How File Viewers Are Instantiated

The system doesn't directly call a file viewer; instead, it starts an instance of QUIKVIEW.EXE for each file to be viewed. QUIKVIEW is a program that starts the file viewing process and creates a message queue for file viewers. The program then associates a path with a file viewer, instantiates the file viewer object, and tells the file viewer to load and display the file. At this point, QUIKVIEW turns over execution of the process it created to the file viewer until the file viewer shuts down. When determining which file viewer to instantiate, QUIKVIEW uses one of these three methods:

• If the file is an OLE compound file, it contains a class identifier (CLSID) that identifies its type. QUIKVIEW uses this CLSID to determine the file type and which viewer to use.
• If the file is not an OLE compound file but begins with a consistent and unique byte pattern, the application that created it can register the byte pattern in the Registry and associate the pattern with a CLSID, which will determine the file type.
• If the file is not an OLE compound file (or if it is but lacks a CLSID), QUIKVIEW uses the filename extension to identify the file type.

If a file type is registered but the system can't find its file viewer, or if the Quick View operation fails, QUIKVIEW displays the message There is no viewer capable of viewing <type of file> files.