PostelsLawDistanceCalculator.cs

public class DistanceCalculator
{
    public int CalculateDistanceInKm(string origin, string destination)
    {
        if (string.IsNullOrEmpty(origin)
        {
            throw new ArgumentNullException("You tried to provide a null or empty origin; please supply a valid city name");
        }
        if (string.IsNullOrEmpty(destination)
        {
            throw new ArgumentNullException("You tried to provide a null or empty destination; please supply a valid city name");
        }
            
        return GoogleMaps.DistanceMatrix.Query(origin, destination);
    }
}