public Activity this[WorkflowIdentity definitionIdentity]
 {
     get
     {
         return(ActivityPersistenceHelper.LoadActivity(InstanceDefinitions[definitionIdentity]));
     }
 }
 public Activity this[Guid id] {
     get
     {
         return(ActivityPersistenceHelper.LoadActivity(InstanceDefinitions[id]));
     }
 }