C# preventing Collection Was Modified exception

Question

Does

 foreach(T value in new List(oldList) )

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

More g

Answer 1

I usually just create a list for all the objects to be removed or added.

Within the foreach I just add the items to the appropriate collections and modify the original collection after the foreach have completed (loop through the removeItems and addItems collection)