Getting Error In Item Added Event In Event Receiver in SharePoint 2010

Question

Hi Guys,
I have written Event Receiver in SharePoint 2010 On ItemAdded event of Document Library. Wants to increment a col value by reading maximum value first. I wrote below code but sometime it works correctly but some time I got below error: Document Set
enabled on this document library and weh we added any document in any document set then this event occurred. I mentioned Code block and Error description both.
Please see and suggest right solution.
Code:
public override void ItemAdded(SPItemEventProperties properties)
                base.ItemAdded(properties);
                SPListItem _currentItem = properties.ListItem;
                string _QUERY = @"<Where><Eq><FieldRef Name='DocumentId'/><Value Type='Text'>21</Value></Eq></Where><OrderBy><FieldRef
Name='RevisionReference' Ascending='False' /></OrderBy><ViewFields><FieldRef Name='RevisionReference' /></ViewFields><QueryOptions><Folder>DocumentLIBB/</Folder></QueryOptions>";
                int maxID = 0;
                string revisionreferencee = string.Empty;
                using (SPWeb web = properties.OpenWeb())
                    SPList list = web.Lists["DocumentLIBB"];
                    SPQuery query = new SPQuery();
                    query.ViewAttributes = "Scope=\"Recursive\"";
                    query.Query = (_QUERY);
                    SPListItemCollection results = list.GetItems(query);
                 if (results.Count > 0)
                        SPListItem item = results[0];
                        revisionreferencee = item["RevisionReference"].ToString();
                        bool result = Int32.TryParse(revisionreferencee, out maxID);
                        _currentItem["RevisionReference"] = maxID + 1;
                        _currentItem["DocumentId"] = item["DocumentId"].ToString();
                properties.ListItem.File.Update();
                _currentItem.Update();
Error Desc:
<nativehr>0x81020015</nativehr><nativestack></nativestack>The file EKENg LIBB/QA_DOC_044/05-22 Emirates Engineering Occurrence_Error Investigation - MEDA Process - 27 AUG 2009-921.doc has been modified by SHAREPOINT\system
on 27 Aug 2012 12:10:13 +0400.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Runtime.InteropServices.COMException: <nativehr>0x81020015</nativehr><nativestack></nativestack>The file EKENg LIBB/QA_DOC_044/05-22 Emirates Engineering Occurrence_Error Investigation - MEDA
Process - 27 AUG 2009-921.doc has been modified by SHAREPOINT\system on 27 Aug 2012 12:10:13 +0400.
Source Error:
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. 

Answer 1

Making the event synchronous (rather than the default) eliminates this error. If such a change suits your requirements you should consider it. You just have to tweak the elements.xml in your feature receiver project;
http://blogs.msdn.com/b/unsharepoint/archive/2010/11/10/sharepoint-event-receivers-making-asynchronous-event-synchronous-to-avoid-save-conflict-error.aspx
w: http://www.the-north.com/sharepoint | t: @JMcAllisterCH | c: http://www.b-i.com