LocationCalculator/API/POCDistance/Util/CalculateDistance.cs

using System;

namespace POCDistance.Util
{
    public static class CalculateDistance
    {
        static double toRadians(
           double angleIn10thofaDegree)
        {
            // Angle in 10th
            // of a degree
            return (angleIn10thofaDegree * Math.PI) / 180;
        }
        public static double GetDistance(double lat1, double lon1, double lat2, double lon2)
        {

            // The math module contains
            // a function named toRadians
            // which converts from degrees
            // to radians.
            lon1 = toRadians(lon1);
            lon2 = toRadians(lon2);
            lat1 = toRadians(lat1);
            lat2 = toRadians(lat2);

            // Haversine formula
            double dlon = lon2 - lon1;
            double dlat = lat2 - lat1;
            double a = Math.Pow(Math.Sin(dlat / 2), 2) +
                       Math.Cos(lat1) * Math.Cos(lat2) *
                       Math.Pow(Math.Sin(dlon / 2), 2);

            double c = 2 * Math.Asin(Math.Sqrt(a));

            // Radius of earth in
            // kilometers. Use 3956
            // for miles
            double r = 6371;

            // calculate the result
            return (c * r);
        }
    }
}