The question of what Windows Workflow Foundation version 4 means for developers currently developing using WF recently came up. As I mentioned before WF 4 is a complete rewrite and doesn’t use any of the existing WF 3 classes. The design of WF 4 is even quite different from the design of WF 3.
So are WF 3 developers completely left in the dark and have to start from scratch? And what about their existing applications, will they still run on .NET 4 or are they stuck in .NET 3.5 until the end of times?
Fist of all let me address why I believe Microsoft could make such a radical change. Keep i mind this is just my personal opinion and I might be completely wrong .
WF 3 was never widely adopted so I believe the team felt they could do this and not create too much of a problem. Personally I don't think the low adoption of WF 3 had anything to do with the technology. Sure it has its problems but most people never got to the point that they ran into real technical issues. Instead most people looked, didn't understand and quit. So IMHO the main problem was far more a question if evangelism that technology.
So what is the big issue with a migration from WF 3 to WF 4?
One big issue with WF 4 is that Microsoft decided not to release a state machine implementation. In WF 3 there was a choice between state machine and sequential workflows. In WF 4 there is a choice between sequential and flow chart. And in WF 3 a state machine turned out to be far more flexible and useful and quite a few cases are not really covered by the WF 4 model. There are a few workarounds to do something like a state machine in WF 4 but they are not pretty. That said, a number of state machine workflows could very well be done using a flow chart.
As far as backward compatibility the story is a bit more complicated. WF 4 ships with an interop activity you can use to host custom WF 3 activities in a WF 4 workflow. How good or bad this is? I really don't know yet but given how different the two designs are I fear it is very limited.
But WF 3 developers need not worry about their existing projects!
There is however a second side to the compatibility story. .NET 4 will ship both with the WF 3 and the WF 4 runtimes. The WF 3 runtime will not be prominent but it will still be there so all existing WF 3 applications will continue to run as is. And because it is part of the current .NET framework I suspect this is a legal requirement (but then I am no lawyer so could be wrong here). So there is no reason for existing workflow developers to panic just yet .
So existing workflow developers should start learning WF 4 for future projects but can continue to keep using their existing WF 3 solutions for the foreseeable future.