using NetTopologySuite.Geometries;
using NetTopologySuite.Geometries.Implementation;
using Npgsql;

NpgsqlConnection.GlobalTypeMapper.UseNetTopologySuite(new DotSpatialAffineCoordinateSequenceFactory(Ordinates.XY), handleOrdinates: Ordinates.XY);

using (NpgsqlConnection con = new NpgsqlConnection(connectionString))
{
    con.Open();

    using (NpgsqlCommand cmd = con.CreateCommand())
    {
        Point p = new Point(new Coordinate(1d, 1d));
        cmd.CommandText = "CREATE TEMP TABLE tmp_geom (geom GEOMETRY)";
        cmd.ExecuteNonQuery();

        cmd.CommandText = "INSERT INTO tmp_geom (geom) VALUES (@p)";
        cmd.Parameters.AddWithValue("@p", NpgsqlTypes.NpgsqlDbType.Geometry, p);
        cmd.ExecuteNonQuery();
    }

    using (NpgsqlCommand cmd = con.CreateCommand())
    {
        cmd.CommandText = "select geom from tmp_geom";

        using (var rdr = cmd.ExecuteReader())
        {
            if (rdr.HasRows)
            {
                rdr.Read();
                var geom = rdr[0];
                Console.WriteLine(geom);
            }
        }
    }
}

Exception message:
System.NotSupportedException
  HResult=0x80131515
  Message=The field 'geom' has type 'postgis.geometry', which is currently unknown to Npgsql. You can retrieve it as a string by marking it as unknown, please see the FAQ.
  Source=Npgsql
Stack trace:
   at Npgsql.Internal.TypeHandlers.UnknownTypeHandler.Read(NpgsqlReadBuffer buf, Int32 byteLen, Boolean async, FieldDescription fieldDescription)
   at Npgsql.Internal.TypeHandling.NpgsqlTypeHandler.<Read>d__4`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at System.Runtime.CompilerServices.ValueTaskAwaiter`1.GetResult()
   at Npgsql.Internal.TypeHandling.NpgsqlTypeHandler`1.<ReadAsObject>d__2.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlDataReader.GetValue(Int32 ordinal)
   at Program.<Main>$(String[] args) in D:\linq2db\newest_everything\newest_everything\Program.cs:line 36

  This exception was originally thrown at this call stack:
    Npgsql.Internal.TypeHandlers.UnknownTypeHandler.Read(Npgsql.Internal.NpgsqlReadBuffer, int, bool, Npgsql.BackendMessages.FieldDescription)
    Npgsql.Internal.TypeHandling.NpgsqlTypeHandler.Read<TAny>(Npgsql.Internal.NpgsqlReadBuffer, int, bool, Npgsql.BackendMessages.FieldDescription)
    System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
    System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
    System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task)
    System.Threading.Tasks.ValueTask<TResult>.Result.get()
    System.Runtime.CompilerServices.ValueTaskAwaiter<TResult>.GetResult()
    Npgsql.Internal.TypeHandling.NpgsqlTypeHandler<TDefault>.ReadAsObject(Npgsql.Internal.NpgsqlReadBuffer, int, bool, Npgsql.BackendMessages.FieldDescription)
    System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
    System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
    ...
    [Call Stack Truncated]