C# preventing Collection Was Modified exception

Question

Does

 foreach(T value in new List<T>(oldList) )

is dangerous (costly) when oldList contains 1 millions of object T ?

More generaly what is the best way to enumerate over oldList given that elements can be added/removed during the enumeration...

Answer 1

foreach(T value in new List(oldList).ToList() ) - give a try

Answer 2

You could iterate through the list without using an enumerator, so do something like...

for(int i = 0;i<oldList.Count;i++) {
   var value = oldList[i];

   ...

   if(itemRemoveCondition) {
     oldList.RemoveAt(i--);
   }
}