Content Classes

A content class is different from a content type. A content type defines the type of data that is stored in an item's stream. For example, a common content type is "application/msword"; this content type value indicates that the item's stream contains a Microsoft® Word document in binary form. However, it neither defines the intent or purpose of the Word file to an application nor specifies what properties are stored with the item. For instance, one such file might be a Ph.D. thesis, and another might be a resume'.

A good example of a content class is an expense report. An expense report can be in the format of a Microsoft Excel worksheet, a Word document, or an e-mail message. However, regardless of the format, there may be a common set of properties stored with the expense report. For example, an expense-tracking application might store the date range in the report, the total dollar amount in the report, and the approval state of the report. Each item stored by the application can have these properties defined, irrespective of the item's content type (e-mail message or Microsoft Office document).

A content class is used frequently by applications to do the following:

• Indicate what processing should occur for the specified item.
• Instantiate an object to render or manipulate the specified item.
• Enhance the user interface, such as by displaying an icon appropriate for the specified content class.
• Render items using forms that are based on the specified item's content class.

The following illustration shows the relationship between property and content class definitions and how you can extend a content class, thereby refining and inheriting both its intent and its associated properties.

The following topic discusses content class definitions and inheritance in further detail:

Content Class and Inheritance