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

you can use a flag to switch the modification to a temporary list while the original is being enumerated.

/// where you are enumerating

isBeingEnumerated = true
foreach(T value in new List(oldList) )
isBeingEnumerated = false
SyncList(oldList with temporaryList)

/// where you are modifying while enumerating

if isBeingEnumerated then
use a temporaryList to make the changes.