SharePoint Workflow CreateTaskActivity and OnTaskChangedActivity must be one-to-one mapping

In a Visual Studio workflow for SharePoint 2010, if you have a series of complex logic that check various conditions to determine what to do if a task is completed, you may be tempted to put in multiple OnTaskChanged activities with the token tied to the same CreateTask activity.  I have recently tried that.  The flow of the workflow looks pretty clear and natural visually.  It built and deployed successfully and started and executed the steps successfully until it hit the second OnTaskChanged.  The Workflow History logged "An error has occurred in ", and the SharePoint log logged this error:

Error in commiting pending workflow batch items: System.ArgumentException: 0x80070057     at Microsoft.SharePoint.Library.SPRequestInternalClass.RegisterEventReceiver(String bstrUrl, String bstrListName, EventReceiverOperation operation, Guid guidId, String bstrName, Guid guidSiteId, Guid guidWebId, Guid guidHostId, Int32 dwHostType, Int32 dwSynchronization, Int32 dwType, Int32 dwSequenceNumber, String bstrRemoteUrl, String bstrAssembly, String bstrClass, Guid solutionId, String bstrData, String bstrFilter, Int32 dwCredential, Guid contextObjectId, Guid contextType, Guid contextEventType, Guid contextId, Guid contextCollectionId)     at Microsoft.SharePoint.Library.SPRequest.RegisterEventReceive.....

THe bottomline is, put one OnTaskChanged activity in a While activity.  With a couple of IfElse in there, the workflow would look confusing visually, but it would work fine at runtime.

FileNotFoundException after SharePoint workflow DelayActivity

I noticed that a SharePoint 2010 workflow that's deployed to my dev SharePoint farm kept logging " failed to run" in the Workflow History after every so often.  Turned out that every time the Delay Activity comes out of its time out, this error is logged.  The workflow is stuck forever in the In Progress status though.  Found in the SharePoint ULS log the following everytime this happened:

03/09/2013 20:50:13.76  OWSTIMER.EXE (0x09F0)                    0x20E0 SharePoint Foundation          Legacy Workflow Infrastructure 75yn Unexpected Load Workflow Assembly: System.IO.FileNotFoundException: Could not load file or assembly 'My.Content.WF.ClientReview, Version=1.0.0.0, Culture=neutral, PublicKeyToken=1f310f3d9eb1728b' or one of its dependencies. The system cannot find the file specified.  File name: ''My.Content.WF.ClientReview, Version=1.0.0.0, Culture=neutral, PublicKeyToken=1f310f3d9eb1728b'     at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)     at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, S... 1746069c-9c8e-2075-8ca7-47e33a38cd91
So what was going on?  My workflow only references SharePoint assemblies, and itself is indeed in the GAC.  A closer look shows that this was thrown from OWSTIMER.EXE.  So it's the timer job service.  Apparently the Delay Activity relyies on the timer job service to fire.  Just like w3wp.exe, it needs a restart to recognize newly changed assemblies in the GAC.  Sure enough, I restarted the timer job service, started a new WF, and the Delay Activity fired successfully after coming out of its timeout. 

System.InvalidOperationException: Correlation value specified does not match the already initialized correlation value on declaration approveRejectTas

Continuing with new discoveries of SharePoint workflow stuff, I ran into this logged exception after I added a SendEmail activity to my previously working workflow. So I know it's gotta be something in this Activity, which runs after a new Task is created in the workflow. Like every other Activity in the pipeline of the task process (Create Task, OnTaskChanged, CompleteTask etc), I gave this one the same correlation token as the task level token. Turn out that's the problem. Apparently the SendEmail activity (and a few other activities such as LogActivity) can only assume the workflow level correlation token.

NullReferenceException on SPWorkflowTask.AlterTask ()

In a custom task edit form for a custom content type, the following code is used to update the task item:

Hashtable taskHash = new Hashtable();
taskHash["Status"] = "Approved";
taskHash["PercentComplete"] = "1";
SPWorkflowTask.AlterTask(this._taskItem, taskHash, true);

This works great most of the time, however, if the custom content type defines no fieldrefs, or more specifically, the content type XML does not have the node, as could be the case if only a custom form is included in the custom content type, the following exception would be thrown on the AlterTask line:

System.NullReferenceException was unhandled by user code
Message="Object reference not set to an instance of an object."
Source="Microsoft.SharePoint"

To fix this cryptic error, make sure that the node is in the content type definition XML, even when there're no child nodes.