c# - Why is this code not threadsafe? -

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i have piece of code this:

public class usercache {     private dictionary<int, user> _users = new dictionary<int, user>();      public user getuser(int id)     {         user u = null;          lock (_users)         {             if (_users.containskey(id))                 return _users[id];         }          //the below line threadsafe, no worries on that.         u = retrieveuser(id); // method retrieve database;          lock (_users)         {             _users.add(id, u);         }          return u;     } }

i'm locking access dictionary, in team told still not threadsafe(without explanation). question - think thread safe @ all?

edit: forgot ask,what solution like.please note i'mnot looking lock whole method, retrieve user time consuming operation.

no, it's not thread-safe. imagine it's called twice @ same time same id, isn't present.

both threads far retrieveuser, , they'd both call _users.add(id, u). second call fail because key exist in dictionary.

(as aside, i'd recommend using braces locks, if statements etc, sake of readability.)


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