using LinqToDBPostGisNpgsqlTypes; // or LinqToDBPostGisNetTopologySuite

using (var db = new PostGisTestDataConnection())
{
    NpgsqlTypes.PostgisGeometry point = db.STGeomFromEWKT("SRID=3857;POINT(0 5)"); // For Npgsql 3.x
    NetTopologySuite.Geometries.Point point = new Point(new Coordinate(0, 5)) { SRID = 3857 }; // For Npgsql 4.x

    var selected = db.Polygons
        .Where(p => p.Geometry.STArea() > 150.0)
        .OrderBy(p => p.Geometry.STDistance(point))
        .ToList();

    var nearestCity = db.Cities
        .OrderBy(c => c.Geometry.STDistance(point))
        .FirstOrDefault();

    var stats = db.Cities
        .Select(c =>
             new
             {
                 Id = c.Id,
                 Name = c.Name,
                 Area = c.Geometry.STArea(),
                 Distance = c.Geometry.STDistance(point),
                 NumPoints = c.Geometry.STNPoints(),
                 Srid = c.Geometry.STSrId(),
                 Wkt = c.Geometry.STAsText(),
             })
        .ToList();
}

[Table("test_geometry", Schema = "public")]
public class PolygonEntity
{
    [Column("geom")]
    public NpgsqlTypes.PostgisGeometry Geometry { get; set; } // For Npgsql 3.x
    public NetTopologySuite.Geometries.Geometry Geometry { get; set; } // For Npgsql 4.x
}

[Table("owm_cities", Schema = "public")]
public class CityEntity
{
    [Column("geom")]
    public NpgsqlTypes.PostgisGeometry Geometry { get; set; } // For Npgsql 3.x
    public NetTopologySuite.Geometries.Geometry Geometry { get; set; } // For Npgsql 4.x
}

class PostGisTestDataConnection : LinqToDB.Data.DataConnection
{
    public ITable<PolygonEntity> Polygons { get { return GetTable<PolygonEntity>(); } }
    public ITable<CityEntity> Cities { get { return GetTable<CityEntity>(); } }
}