distance between two latitude longitude points in c#

/// Loc A Longitude

/// Loc A Latitude

/// Loc B Longitude

/// Loc B Latitude

/// 1=Km,2=mtr

///

public double distance(double X1,double Y1,double X2,double Y2,int Flg)

{

  double R = 6371;

  double dLat = this.toRadian(Y2 - Y1);

  double dLon = this.toRadian(X2 - X1);

  double a = Math.Sin(dLat / 2) * Math.Sin(dLat / 2) +  Math.Cos(this.toRadian(Y1)) * Math.Cos(this.toRadian(Y2)) * Math.Sin(dLon / 2) * Math.Sin(dLon / 2);

  double c = 2 * Math.Asin(Math.Min(1, Math.Sqrt(a)));

  double d = R * c;

  switch (Flg)

  {

      case 1:                 //km

            return d;

            break;

      case 2:                 //mtr

            return d * 1000;

            break;

      default:

            return d;

            break;

   }

}

private double toRadian(double val)

{

     return (Math.PI / 180) * val;

}