Workflow Foundation 4.0 is introduced a significant amount of change from the previous versions of the technology. In WF4.0, we are using Extensions for communication between the host and workflow, the workflow and its child activities and between child activities. There are two types of extensions in WF4.0, for more details on normal extensions, please refer http://www.dotnetfunda.com/articles/article1091-workflow-foundation-40–extension-.aspx