public int Compare(Object a, Object b) {
if (a == b) return 0;
if (a == null) return -1;
if (b == null) return 1;
if (_comparer != null)
return _comparer.Compare(a,b);
IComparable ia = a as IComparable;
if (ia != null)
return ia.CompareTo(b);
throw new ArgumentException(Environment.GetResourceString("Argument_ImplementIComparable"));
}