sql = $"insert into users(id, name, age) VALUES(@id, @name, @age)";
var cmd = new NpgsqlCommand(sql, conn);
cmd.Parameters.AddWithValue("@id", u.id);
cmd.Parameters.AddWithValue("@name", u.name);
cmd.Parameters.AddWithValue("@age", u.age);
var result = cmd.ExecuteScalar();