Question

edit: in case anyone is wondering, the actionhandler invokes code that creates and disposes the same kind of datacontext, in case that might have anything to do with this behaviour. the code doesn’t touch the MatchUpdateQueue table, but i figure i should mention it just in case.

double edit: everyone who answered was correct! i gave the answer to the respondent who suffered most of my questioning. fixing the problem allowed another problem (hidden within the handler) to pop up, which happened to throw exactly the same exception. whoops!

I’m having some issues with deleting items in LINQ. The DeleteOnSubmit call in the code below causes a LINQ Exception with the message ‘Cannot add an entity with a key that is already in use.’ I’m not sure what I’m doing wrong here, it is starting to drive me up the wall. The primary key is just an integer autoincrement column and I have no other problems until I try to remove an item from the database queue. Hopefully I’m doing something painfully retarded here that is easy to spot for anyone who isn’t me!

static void Pacman() {     Queue<MatchUpdateQueue> waiting = new Queue<MatchUpdateQueue>();      events.WriteEntry('matchqueue worker thread started');      while (!stop)     {         if (waiting.Count == 0)         {             /* grab any new items available */             aDataContext db = new aDataContext();             List<MatchUpdateQueue> freshitems = db.MatchUpdateQueues.OrderBy(item => item.id).ToList();              foreach (MatchUpdateQueue item in freshitems)                 waiting.Enqueue(item);             db.Dispose();         }         else         {             /* grab & dispatch waiting item */             MatchUpdateQueue item = waiting.Peek();             try             {                 int result = ActionHandler.Handle(item);                 if (result == -1)                     events.WriteEntry('unknown command consumed : ' + item.actiontype.ToString(), EventLogEntryType.Error);                  /* remove item from queue */                 waiting.Dequeue();                  /* remove item from database */                 aDataContext db = new aDataContext();                 db.MatchUpdateQueues.DeleteOnSubmit(db.MatchUpdateQueues.Single(i => i == item));                 db.SubmitChanges();                 db.Dispose();             }             catch (Exception ex)             {                 events.WriteEntry('exception while handling item : ' + ex.Message, EventLogEntryType.Error);                 stop = true;             }         }          /* to avoid hammering database when there's nothing to do */         if (waiting.Count == 0)             Thread.Sleep(TimeSpan.FromSeconds(10));     }      events.WriteEntry('matchqueue worker thread halted'); } 

Answer 1

You could do something to the effect of

db.MatchUpdateQueues.DeleteOnSubmit(db.MatchUpdateQueues.Single(theItem => theItem == item)); 

Just a note as other answers hinted towards Attach.. you will not be able to use attach on a context other then the original context the item was received on unless the entity has been serialized.