The IMSLogon::CompareEntryIDs method compares two entry identifiers to determine if they refer to the same object. MAPI only refers this call to a service provider if the unique identifiers (UIDs) in both entry identifiers to be compared are handled by that provider.
See IMSLogon : IUnknown.
HRESULT CompareEntryIDs(
ULONG cbEntryID1,
LPENTRYID lpEntryID1,
ULONG cbEntryID2,
LPENTRYID lpEntryID2,
ULONG ulFlags,
ULONG FAR * lpulResult
);
Message store providers implement the IMSLogon::CompareEntryIDs method to compare two entry identifiers for a given entry within a message store to determine whether they refer to the same object. If the two entry identifiers refer to the same object, then CompareEntryIDs sets the lpulResult parameter to TRUE; if they refer to different objects, CompareEntryIDs sets lpulResult to FALSE.
CompareEntryIDs is useful because an object can have more than one valid entry identifier; such a situation can occur, for example, after a new version of a message store provider is installed.