCB740-6119-101A-BCB7-00DD010655AF yields the IFilter Persistent Handler GUID for this document type.

Let this be . 89BCB740-6119-101A-BCB7-00DD010655AF is the IFilter interface GUID.

\ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes \ CLSID
{EEC97550-47A9-11CF-B952-00AA0051FE20}
= REG_SZ HTML File Persistent Handler
PersistentAddinsRegistered
{89BCB740-6119-101A-BCB7-00DD010655AF}
= REG_SZ {E0CA5340-4534-11CF-B952-00AA0051FE20}

Determine the filter DLL. Using determined in Step 4, the filter DLL can be found under the entry \ HKEY_LOCAL_MACHINE \ SOFTWARE \ Classes \ CLSID \ \ InprocServer32.

For further information on Ifilters see the Ifilter section on MSDN