Optimizing an Event Provider
An event provider may devote a great deal of time to creating events. If no client applications use the created events, then the provider is wasting system resources. Further, WMI spends a considerable amount of resources parsing and sending complex queries to the appropriate provider. To avoid the waste of system resources and to improve the performance of your event provider, you can implement the IWbemEventProviderQuerySink interface. IWbemEventProviderQuerySink monitors the queries that client applications register with WMI by using the NewQuery and CancelQuery methods. By monitoring the registered client queries, your provider can determine what if any messages must be sent to WMI.
WMI calls NewQuery on an event provider when a client consumer registers an event filter query that contains references to events supported by that event provider. So the event provider responsible for instance modification events for the EmailClass class can be set up to generate notifications only for sender. When the provider receives a query requesting notification of changes to the subject property, the provider can start generating those notifications. In this scenario, WMI is not required to discard the notifications that report recipient changes only.
Similarly, WMI calls CancelQuery on an event provider when a client consumer deregisters an event filter query that contains references to events supported by the event provider. The purpose of CancelQuery is for the event provider to update the list of what events should be sent out.