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About Window Properties

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A window property is any data assigned to a window. A window property is usually a handle of the window-specific data, but it may be any value. Each window property is identified by a string name. There are several functions that enable applications to use window properties. This overview discusses the following topics:

Advantages of Using Window Properties

Window properties are typically used to associate data with a subclassed window or a window in a multiple-document interface (MDI) application. In either case, it is not convenient to use the extra bytes specified in the CreateWindow function or class structure for the following two reasons:

  • An application might not know how many extra bytes are available or how the space is being used. By using window properties, the application can associate data with a window without accessing the extra bytes.
  • An application must access the extra bytes by using offsets. However, window properties are accessed by their string identifiers, not by offsets.

For more information about subclassing, see Window Procedure Subclassing. For more information about MDI windows, see Multiple Document Interface.

Assigning Window Properties

The SetProp function assigns a window property and its string identifier to a window. The GetProp function retrieves the window property identified by the specified string. The RemoveProp function destroys the association between a window and a window property but does not destroy the data itself. To destroy the data itself, use the appropriate function to free the handle that is returned by RemoveProp.

Enumerating Window Properties

The EnumProps and EnumPropsEx functions enumerate all of a window's properties by using an application-defined callback function. For more information about the callback function, see PropEnumProc.

EnumPropsEx includes an extra parameter for application-defined data used by the callback function. For more information about the callback function, see PropEnumProcEx.