C# 4 and .NET 4

1314 ❘ ChaPTer 44 windOws wOrkflOw fOundAtiOn 4
At runtime, the Parallel activity schedules each immediate child for execution. The underlying runtime
execution engine will then schedule these children in a FIFO (first in, first out) manner, thereby providing
the illusion of parallel execution; however, they run only on a single thread.
To include true parallel execution, the activities you drop into the Parallel activity must be derived from
the AsyncCodeActivity class. The sample code in 02_ParallelExecution includes an example that shows
how to asynchronously process code within two branches of a Parallel activity. Figure 44-6 shows the use
of two InvokeMethod activities within a Parallel activity.
figure 44-6
The InvokeMethod activities used here call two simple methods, DoJob1 and DoJob2, which sleep for two
and three seconds, respectively. In order to run these methods asynchronously, there is one final change that
is needed. The InvokeMethod activity has a Boolean RunAsynchronously property that defaults to False.
Setting this in the UI to True then calls the target method asynchronously, thereby allowing the Parallel
activity to execute more than one activity at the same time. With a uni-processor machine, two threads will
execute, giving the illusion of simultaneous execution; however, on a multiprocessor machine, these threads
may well be scheduled on different cores, thereby providing true parallel execution. If you are creating your
own activities, it is worthwhile creating these as asynchronous activities, as then the end user can get the
benefits of true parallel execution.
delay activity
Business processes often need to wait for a period before completing. Consider using a workflow for expense
approval. Your workflow might send an e-mail to your immediate manager asking him or her to approve
your expense claim. The workflow then enters a waiting state where it waits for approval (or, horror of
horrors, rejection); but it would also be nice to define a timeout so that if no response is returned within,
say, one day, the expense claim is then routed to the next manager up the chain of command.
The Delay activity can form part of this scenario (the other part is the Pick activity defined in the next
section). Its job is to wait for a predefined time before continuing execution of the workflow.
The Delay activity contains a Duration property, which can be set to a discrete TimeSpan value, but, as it
is defined as an expression, this value could be linked to a variable within the workflow or computed from
some other values as required.
When a workflow is executed, it enters an Idle state in which it runs a Delay activity. Workflows that are
idle are candidates for persistence — this is where the workflow instance data is stored within a persistent
medium (such as a SQL Server database), and the workflow itself can then be unloaded from memory.
This conserves system resources, as only running workflows need to be in memory at any given time. Any
workflows that are delayed will be persisted to disk.
Pick activity
A common programming construct is to wait for one of a set of possible events — one example of this is the
WaitAny method of the WaitHandle class in the System.Threading namespace. The Pick activity is the way
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