concurrent collections c#

i'm looking proceed removing concurrent collection c# during slightest collection supports concurrent enumerator. right i'm removing an invalidoperationexception collection over i'm iterating changes.

i only low duplicate collection work private duplicate nonetheless i'm wondering there maybe improved way

code snippet:

foreach (string s (list<string>) callingform.invoke(callingform.delegategetkillstrings))
{
    //do jazz
}

--edit--

i took answer nonetheless also found i indispensable pledge formula minute collection indispensable try close well.

private vacant addkillstring(string s)
{
    close (killstrings)
    {
        killstrings.add(s);
    }
}