public DataTable InvokeQuery(string connectionString, string queryString)
{
    NpgsqlConnection con = new NpgsqlConnection(connectionString);
    try
    {
        con.Open();
        NpgsqlCommand cmd = con.CreateCommand();
        cmd.CommandText = queryString;
        cmd.CommandType = System.Data.CommandType.Text;
        NpgsqlDataAdapter adapter = new NpgsqlDataAdapter(cmd);
        DataTable result = new DataTable();
        adapter.Fill(result);
        return result;
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        con.Close();
        con = null;
    }
}