.NET Standard 2.0 library with OGC extensions methods on geometry (NpgsqlTypes.PostgisGeometry or NetTopologySuite.Geometries.Geometry) instances, providing strongly typed access to PostGIS functions on server side while using linq2db LINQ to database provider.

Property	LinqToDBPostGisNpgsqlTypes	LinqToDBPostGisNetTopologySuite
Extending type	NpgsqlTypes.PostgisGeometry	NetTopologySuite.Geometries.Geometry
Npgsql version	3.x	4.x
PostGIS reference version	1.5	3.0/3.1/3.2/3.3
Status	Legacy	Active
NuGet	—
Dependencies	linq2db, Npgsql	linq2db (>= 3.0.0), NetTopologySuite (>= 2.0.0), NetTopologySuite.IO.PostGis (>= 2.0.0), Npgsql (>= 4.1.0), Npgsql.NetTopologySuite (>= 4.1.0)

NpgsqlTypes.PostgisGeometry or NetTopologySuite.Geometries.Geometry gets additional methods, similar to Microsoft.SqlServer.Types.SqlGeometry OGC Methods on Geometry Instances or NetTopologySuite plugin for Entity Framework Core for PostgreSQL. These methods will be translated into PostGIS SQL operations, so evaluation will happen on the server side. Calling these methods on client side results in throwing InvalidOperationException. Naming convention follows OGC methods names, starting with ST* prefix.

Using extensions methods inside LINQ expression (Npgsql 4):

using LinqToDBPostGisNetTopologySuite

using (var db = new PostGisTestDataConnection())
{
    NetTopologySuite.Geometries.Point point = new Point(new Coordinate(1492853, 6895498)) { SRID = 3857 };

    var dms = db.Select(() => GeometryOutput.STAsLatLonText(point));

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

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

    var stats = db.Polygons
        .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 NetTopologySuite.Geometries.Geometry Geometry { get; set; }
}

[Table("owm_cities", Schema = "public")]
public class CityEntity
{
    [Column("geom")]
    public NetTopologySuite.Geometries.Geometry Geometry { get; set; }
}

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

Developed using MS Visual Studio 2019. Tested on PostgreSQL version 9.6/11, PostGIS version 2.5/3.0/3.1/3.2. Depends on linq2db, Npgsql.

• Make sure you have PostgreSQL DBMS with PostGIS extension installed. Execute SELECT PostGIS_Full_Version() query to check PostGIS version.
• Create new database named "postgistest" (or any other name), add support of spatial features for this database.
• Execute SQL script Sql\create_tables.sql in this database.
• Open solution LinqToDBPostGis.sln in Visual Studio.
• Check database connection string in App.config of all projects.
• Run application, view table data along with PostGIS functions results in console output.

• Implement full set of PostGIS methods.
• Add support for PostGIS geography data type.
• Test on various versions of PostgreSQL/PostGIS and platforms (including .NET 5 and later).
• More tests for corner cases.

• PostGIS Reference
• PostGIS/NetTopologySuite Type Plugin
• Spatial Mapping with NetTopologySuite in Entity Framework Core
• How to teach LINQ to DB convert custom .NET methods and objects to SQL - Sql.Function attribute