Developing custom state activities

I've banged on before about how extensible Windows Workflow is (assuming you can work out how to extend it), but there is at least one place where it isn't quite as extensible as one would like, state activities.

I was trying to create a custom state activity that contained a state initialization activity and an event driven activity and various other activities within those. The first problem I encountered was when I inherited from StateActivity. Trying to add any activities at design-time failed. This seems to be down to the state activity designer. One solution would have been to write my own designer class, but this is complicated by the fact that the standard state designer is internal so can't be inherited from. I guess I could have decompiled the code using Reflector and made the required mods in my own code, but that seemed too much like hard work. So I decided to construct the activity in code instead.

Then I bumped into the second problem. Adding activities in code is pretty straightforward, just do it in the constructor after the call to InitializeComponent, like so.

public UserActivity()
{
  InitializeComponent();
  CanModifyActivities = true;
  try
  {
    // add activities here
  }
  finally
  {
    CanModifyActivities = false;
  }
}

Having said that, if you're doing all your activity construction in code, you can actually get rid of the call to InitializeComponent and get rid of the designer.cs file, since you're not going to need any design-time support.

Anyway that's the theory. In practice, it doesn't quite work out like this, because the StateActivity's validator doesn't like you to add child activities to a state activity. At least this class is public so I could inherit from it, but there aren't really any extensibility points to allow for modification of its behaviour. So I initially wrote my own validator class, that doesn't do anything at all. I later realised that if the validator wasn't going to do anything, I could actually just use ActivityValidator instead. This isn't perfect since it means none of the child activities I've added will be validated, but it works well enough for my scenario.

One further problem (although it isn't an issue for me) is that an end user of your custom state activity won't be able to add their own child activities. I guess the solution to this once again is to write your own designer.

Another related problem is the addition of a set state activity. Since my users can't add one themselves, I have to add one but I don't know the name of the state to transition to until the activity is in use. This was pretty easy to solve, just add a TargetStateName to my activity and pass that value through to my own set state activity.

public string TargetStateName
{
  get
  {
    return setState.TargetStateName;
  }
  set
  {
    setState.TargetStateName = value;
  }
}

My final problem was that the state activity was displayed with all of its child activities visible to the end user, although they are not editable. I still haven't solved this issue, although I suspect the solution will yet again involve writing my own designer class. If only that standard state designer wasn't internal...