c# - Hashset First() + Remove() performance -

i encountered performance issue today involving hashset.remove , i'm still unclear on problem was. question i'm left is, why approach2 method faster approach1 method? i'm assuming it's calls hashset.remove msdn docs hashset.remove o(1).

public class hashsettester {     int testnum = 20000;      public void run()     {         var hashset2 = createtesthashset();         var watch2 = new stopwatch();         watch2.start();         approach2(hashset2);         watch2.stop();          var hashset1 = createtesthashset();         var watch1 = new stopwatch();         watch1.start();         approach1(hashset1);         watch1.stop();          console.writeline("approach1 {0:0.0}x slower approach2", watch1.elapsed.totalseconds / watch2.elapsed.totalseconds);     }      hashset<object> createtesthashset()     {         var result = new hashset<object>();         var rnd = new random();          (int = 0; < testnum; i++)         {             result.add(rnd.next());         }         return result;     }      void approach1(hashset<object> hashset)     {         while (hashset.any())         {             var instance = hashset.first();             hashset.remove(instance);             dosomething(instance, hashset);         }     }      void approach2(hashset<object> hashset)     {         var tempitems = new list<object>();         while (hashset.any())         {             tempitems.clear();             tempitems.addrange(hashset);             hashset.clear();             foreach (var instance in tempitems)             {                 dosomething(instance, hashset);             }         }     }      void dosomething(object obj, hashset<object> hashset)     {         // in cases, hashset added here     }      public static void main()     {         new hashsettester().run();     } } 

if need every entry of hashset , remove entries why not use linq instead?

hashset.foreach(item => dosomething(item, hashset)); hashset.clear(); 

this give acceptable performance when dealing big collections.