The wpAddToObjUseList method adds items to the object's in-use list. When an object's memory is allocated, wpAllocMem calls wpAddToObjUseList to add a USAGE_MEMORY item to the object's in-use list. The allocated memory immediately follows the MEMORYITEM data structure. The following figure shows the syntax of the MEMORYITEM data structure:

typedef struct _MEMORYITEM
{
  ULONG   cbBuffer;   /* Number of bytes in memory block */
}MEMORYITEM;

The wpOpen method calls wpAddToObjUseList to add a USAGE_OPENVIEW item to the object's in-use list. The wpSwitchTo method scans the in-use list for USAGE_OPENVIEW items to give focus to an already existing view. In general, wpViewObject is used instead of wpOpen because wpViewObject considers the setting in the Object open behavior group-box control on the Window page of the Settings notebook, and calls either wpOpen or wpSwitchTo. The following figure shows the syntax of the VIEWITEM data structure:

typedef struct _VIEWITEM
{
  ULONG   view;              /* Object view that this represents */
  LHANDLE handle;            /* Open handle                      */
  ULONG ulViewState;         /* View state flags                 */
  HWND hwndCnr;              /* Used only by system              */
  PMINIRECORDCORE pRecord;   /* Used only by system              */
}VIEWITEM;

When wpSetLinkToObject is called to link a shadow to the object it points to, wpAddToObjUseList is called to add the USAGE_LINK item to the original object's in-use list. The following figure shows the syntax of the LINKITEM data structure:

typedef struct _LINKITEM
{
  WPObject *LinkObj;         /* The link object  */
}LINKITEM;

When a data file is opened, its associated program is started and a USAGE_OPENFILE use-item is added to the data file's in-use list. The following figure shows the syntax of the VIEWFILE data structure:

typedef struct _VIEWFILE
{
  ULONG   ulMenuId;          /* Menu id if association or menu page */
  LHANDLE handle;            /* Open handle                         */
  HWND    hwndCnr;           /* Used only by system                 */
  PMINIRECORDCORE pRecord;   /* Used only by system                 */
}VIEWFILE;