Calculate Distance by Latitude and Longitude using C# 2

public static double distance (double lat1, double lon1, double lat2, double lon2) { 
    double lat1 = Convert.ToDouble(latitude);
    double lon1 = Convert.ToDouble(longitude);
    double lat2 = Convert.ToDouble(destlat);
    double lon2 = Convert.ToDouble(destlon);

    double theta = toRadians(lon1-lon2); 
    lat1 = toRadians(lat1); 
    lon1 = toRadians(lon1); 
    lat2 = toRadians(lat2); 
    lon2 = toRadians(lon2); 

    double dist = sin(lat1)*sin(lat2) + cos(lat1)*cos(lat2)*cos(theta); 
    dist = toDegrees(acos(dist)) * 60 * 1.1515 * 1.609344 * 1000; 

    return dist; 
}