Npgsql.NpgsqlDataAdapter adapter = new Npgsql.NpgsqlDataAdapter(
                 "SELECT myquery blah blah blah blah...",
                 ConnString);
        DataSet ds = new DataSet();
        adapter.Fill(ds);
        DataTable Definitions = ds.Tables["Table"];

produces:

System.NotImplementedException: The requested feature is not implemented.
in <0x00029> System.Data.Common.DbDataAdapter:get_SelectCommand ()
in <0x0002b> System.Data.Common.DbDataAdapter:Fill (System.Data.DataSet dataSet)