One of the cool things about WF is that you can specify the Guid it uses to identify a workflow instance. WorkflowRuntime.CreateWorkflow has an overload (actually two) where you can specify said workflow instance identifier. This is awesome for using WF with Service Broker, as Service Broker already has the idea of a conversation group which is roughly analogous to a workflow instance. Conversation groups even use a Guid identifier, so there's not even any mapping required to go from conversation group to workflow instance.
However, things get less cool when you call WorkflowRuntime.GetWorkflow. If you call GetWorkflow with a Guid that has no corresponding workflow instance, it throws an InvalidOperationException instead of just returning null. That seems like an odd choice. If you're going to support specifying the instance identifier when you create the workflow instance, doesn't it make sense that you should also gracefully support the scenario where an instance identifier is invalid?
I see two ways to deal with this:
I ended up picking the "Swallow the Exception" approach as I can't imagine the iteration thru every loaded and persisted instance would be very performant. But swallowing exceptions always makes me feel icky. I'm a fan of the "exceptions only for exceptional situations" approach and as far as I'm concerned, an invalid instance identifier isn't that exceptional. Still, it's a minor annoyance, especially given how cool it is to be able to specify the workflow instance identifier in the first place.