The GetService function can be used inside of an activity to return a reference to one of the BaseRuntimeServices derived classes that have been added to the WorkflowRuntime.

The GetService function is defined on a number of classes and interfaces like WorkflowRuntime, ActivityExecutionContext, ExternalDataExchangeService and IServiceProvider.

Depending on the object used to call GetService a number of RuntimeServices might not be returned, a null/Nothing is returned instead. This is the case with a ActivityExecutionContext which will not return any of the following: WorkflowSchedulerService, WorkflowPersistenceService, TrackingService, WorkflowCommitWorkBatchService, WorkflowLoaderService or the WorkflowRuntime itself.

http://msdn.microsoft.com/en-us/library/ms592893.aspx