Storing Property Sets

Applications can expose some of the state of their documents so that other applications can locate and read that information. Some examples are a property set describing the author, title, and keywords of a document created with a word processor, or the list of fonts used in a document. This facility is not restricted to documents; it can also be used on embedded objects. Generally, access to property sets should be through the IPropertySetStorage and IPropertyStorage interfaces, but this section describes the previously recommended way.

Note If you are storing a property set that is internal to your application, you might not want to follow the guidelines described below. If you want to expose your property set to other applications, you need to follow these guidelines.

To store a property set in a compound file:

Create an IStorage or IStream instance in the same level of the storage structure as its data streams. Prepend the name of your IStorage or IStream instance with "\005." Stream and storage names that begin with 0x05 are reserved for common property sets that can be shared among applications. Also, streams beginning with that value are limited to 256K. The names can be selected from either published names and formats, or by assigning the property set a FMTID and deriving the name from the FMTID according to the conventions described in the section "Naming Property Sets".
A property set may be stored in a single IStream instance or in an IStorage instance containing multiple streams and storages. In the case of an IStorage instance, the contained stream named "Contents" is the primary stream containing property values, where some values may be names of other streams or storage instances within the storage for this property set.
Specify the CLSID of the object class that can display and/or provide programmatic access to the property values. If there is no such class, the CLSID should be set to the property set's format identifier. For a property set that uses an IStorage instance, either set the CLSID of the IStorage instance to be the same as that stored in the Contents stream or to CLSID_NULL (the value in a newly created IStorage instance).
You have the option of specifying displayable names that form the contents of the dictionary.

Some applications can read only implementations of property sets stored as IStream instances. Applications should be written to expect that a property set may be stored in either an IStorage or IStream instance, unless the property set definition indicates otherwise. For example, the Summary Information property set's definition says that it can only be stored in a named IStream instance. In cases where you are searching for a property set and don't know whether it is a storage or stream, look for an IStream instance with your property set name first. If that fails, look for an IStorage instance.

To better understand storing property sets in an IStorage implementation, suppose there is a class of applications that edit information about animals. First, a CLSID (CLSID_AnimalApp) is defined for this set of applications, so they can indicate that they understand property sets containing animal information (FMTID_AnimalInfo), and others containing medical information (FMTID_MedicalInfo).

IStorage (File): "C:\OLE\REVO.DOC" 
  IStorage: "\005AnimalInfo", CLSID = CLSID_AnimalApp 
    IStream: "Contents" 
      WORD wByteOrder, WORD wFmtVersion, DWORD dwOSVer, 
      CLSID CLSID_AnimalApp, DWORD cSections... 
      ... 
      FMTID = FMTID_AnimalInfo 
      Property: Type = PID_ANIMALTYPE, Type = VT_LPWSTR, Value = L"Dog" 
      Property: Type = PID_ANIMALNAME, Type = VT_LPWSTR, Value = L"Revo" 
      Property: Type = PID_MEDICALHISTORY, Type = VT_STREAMED_OBJECT, 
              Value = "MedicalInfo" 
      ... 
    IStream: "MedicalInfo" 
      WORD wByteOrder, WORD wFmtVersion, DWORD dwOSVer, 
      CLSID CLSID_AnimalApp, DWORD cSections... 
      ... 
      FMTID = CLSID_MedicalInfo 
      Property: Type = PID_VETNAME, Type = VT_LPWSTR, Value = L"Dr. Woof" 
      Property: Type = PID_LASTEXAM, Type = VT_DATE, Value = ...

Note that the class identifiers of the IStorage interface and both property sets is CLSID_AnimalApp. This identifies any application that can display and/or provide programmatic access to these property sets. Any application can read the information within the property sets (the point behind property sets), but only applications identified with the class identifier of CLSID_AnimalApp can understand the meaning of the data in the property sets.