Two Minute WF: WorkflowInstance
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.)
