c# - Why don't I get an EF5 Concurrency Error? -

in database i'm working on company keeps track of order numbers each regional office. have table named [company] has single record each regional office. table has field holds next available order number office. orders created in single [order] table. realized possible more 1 user @ given office ask new order @ same time. have set optimistic concurrency. [company] table has column named rowmodtracker used concurrency purposes (see images @ bottom properties in ef , sql).

in code below, record regional office, increment field has order number , put copy in local variable. save off table. expect if 2 users hit method @ same time first user save off valid order number. expect second user save off optimistic concurrency error , null should returned (which causes prompt asks them try again). users tell me few times in last 8 months have indeed gotten orders same order number. no messages , no errors happen until later in process when go save order , unique constraint on order number violated.

i don’t understand fundamental ef concurrency checking , i’m hoping can show me light. wrote test version of getnextordernumberandupdate wrapped second using context around first , did indeed generate expected concurrency error. doesn't seem happen when 2 different clients it.

 private int? trytogetnextordernumber()  {      int? order_number;       bool keep_trying = true;       while (keep_trying)       {           order_number = _entitymanager.getnextordernumberandupdate();           if (order_number != null) return order_number;           keep_trying = keeptryingforordernumberprompt();       }       return null;   }     public int? getnextordernumberandupdate()   {      try      {               using (var db_context = new hotentities(entityconnectionstring))          {                                  var hss_company = db_context.companies.find(hsscompanyinuse.hsscompanyid);               int? next_order_num = ++hss_company.nextordernum;               db_context.savechanges();               return next_order_num;           }        }        catch (dbupdateconcurrencyexception)        {            return null;        }        catch (exception ex)        {            return null; //actual code puts out alert         }   } 

i using entity framework 5 , sql server 2008 r2 .net 4.5.

edit

the test version looked this, i've elided try/catch stuff same above

//test force failure  using (hotentities test_context = new hotentities(entityconnectionstring)) {     var test_hss_company = test_context.companies.find(hsscompanyinuse.hsscompanyid);     int? test_next_order_num = ++test_hss_company.nextordernum;       //======= normal code ============     using (var db_context = new hotentities(entityconnectionstring))     {         var hss_company = db_context.companies.find(hsscompanyinuse.hsscompanyid);         var next_order_num = ++hss_company.nextordernum;         db_context.savechanges();         return next_order_num;     }     //======= end normal code ============  test_context.savechanges(); //dbupdateconcurrencyexception exception thrown here } //end test using