c# - Is not checking key existence in a static dictionary thread-safe? Resharper thinks it's not -

resharper suggests wrapping return dic.contains("v1"); in lock statement saying "the field used inside synchronized block , used without synchronization":

public class myclass {     private static dictionary<string, string> _dic = new dictionary<string, string>();      protected bool v1exist()     {         return dic.contains("v1");                 } } 

however not see why lock needed in example. looks safe-thread me. please advise

it's known dictionary not thread-safe, should synchronize both writes , reads. if want specific example of can go wrong - consider small application:

static void main(string[] args) {     var dict = new dictionary<int, int>();     dict.add(0, 0);     new thread(() => {         (int = 1; < int.maxvalue; i++) {             lock (dict) {                 dict.add(i, i);                 dict.remove(i);             }         }     }).start();     new thread(() => {         while (true) {             if (!dict.containskey(0))                 throw new exception("cannot happen?");         }     }).start();     console.readkey(); } 

we create dictionary 1 entry key 0, run 2 threads. first thread adds , removes keys dictionary, note not remove item key 0. item key 0 present.

second threads checks if there item key 0 , throws exception if not. might think can never happen, because never remove item key 0, not true. if run application throw "cannot happen?" exception immediately. if add lock around containskey - never happen.

so in short, things might go horribly wrong if not synchronizing access not thread safe structures properly, reads. , might not notice that, , have hard time debugging such issues, because application might behave fine.