Make an extension method that does the appropriate comparison:
public static class ExtensionMethods
{
public static bool IsCharacter(this char c, char thatChar)
{
return (c == thatChar);
}
}
And call it this way:
e.Handled = (!char.IsDigit(e.KeyChar) &&
!char.IsControl(e.KeyChar) &&
!char.IsCharacter(System.Threading.Thread.CurrentThread.CurrentCulture.NumberFormat.CurrencyDecimalSeparator.GetAt(0)));
If you want to, you can also account for the CurrencyGroupSeparator.