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Client applications and message store providers
Property attributes can only be accessed on property objects, that is, objects implementing the IMAPIProp : IUnknown interface. To make MAPI properties available on an OLE structured storage object, OpenIMsgOnIStg builds an IMessage : IMAPIProp object on top of the OLE IStorage object. The property attributes on such objects can be set or altered with SetAttribIMsgOnIStg and retrieved with GetAttribIMsgOnIStg.
GetAttribIMsgOnIStg and SetAttribIMsgOnIStg do not operate on all IMessage objects. They are only valid for IMessage-on-IStorage objects returned by OpenIMsgOnIStg.
The number and positions of the attributes in the lppPropAttrArray parameter correspond to the number and positions of the property tags in the lpPropTagArray parameter.