Concurrent collections in C#

foreach (String s in (List<String> )callingForm.Invoke(callingForm.delegateGetKillStrings))
    {
        //do some jazz
    }

private void addKillString(String s)
	{
		lock (killStrings)
		{
			killStrings.Add(s);
		}
	}

List<string> theList = (List<String> )callingForm.Invoke(callingForm.delegateGetKillStrings);
    lock(theList.SyncRoot) {
        foreach(string s in theList) {
               // Do some Jazz
        }
    }