Creating a Publisher Filter

There are two methods to establish the publisher filter: using the MultiPublisherFilterCLSID property of the event class, or using IEventControl::SetPublisherFilter.

• Because it allows you to compose the event object with the COM+ queued components service, the preferred method is to use the MultiPublisherFilterCLSID property in the event class to set the publisher filter. This establishes one filter object for all the methods of the event interfaces for an event object. The event object activates the publisher filter when the event class object is instantiated using CoCreateInstance.
• You can also use SetPublisherFilter. However, if you choose this method, the interface is not queuable and therefore cannot use the event object with the COM+ queued components service between the publisher and the event class object. For additional information about using the queued components service with COM+ Events, see Using COM+ Events with COM+ Queued Components.

An event can have one or more filter objects or none at all. The publisher filter objects must support either IPublisherFilter or IMultiInterfacePublisherFilter, depending on whether you have a single firing interface or multiple firing interfaces on the event class object. The IPublisherFilter and IMultiInterfacePublisherFilter interfaces both specify an Initialize method. The Initialize method is called by the event class object immediately after creating the filter object.

COM+ Events tries to invoke two methods on the filter. First it calls IPublisherFilter::PrepareToFire and passes an IFiringControl interface pointer to the filter. Then it queries the filter object for the event interface. If the filter supports the event interface, it invokes a method on it. This provides access to the publisher parameters for an event. The filter can use these parameters to determine which subscriptions to fire.

Related topics

Filtering Events in COM+