InfoQ Article: The HandleExternalEvent in Windows Workflow
Windows Workflow Foundation provides a powerful runtime environment that you can host within your own applications. In this article, Scott Allen takes a look at one method of communicating from your host application back into the workflow to signal that something has occurred to which the workflow needs to respond.
Windows Workflow has several ways to communicate with the outside world and for the outside world to communicate with it. For communication with WSDL based remote services, you can use the WebServiceInput and InvokeWebService activities. The CallExternalMethod and HandleExternalEvent activities are used for local, in-process communications.
The HandleExternalEvent activity allows the workflow to listen for events raised by its host process and is the focus of this article.
Jon Brisbin,Stephane Maldini Nov 26, 2014