C# 4 and .NET 4

OC306 ❘ ChaPTer 57 windOws wOrkflOw fOundAtiOn 3.0
can be correlated with the activities to produce the image in Figure 57-20.
It is also possible to extract data from the workflow as it executes, which
could be used to form an audit trail of the execution flow of the workflow.
Custom services
In addition to built-in services such as the persistence service and the
tracking service, you can add your own objects to the services collection
maintained by the WorkflowRuntime. These services are typically defined
using an interface and an implementation, so that you can replace the
service without recoding the workflow.
The state machine presented earlier in the chapter uses the following
interface:
[ExternalDataExchange]
public interface IDoorService
{
void LockDoor();
void UnlockDoor();
event EventHandler RequestEntry;
event EventHandler OpenDoor;
event EventHandler CloseDoor;
event EventHandler FireAlarm;
figure 57-20
}
void OnRequestEntry(Guid id);
void OnOpenDoor(Guid id);
void OnCloseDoor(Guid id);
void OnFireAlarm();
The interface consists of methods that are used by the workflow to call the service and events raised by the
service that are consumed by the workflow. The use of the ExternalDataExchange attribute indicates to
the workflow runtime that this interface is used for communication between a running workflow and the
service implementation.
Within the state machine, there are a number of instances of the CallExternalMethodActivity that are
used to call methods on this external interface. One example is when the door is locked or unlocked — the
workflow needs to execute a method call to the UnlockDoor or LockDoor methods, and the service responds
by sending a command to the door lock to unlock or lock the door.
When the service needs to communicate with the workflow, this is done by using an event, because the
workflow runtime also contains a service called the ExternalDataExchangeService, which acts as a proxy
for these events. This proxy is used when the event is raised, because the workflow may not be loaded in
memory at the time the event is delivered. So the event is first routed to the external data exchange service,
which checks to see if the workflow is loaded, and, if not, rehydrates it from the persistence store and then
passes the event on into the workflow.
The code used to construct the ExternalDataExchangeService and to construct proxies for the events
defined by the service is shown here:
WorkflowRuntime runtime = new WorkflowRuntime();
ExternalDataExchangeService edes = new ExternalDataExchangeService();
runtime.AddService(edes);
DoorService service = new DoorService();
edes.AddService(service);
This constructs an instance of the external data exchange service and adds it to the runtime. It then creates
an instance of the DoorService (which itself implements IDoorService) and adds this to the external data
exchange service.
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