COM & .NET Component Services

Subscription Types

As I mentioned earlier in the chapter, there are two types of subscribers. The first type is an existing instance of a class that supports a sink interface. That instance can be added at runtime to the list of subscribers of a particular event class. This type of subscription is called transient subscription because it exists as long as the subscriber is running and will not persist or survive a system reboot or a crash.

Note that when a particular instance of a class subscribes to an event class, only that instance will receive events published using that class. Other instances will receive the events only if they transiently subscribe themselves.

Adding a transient subscription can only be done programmatically using theCOM+ Catalog interfaces and objects. There is no administrative Component Services Explorer support. On the other hand, since all you give the COM+ Catalog is a pointer to a sink, even a nonconfigured COM component can register as a transient subscription, as long as it supports the sink interface.

Tip

A transient subscription does not even need to be on a Windows 2000 machine, as long as it is registered with a COM+ Catalog on the Windows 2000 machine where the event class resides.

The second type of subscription is used when you want COM+ to instantiate an object of a particular class when an event is published, let it handle the event, and release it. This type of subscription is called persistent subscription. Every event class has a list ...