In .NET 4 the Workflow Team released an all new Workflow engine in the System.Activities namespace. This new Workflow technology, which we refer to as WF4, responded to key customer feedback by providing significant programming model, runtime, and authoring enhancements.
With the release of .NET 4.5 Beta we are marking the types in the “WF3”, System.Workflow, namespace as obsolete. The following assemblies have one or more public types that will be deprecated:
As a result, customers who are using the deprecated WF3 APIs will encounter build warnings with a message similar to “ Warning BC40000: X is obsolete: ‘WF 3 types are deprecated. Please use WF 4 instead.’” We will remove the types from the .NET Framework in a future release, but we have not yet determined that timeframe (post-.NET 4.5). This current step allows us to communicate our direction and give our customers sufficient time to move to the WF4 model. We will, of course, continue to support these WF3 types under the Microsoft Support Lifecycle Policy. Existing WF3 applications will run without issue on .NET 4.5, and Visual Studio 11 will support new and existing WF3-based solutions.
Rules related types (e.g. System.Workflow.Activities.Rules), which do not have a replacement in WF 4.5, have not been deprecated and can be used with WF3 or WF4 solutions.
– The Workflow Team