c# - Retrieve serial ID with Npgsql when inserting with ExecuteScalar

Question

I'm trying to insert a row into a PostgreSQL table with a serial primary key and I need to retrieve this column after it was inserted. I got something like this:

The table "pais" has 3 columns: id, pais, capital; id is a serial column and is its primary key.

NpgsqlCommand query = new NpgsqlCommand("insert into pais(nombre, capital) values(@nombre, @capital)", conn);
query.Parameters.Add(new NpgsqlParameter("nombre", NpgsqlDbType.Varchar));
query.Parameters.Add(new NpgsqlParameter("capital", NpgsqlDbType.Varchar));
query.Prepare();
query.Parameters[0].Value = this.textBox1.Text;
query.Parameters[1].Value = this.textBox2.Text;
Object res = query.ExecuteScalar();
Console.WriteLine(res);

It inserts the row on the table but "res" value is null. If I insert with the nexval('table_sequence') also returns null.

Any idea of how can I return the id of the table? Am I missing something?

Thanks in advance

Answer 1

Is that thread safe?
What if another insert happens between your insert and select? Why not use:

INSERT INTO table (fieldnames) VALUES (values) RETURNING idcolumn?