Property ID Zero

To enable users of property sets to attach meaning to properties beyond those provided by the type indicator, property ID zero is reserved for an optional dictionary of displayable names for the property set.

The dictionary contains a count of entries in the list followed by a list of dictionary entries.

typedef struct tagDICTIONARY


DWORD cbEntries ; // Count of entries in the list

ENTRY rgEntry[ cbEntries ] ; // Property ID/String pair


Each dictionary entry in the list is a Property ID/String pair. This can be illustrated using the following pseudo-structure definition for a dictionary entry (it's a pseudo-structure because the sz[] member is variable in size):

typedef struct tagENTRY


DWORD dwPropID ; // Property ID

DWORD cb ; // Count of characters in the string

char sz[cb]; // Zero-terminated string


Note the following about property set dictionaries:

  • Property ID Zero does not have a type indicator. The DWORD that indicates the count of entries sits in the type indicator position.

  • The count of characters in the string (cb) includes the zero character that terminates the string.

  • The dictionary is entirely optional. Not all the names of properties in the set need appear in the dictionary. The dictionary can omit entries for properties assumed to be universally known by clients that manipulate the property set. Typically, names for the base property sets for widely accepted standards are omitted, but special purpose property sets may include dictionaries for use by browsers.

  • Property names in the dictionary are stored in the code page indicated by Property ID One (see below). For all code pages other than the Unicode code page (1200), each name is byte aligned. Thus, there is no padding between property names with Property ID Zero. If the Unicode code page is specified, the property name strings must be aligned on 32-bit boundaries.

  • Property names that begin with the binary Unicode characters 0x0001 through 0x001F are reserved for future use.

The dictionary is stored as a VT_BLOB | VT_VECTOR value. Each BLOB in the vector contains the prerequisite DWORD count of bytes followed by a DWORD indicating the Property ID followed by the name (a zero-terminated string).

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