Returning Properties from a Filter Handler

Properties are extracted from items using registered property handlers, or using filters registered for specific file types. A filter handler (an implementation of the IFilter interface) can interpret the contents of a file type in any number of ways.

This topic is organized as follows:

Property Filtering

Best practices for property filtering are listed in the following table.

Method Description
IFilter::Init Returns the IFILTER_FLAGS enumeration. If the IFILTER_FLAGS_OLE_PROPERTIES member of this enumeration is set to one, then Windows Search uses the IPropertySetStorage and IPropertyStorage interfaces interfaces to enumerate and access external value-type properties.
IFilter::GetChunk Returns information from a document in "chunks" with chunk type (text or value), name, and locale. A chunk contains one document property.
IFilter::GetText Gets a text-type property from a chunk.
IFilter::GetValue Gets a value-type property from a chunk.

The following illustration shows an example document. The external value-type property DocTitle (obtained using methods of the IPropertySetStorage and IPropertyStorage interfaces) and the internal value-type property Book (obtained as a result of a custom IFilter implementation) describe the document as a whole. The text-type properties Contents and Chapter describe the content of the document. When processing this document, the filter handler (an implementation of the IFilter interface) identifies and extracts these properties.

diagram showing the elements of a typical document

Property Size Limitations

There are two potential limitations to property size:

  • The maximum size of data that Windows Search accepts per file.
  • The maximum size per property as defined in the property description file.

Currently, Windows Search does not use the defined property size when calculating the amount of data it accepts from an item. Instead, the limit Windows Search uses is the product of the size of the file and the MaxGrowFactor (file size N * MaxGrowFactor) read from the registry. The default MaxGrowFactor is four.

         Gathering Manager

Consequently, if your file type tends to be small in total size but have larger properties, Windows Search may not accept all the property data you want to emit. However, you can increase the MaxGrowFactor to suit your needs.

Additional Resources

Developing Filter Handlers

About Filter Handlers in Windows Search

Best Practices for Creating Filter Handlers in Windows Search

Filter Handlers that Ship with Windows

Implementing Filter Handlers in Windows Search

Registering Filter Handlers

Testing Filter Handlers