Handling save conflict within SPList EventReceiver

Background

Publishing an InfoPath form to a SharePoint site creates a new contenttype in the site. Via the InfoPath publishing wizard you have some control over the structure of the created contenttype. But in essence the control is very limited when compared with explicitly provision a contenttype yourself [see also previous post Administrate data from InfoPath form in a self-provisioned ContentType for further explanation why I prefer this above the automatic created information architecture entities]. However, it is not always possible or viable to explicit provision the InfoPath utilized IA entities. The big selling point of InfoPath is that it enables self-service (web)form-definition by functional management. A key cornerstone in this self-services proces is the InfoPath publishing functionality to allow functional managers to themselves distribute the InfoPath forms to a SharePoint site.
One of the aspects missing with the InfoPath publishing managed contenttypes is including the InfoPath promoted fields in the display form. Displaying all InfoPath form fields is in particular handy when quickly browsing the data submitted to a SharePoint forms library.

Solution approach

To fix the functional shortcoming requires to afterwards make the promoted fields displayed. This is not something you want to burden the functional managers with. The aim is therefore to execute it automatically in the context of the target SharePoint site. The ideal moment would be when the InfoPath managed contenttype is created or updated. SharePoint however does not provide an eventreceiver for these events. The next best moment is when the contenttype is associated with the forms library to which the InfoPath form data is submitted. Here you neither have a direct event notifying the contentype association. But you can receive indirect notification via SPListEventReceiver::FieldAdded. The idea is then to check each added field whether it is an InfoPath promoted field, and if so to alter its definition so that the field will appear in the standard SharePoint display form.

Issues encountered

So, easily said and done. Associate the custom ListTemplate with a SPListEventReceiver, and fill in the FieldAdded method. In my local development image it worked like a charm: after you associate via the SharePoint GUI a document library with an InfoPath contenttype, all promoted fields of that contenttype appear in the document library display form. But after deploying to the central SharePoint development farm, I ran into the first issue. Upon associating a library with a contenttype, SharePoint faulted with a weird message: The specified program requires a newer version of Windows.

Despite this misleading error message, it was immediate clear that the issue was caused by the custom FieldAdded eventhandler:

Apparently, SharePoint does not allow to update a list field in the runtime context of that field being added to a list.

Solution outline

Solution is to isolate the execution of the field-update from the field-addition runtime context. Instead of direct updating the field, schedule this work via a worker thread. One extra aspect to take into account is that the scheduled worker threads can still run into a parallel update conflict:

Save Conflict

Your changes conflict with those made concurrently by another user. If you want your changes to be applied, click Back in your Web browser, refresh the page, and resubmit your changes.

  at Microsoft.SharePoint.Library.SPRequest.UpdateField(
        String bstrUrl,String bstrListName, String bstrXML)
  at Microsoft.SharePoint.SPField.UpdateCore(Boolean bToggleSealed)
  at Microsoft.SharePoint.SPField.Update()
  at WebformulierFieldsHandler.Worker.b__1()
 

The likelihood of these save conflicts increases with the number of promoted fields in the contenttype, and can already manifest itself when that number is 4 to 5. It can be made functional robust by implementing a retry mechanism in the worker thread.