My Introduction to Windows Workflow Foundation (WF)
Team Build is a build workflow tool. It manages how and when builds are performed. Currently (in VS2005 and VS2008), that workflow was written in the language of MSBuild. This allows us a lot of flexibility and power on the machine that runs the build. However, modifying the MS.TF.Build.Targets file that contained the description of the workflow was heavily discouraged and by no means easy. So, how can we make changing the workflow of the build easier. The first step (we believe) is to change the language that the workflow is described in. Windows Workflow is a natural choice. As we look at using Windows Workflow within Team Build, I will attempt to write some useful posts on things that we have discovered or realized. Mostly things that the books don't talk about. This is my first attempt...
First let me tell you about the books that I have read on WF. I read Bruce Bukovics' "Pro WF" first. This is a great starter for a developer wanting to write WF solutions. Bruce covers all of the functionality you would need to know. But, if you want to host WF and allow your users to manipulate the workflow directly, you need more details about how it works. The second and last book that I read on WF was "Essential Windows Workflow Foundation" by Dharma Shukla and Bob Schmidt. The Essentials book is really brief, but explains how WF Activities really work. If you plan on writing your own custom activities, this book is very useful.
So, what did I learn. More importantly, what have I learned that I didn't get from the books. Well the first thing you should know is that you can represent a workflow in many different ways. The workflow is basically a tree of classes derived from Activity. There are two ways to represent the tree in the Visual Studio world: in code; and in XAML (basically XML). Workflow uses the extension XOML, however, to let VS know that its a workflow and not some kind of WPF stuff (more on WPF in another post). Working with code is the most natural way to deal with the Activity tree. In fact, even if you choose to create your workflow in XOML, VS will create a code behind file for you and compile your XOML into a dll.
The first lesson we learned was... If you want to use XOML only and not do any code behind, you must create the XOML yourself or hand edit the XOML created by Visual Studio. When Visual Studio creates the XOML file, it assumes you will compile it. To compile XOML you need a special attribute on the root element in the XOML that tells the compiler to create a class. The attribute is "x:Class". If you create a XOML workflow in Visual Studio and open that XOML in the XML editor or in notepad, this should be the first attribute of the root element.
When you load a workflow in the workflow runtime, there are two ways to do it. Call load with a type, this implies that the XOML or straight code workflow is compiled. Or you can call load with an xml reader, that has the XOML loaded. If you choose the xml reader option and that x:Class attribute is in the XOML, an error will be thrown by the runtime. However, if you try to compile XOML without that attribute, compilation will fail. So, the first choice you have to make in using XOML files is "To compile or not to compile" - that is the question ;)