How to modify a hashtable in a foreach loop

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If you try to modify a collection while enumarte over it you will get an Exception : "Collection was modified; enumeration operation may not execute."

Personally, I simply copy the keys:

Hashtable t = new Hashtable();
t.Add(1, 1);
t.Add(2, 2);
ArrayList a = new ArrayList(t.Keys);
foreach (int i in a)
{
  t.Remove(i);
}

Eric Gunnerson has another aproach. You can read it here.