vb.net - Parallel.ForEach gives different results each time -

please me convert following loop parallel loop. tried using parallel.foreach , concurrentbag instead of hashset, wied thing "matched" returns every time different results.

i can't figure out... because of thread safety issues?

keywords list contains 500 unique strings, each 1-3 words in lenght.

items contains 10000 items.

original code:

    dim items ienumerable(of item) = db.items.getall      dim keywords hashset(of string)      dim matched hashset(of item)      each item in items          each keyword in keywords              if regex.ismatch(headline, string.format("\b{0}\b", keyword), regexoptions.ignorecase or regexoptions.cultureinvariant)                  if not matched.contains(item)                     matched.add(item)                 end if              end if          next      next 

attempt convert

dim items ienumerable(of item) = db.items.getall  dim keywords hashset(of string)  dim matched concurrent.concurrentbag(of item)  threading.tasks.parallel.foreach(of item)(items, sub(item)     each keyword in keywords         if regex.ismatch(item.title, string.format("\b{0}\b", keyword), regexoptions.ignorecase or regexoptions.cultureinvariant)             if not matched.contains(item)             matched.add(item)             end if        continue       end if    next end if 

yes, code isn't thread-safe. using thread-safe collections won't make code automatically thread-safe, still need use them correctly.

your problem after contains() finishes before add() called on 1 thread, add() can called on thread (and same can possibly happen while contains() executes).

what need to:

1. use locking (this means won't need use thread safe collection anymore); or
2. use concurrenthashset. there no such class in .net, can use concurrentdictionary instead (even if doesn't fit needs exactly). instead of call contains() , add(), matched.tryadd(item, true), true there because concurrentdictionary needs value.