One of the new power features in .NET 3.5, sorely missing when .NET 3.0 was released is the integration between Windows Communication Foundation and Windows Workflow Foundation, in the way of the new Workflow Services model. Besides providing an easy way to invoke WCF services from WF without resorting to using BasicHttpBinding on the services or invoking a [wrapper] client class form a code activity in your workflow, it also provides a way to expose a workflow as a WCF service to the outside. The second functionality is the one that, naturally, interests me the most. After looking at it for a while, I've made a few observations that I'd like to share. Implementing Services A very interesting thing is how you implement services as workflows. If you know how to create WCF services the right way, then you [mostly] know how to implement services as workflows. The reason for this is that most of the tasks you'd normally when creating a WCF service still apply when creating a workflow service. That is, you will still declare your service contract (interface) as well as your message/data contracts. The only key differences really is how you implement that contract. When using workflow services, you implement a contract by using the new Receive activity introduced in .NET 3.5, and select which operation of the contract you'd be implementing. You still need to do the activity binding dance for parameters as well as (possibly) create response messages for the incoming requests. Basically, the Receive activity works as a kind of sequential activity where the child activities represent the tasks to execute after the request message is received until the response message is sent (if this is a request/response service). You'd also need to do a lot of the usual work in setting up your service host, only instead of using the regular ServiceHost class you'll be using the new WorkflowServiceHost class. Guy Burstein has a good entry on getting started on exposing workflows as services here you might want to check out. Starting Workflows One very interesting option in the Receive Activity is the CanCreateInstance property. You can use this property to have the WorkflowServiceHost automatically create and start a new instance of your Workflow when a new message arrives at the service endpoint for the implemented operation, without having to write manual code to receive the message and create the new workflow instance. People familiar with BizTalk 2004/6 will recognize this option as being similar to the Activate property of the Receive Shape in BizTalk orchestrations. It also serves, in a way, a similar purpose as the CanCreateInstance property of the new [DurableOperationBehavior] attribute introduced for WCF in .NET 3.5 as part of WCF Durable Services infrastructure. Jesus Rodriguez has an excellent article introducing this new feature. Something I found a bit curious about the CanCreateInstance property of the Receive Activity is that there's currently no validation Read More...