Thread Safety in ManualWorkflowSchedulerService.RunWorkflow

No, I do not think so. The following is a related excerpt from the book 'Essential Windows Workflow Foundation'

Although operations with WorkflowInstance can be invoked by arbitrary threads, the WF scheduler, hosted within a program instance, is served by a single thread. The WF runtime ensures that no other thread can interfere with or service the scheduler while its dispatch loop is actively processing work items. To be clear, a hosting application can call the WorkflowInstance on methods on separate threads at the same time - this does not affect the scheduler for executing activities on a dedicated thread (for a sequence of execution).

EDIT: To further explore the issue, I created a wf with ParallelActivity

which contains two steps HandleExternalEvent

. The handler for invoked

each action simply makes its thread sleep for 3 seconds. In the host program, I created two threads and triggered two events through the service. Also, I will subclass ManualWorkflowSchedulerService

to keep track of its method Schedule

. Here are the results (time in 10ms):

Src    Time Thread
HOST   7616      1 CreateWorkflow
MWSS   7642      1 Schedule workflow
HOST   8297     12 Trigger event 1 and wait for RunWorkflow
MWSS   8316     12 Schedule workflow
  WF   8327     12 Handler 1 Invoked...wait 3 sec
HOST   8327      1 Press any key to exit...
HOST   8767     13 Trigger event 2 and wait for RunWorkflow
MWSS   8784     13 Schedule workflow
  WF  38319     12 Handler 1 Completed
  WF  38406     12 Handler 2 Invoked...wait 3 sec
  WF  68396     12 Handler 2 Completed
HOST  68573     13 RunWorkflow for event 2 completed in 5,98 sec
HOST  68794     12 WorkflowCompleted
HOST  68795     12 RunWorkflow for event 1 completed in 6,05 sec

      

        
        
        
      

    

Some notes:

• The scheduler always uses the host thread to schedule the work item.
• A workflow instance does not always use the host thread to perform actions. If another activity is already in progress on a thread, that thread is used to perform all scheduled activities.
• The execution of the handlers is thread safe, but both threads wait for both handlers to finish!

If the latter bothers you, I would suggest the following posts:

• Understanding ParallelActivity in a Windows Workflow
• Use Workflow to invoke web services in parallel

By the way, can you share some information about the scenario you are facing?