public async Task MergeRatesAsync(ExchangeRates toMerge, CancellationToken cancellationToken)
{
using (await fLock.LockForWritingAsync(cancellationToken))
{
var newRates = new ExchangeRates(fRates);
foreach (var rate in toMerge)
{
newRates.Add(rate.Key, rate.Value);
}
fRates = newRates;
}
}