In this Two Minute WF, I’m going to tell you about WorkflowInstance.

This is a really simple one: A WorkflowInstance is - wait for it - a single instance of a workflow. Yes, it really is that simple. You pick your workflow type, fill in the activities it’ll use, and you have a workflow definition. Once you have the definition, you instantiate it and start it running. It’s sort of like defining your class/type and then newing one up. For objects it’s like this:

MyCustomType myVar = new MyCustomType();

For workflows, it’s like this:

using(WorkflowRuntime workflowRuntime = new WorkflowRuntime())
{
  AutoResetEvent wh = new AutoResetEvent(false);
  workflowRuntime.WorkflowCompleted +=
    delegate(object sender, WorkflowCompletedEventArgs e) {wh.Set();};
  workflowRuntime.WorkflowTerminated +=
    delegate(object sender, WorkflowTerminatedEventArgs e) {wh.Set();};
  WorkflowInstance instance = workflowRuntime.CreateWorkflow(typeof(MyCustomWorkflowType));
  instance.Start();
  wh.WaitOne();
}

In the example, I not only create an instance of my custom workflow type, but I also start it running. It’s a little more complex than just creating a new object, but it’s not that much worse.

The WorkflowRuntime is responsible for instantiating workflows and starting them running. Since workflow instances run on a different thread than the main program, I’m using an AutoResetEvent to do thread synchronization - we don’t want to end the program or dispose of the WorkflowRuntime until the WorkflowInstance has had a chance to complete its run.

Something to watch for when you’re working in a larger environment with lots of instances - you only get one WorkflowRuntime for a given AppDomain. What that means is the WorkflowCompleted and WorkflowTerminated events are for all WorkflowInstances - you don’t get one for each instance. Be careful when subscribing to these events when you just want info on a single instance. If you forget to unsubscribe from the event and the event subscriber goes out of scope, you may have a memory leak on your hands. (The Garbage Collector won’t clean up the now-unreferenced event subscriber because there’s still one reference to it - the subscription to the global WorkflowRuntime event.)