sql server 

 --[dbo].[sp_DuAnGetKhoangCach] 10.7756587, 106.70042379999995, 0.5

CREATE PROCEDURE [dbo].[sp_DuAnGetKhoangCach]

    @ZipLatitude float, 
    @ZipLongitude float,
    @Radius  float
AS  
SELECT * FROM DuAn

WHERE dbo.TinhKhoanCach2Diem(lat, lng, @ZipLatitude, @ZipLongitude) < @Radius



CREATE FUNCTION TinhKhoanCach2Diem(
    @Latitude1 float,
    @Longitude1 float,
    @Latitude2 float,
    @Longitude2 float
)
RETURNS float 
AS  
    BEGIN 
    -- CONSTANTS
    DECLARE @EarthRadiusInMiles float;
    SET @EarthRadiusInMiles = 6371 -- 3959 for miles, 6371 for KM

    DECLARE @PI  float;
    SET @PI = PI();
    -- RADIANS conversion
    DECLARE @lat1Radians float;
    DECLARE @long1Radians float;
    DECLARE @lat2Radians float;
    DECLARE @long2Radians float;
    SET @lat1Radians = @Latitude1 * @PI / 180;
    SET @long1Radians = @Longitude1 * @PI / 180;
    SET @lat2Radians = @Latitude2 * @PI / 180;
    SET @long2Radians = @Longitude2 * @PI / 180;
    RETURN Acos(
    Cos(@lat1Radians) * Cos(@long1Radians) * Cos(@lat2Radians) * Cos(@long2Radians) + 
    Cos(@lat1Radians) * Sin(@long1Radians) * Cos(@lat2Radians) * Sin(@long2Radians) + 
    Sin(@lat1Radians) * Sin(@lat2Radians)
    ) * @EarthRadiusInMiles;
END

C#

/// <summary>
/// Specifies a Latitude / Longitude point.
/// </summary>
public class LatLng
{
    public double Latitude { get; set; }
    public double Longitude { get; set; }

    public LatLng(){
    }

    public LatLng(double lat, double lng)
    {
        this.Latitude = lat;
        this.Longitude = lng;
    }
}

/// <summary>
/// Returns the distance in miles or kilometers of any two
/// latitude / longitude points.
/// </summary>
/// <param name="pos1">Location 1</param>
/// <param name="pos2">Location 2</param>
/// <param name="unit">Miles or Kilometers</param>
/// <returns>Distance in the requested unit</returns>
public double HaversineDistance(LatLng pos1, LatLng pos2, DistanceUnit unit)
{
    double R = (unit == DistanceUnit.Miles) ? 3960 : 6371;
    var lat = (pos2.Latitude - pos1.Latitude).ToRadians();
    var lng = (pos2.Longitude - pos1.Longitude).ToRadians();
    var h1 = Math.Sin(lat / 2) * Math.Sin(lat / 2) +
                  Math.Cos(pos1.Latitude.ToRadians()) * Math.Cos(pos2.Latitude.ToRadians()) *
                  Math.Sin(lng / 2) * Math.Sin(lng / 2);
    var h2 = 2 * Math.Asin(Math.Min(1, Math.Sqrt(h1)));
    return R * h2;
}

public enum DistanceUnit { Miles, Kilometers };

var Offices = GetMyOfficeList();
for(int i = 0; i< Offices.Count; i++)
{
    Offices[i].Distance = HaversineDistance(
                            coord,
                            new LatLng(Offices[i].Lat, Offices[i].Lng),
                            DistanceUnit.Miles);
}

var closestOffice = Offices.OrderBy(x => x.Distance).Take(1).Single();