Datasheet

Chapter 1: Workﬂ ow Programming Principles

foreach (Activity activity in this.EnabledActivities)

{

activity.Closed += Activity_Closed;

executionContext.ExecuteActivity(activity);

}

3. Finally, it returns ActivityExecutionStatus.Executing to inform the workflow run time that

ParallelActivity has not completed its execution yet because it is waiting for its child activities to

complete their execution. As mentioned earlier, a composite activity is not allowed to report the

completion of its execution unless every single one of its child activities is in either Initialized or

Closed state:

return ActivityExecutionStatus.Executing;

ParallelActivity scheduled all its child activities with the WF scheduler for execution in one shot. This

means that the WF scheduler now contains one work item for each child activity of ParallelActivity.

Each work item is basically a delegate that encapsulates a reference to the Execute method of the

respective child activity. Because the WF scheduler uses the FIFO algorithm to dispatch work items, the

child activities are executed in the order in which they were scheduled, which is basically the order in

which they were added to ParallelActivity.

The Execute method of each child activity may need to suspend the execution of the child activity

waiting indefinitely for external entities to deposit data into the appropriate workflow queues before the

child activity can resume its execution. When a child activity finally completes its execution, it raises its

Closed event, which in turn triggers the scheduling of the Activity_Closed event handler for execution.

This means that each child activity will end up adding a work item to the WF scheduler, where each

work item is a delegate that references the Activity_Closed event handler. In other words, the same

Activity_Closed event handler is invoked once for each child activity in the ParallelActivity activity.

When the WF scheduler invokes the Activity_Closed event handler, it passes an

ActivityExecutionContext object into it as its first argument. This object represents the execution context

of the event handler.

Let ’ s walk through the implementation of this event handler (refer to Listing 1 - 8 ). The event handler first

removes itself from the list of event handlers registered for the Closed event of the child activity that

triggered the invocation of the event handler:

e.Activity.Closed -= this.Activity_Closed;

Next, the event handler iterates through the child activities to check whether all child activities are in

either Initialized or Closed state. If even one child activity is not in one of these two states, the event

handler returns without reporting any change in the execution status of ParallelActivity. This means that

as far as the workflow run time is concerned, ParallelActivity is still in the Executing state because the

last time the workflow run time heard of a change in the execution status of ParallelActivity was when

the Execute method returned the ActivityExecutionStatus.Executing enumeration value:

foreach (Activity activity in this.EnabledActivities)

{

if (activity.ExecutionStatus != ActivityExecutionStatus.Initialized ||

activity.ExecutionStatus != ActivityExecutionStatus.Closed)

return;

}

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