private static ExchangeRate GetRate(string from, string to, DateTime?date = null)
{
from = from.ToUpper();
to = to.ToUpper();
if (!Symbols.IsValid(from))
{
throw new ArgumentException("Symbol not found for provided currency", "from");
}
if (!Symbols.IsValid(to))
{
throw new ArgumentException("Symbol not found for provided currency", "to");
}
var rates = GetLatestRates(date);
var fromRate = rates.Value <double>(from);
var toRate = rates.Value <double>(to);
var rate = toRate / fromRate;
return(new ExchangeRate(from, to, rate));
}