Every program answers two questions “what to do” and “when to do it.” Windows Workflow Foundation or WF is a part of the WinFX suite of tools which allows programmers to seperate the “what to do” and the “when to do it” and specifically allows you to declaratively model the “when to do it” allowing for easy creation of visual tools for working with the workflow logic. When using WF in this way one of the things you will need to learn to do is how to communicate back and forth between your WF code and your client code. This post will be covering the ExternalDataExchange service and how you can use it with built in activities to provide easy client\WF communications.