Datasheet

Chapter 1: Workﬂ ow Programming Principles

As Listing 1 - 7 shows, this event handler takes the following steps:

1. It unregisters the event handler:

e.Activity.Closed -= this.Activity_Closed;

2. It casts the first argument to the ActivityExecutionContext type:

ActivityExecutionContext executionContext =

sender as ActivityExecutionContext;

3. It determines the index of the child activity that raised the Closed event and increments the

index:

int index = this.EnabledActivities.IndexOf(e.Activity);

index++;

4. If the child activity that raised the Closed event is the last child activity of the SequenceActivity

activity, it invokes the CloseActivity method on the ActivityExecutionContext object to inform

the workflow run time that the SequenceActivity activity has now completed its execution:

if (index == this.EnabledActivities.Count)

executionContext.CloseActivity();

The SequenceActivity activity, like any other composite activity, notifies the workflow run time

that it has completed its execution only after each and every single child activity is in either

Initialized or Closed state. In general, a composite activity mustn ’ t report that it has completed

its execution if one or more of its child activities are still executing. Imagine how weird it would

be if a C# { } statement block were to complete its execution while one or more program state-

ments inside the block were still executing! If a composite activity attempts to report the

completion of its execution while one or more of its child activities is not in either Closed or

Initialized state, the workflow run time raises an exception. In other words, the workflow run

time does not allow a composite activity to transition to the Closed state until all its child

activities are either in Closed or Initialized state.

You may be wondering how it would be possible for one or more child activities of a composite

activity to remain in the Initialized state. This is very similar to the C# { } statement block that

contains “ if ” and “ else if ” flow control program statements. Obviously, only the program

statements contained in one of these flow control program statements are executed. Similarly, a

logical { } flow control program statement may contain logical “ if ” and “ else if ” flow control

program statements such that only the child activities of one of these flow control program

statements are bound to execute — that is, only the Execute methods of the child activities of

one these two flow control program statements are bound to be executed. The child activities of

the flow control program statements that are not executed will remain in the Initialized state.

Keep in mind that the Initialize methods of all activities, including those that are never executed,

are invoked when the workflow is loaded into memory. As such, all activities are initially in the

Initialized state.

5. If the child activity that raised the Closed event is not the last logical program statement in the

logical { } flow control program statement, the Activity_Closed method takes the same steps

c01.indd 20c01.indd 20 8/25/08 4:02:57 PM8/25/08 4:02:57 PM