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
typedef struct tagDICTIONARY
DWORD cbEntries ; // Count of entries in the list
ENTRY rgEntry[ cbEntries ] ; // Property ID/String pair
} DICTIONARY ;
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
} ENTRY ;
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
- 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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