Sharepoint create task workflow error - Not supported exception

If you've tried to create tasks in workflows hosted in Sharepoint using the CreateTask activity you may have come across this error in the Sharepoint logs.

System.NotSupportedException: Specified method is not supported.     at Microsoft.SharePoint.Workflow.SPWorkflowTask.SetWorkflowData(SPListItem task, Hashtable newValues, Boolean ignoreReadOnly)     at Microsoft.SharePoint.Workflow.SPWinOETaskService.UpdateTaskInternal(Guid taskId, SPWorkflowTaskProperties properties, Boolean fSetWorkflowFinalize, Boolean fCreating, HybridDictionary specialPermissions)     at Microsoft.SharePoint.Workflow.SPWinOETaskService.CreateTaskWithContentTypeInternal(Guid taskId, SPWorkflowTaskProperties properties, Boolean useDefaultContentType, SPContentTypeId ctid, HybridDictionary specialPermissions)     at Microsoft.SharePoint.Workflow.SPWinOETaskService.... 

Although saying that, my own searches on the Internet didn't bring up anything, so I may be the only one. Or there may not be many people using workflow in Sharepoint, which is odd given that it's free, which compares very favourably with most workflow products.

But I digress. I thought this would be pretty easy to solve. First up, have a look at the implementation of SPWorkflowTask.SetWorkflowData in Reflector and see what causes it to throw the exception. Which is where things got difficult. Reflector told me the method was obfuscated. I don't know if this is true or if Reflector was getting confused but I left it for a while since I had other things to do. Then I had a thought, perhaps I could still have a look at the code in Reflector if I chose IL as my language of choice. And indeed I could. And this is what the area of interest looked like.

    L_01e8: br.s L_01f0
    L_01ea: newobj instance void [mscorlib]System.NotSupportedException::.ctor()
    L_01ef: throw
    L_01f0: ldloca.s id
    L_01f2: ldsfld valuetype Microsoft.SharePoint.SPContentTypeId       Microsoft.SharePoint.SPBuiltInContentTypeId::WorkflowTask
    L_01f7: call instance bool Microsoft.SharePoint.SPContentTypeId::IsChildOf(valuetype Microsoft.SharePoint.SPContentTypeId)
    L_01fc: brtrue.s L_0200
    L_01fe: br.s L_01ea
    L_0200: ldarg.0

At lines L_01f0 to L_01f7, the method checks to see if the content type is a child of workflow task. If not, it throws the exception. Turns out my workflow.xml file had its TaskListContentTypeId attribute set to 0x0108 (task), rather than 0x010801 (workflow task). I changed that and everything worked, the task was created and I was on my way to workflow nirvana.

It's just a shame that whoever implemented this bit of code didn't use the NotSupportedException that takes an error message as a parameter, then they could have told me what the problem was...