c# - Do I need a lock in AddOrUpdate of a concurrent dictionary? -


in class have

public static concurrentdictionary<string, hashset<string>> connecteduserconnections = new concurrentdictionary<string, hashset<string>>();

when adding or updating, should update via:

connecteduserconnections.addorupdate(userid, new hashset<string>(), (key, existingval) =>                 {                     existingval.add(connectionid);                     return existingval;                 });

or 

connecteduserconnections.addorupdate(userid, new hashset<string>(), (key, existingval) =>                 {                    lock(connecteduserconnections)                      {                         existingval.add(connectionid);                         return existingval;                     }                 });

many all.

by looking @ public tvalue addorupdate(tkey key, func<tkey, tvalue> addvaluefactory, func<tkey, tvalue, tvalue> updatevaluefactory) reference source, 

 while (true)  {      tvalue oldvalue;      if (trygetvalue(key, out oldvalue))      //key exists, try update      {          newvalue = updatevaluefactory(key, oldvalue);          if (tryupdate(key, newvalue, oldvalue))

there no lock until trygetvalue, if there value key, multiple threads arrive @ trygetvalue, execute it, return true, execute updatevaluefactory (your method) @ same time , try adding existingval.add(connectionid);...

so yes, need lock.


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