One standard mechanism to access and extract text from documents is provided by the IFilter plug-in interface used in Microsoft search engines. There are a few IFilter implementations developed by Microsoft and other vendors that cover a variety of file formats. The standard or reliability and text extraction quality varies across multiple IFilter developers.
Opait Text Filters is a small utility program with a simple interface to IFilters that are already installed on the host computer as well as a few custom text extraction filters which work directly with file formats and improve upon the default IFilter implementations.
The interface to extract text is provided by a small class library called Opait.Filters which is included and can be used to integrate text filters into .NET applications.