Error when using ItemAdded event handler to update columns on newly uploaded document

Question

I have an ItemAdded event handler connected to a document library, in order to automatically populate certain fields when a new document is uploaded:

public class AutoPopulateDocumentFields : SPItemEventReceiver
{
   public override void ItemAdded(SPItemEventProperties properties)
   {
       base.ItemAdded(properties);

       SPList sourceList = properties.List;

       sourceList["Title"] = "Auto-populated title value";

        try {
            this.EventFiringEnabled = false;
            sourceList.Update();
        }
        finally {
            this.EventFiringEnabled = true;
        }
   }
}

This usually works when uploading a new file. I get an intermittent error, though: Sometimes the update list item form which shows up after upload does not have the "Title" field autofilled, and when clicking "OK" I get the error message

The file "Documents\testfile.txt" was changed by Geir Smestad on the 18. Nov 2011 11:28:47 +0100

This is a SharePoint error, not an error in my code. After closing the error, the Title has the value "Auto-populated title value" but all other fields have their default values. What is wrong?

Answer 1

This is caused by a race condition in the version control system. Events are as follows:

1. Your file finished uploading
2. Your event handler code runs (in a different thread)
3. The update list item form is populated and displayed
4. You click OK in the update list item form and the changes are submitted

Since ItemAdded is asynchronous, sometimes your event handler code doesn't have time to finish executing before the Update List Item form is loaded. Therefore, the Update List Item form is populated with the old version of the file's columns. When clicking OK (and your code has already had time to save the new version of the file), SharePoint attempts to submit the old version of the file, resulting in an error.

To solve the problem, call sourceList.SystemUpdate(false) instead of Update(), which updates the file without incrementing the version number.

Answer 2

Instead of properties.List use properties.ListItem as below,

SPList sourceListItem = properties.ListItem;

sourceListItem["Title"] = "Auto-populated title value";

sourceListItem.SystemUpdate(false);