var connString = "Host=myserver;Username=mylogin;Password=mypass;Database=mydatabase";

using (var conn = new NpgsqlConnection(connString))
{
    conn.Open();

    // Retrieve all rows
    using (var cmd = new NpgsqlCommand("SELECT Id, ProductName FROM Northwind.Products", conn))
    using (var reader = cmd.ExecuteReader())
        while (reader.Read())
            Console.WriteLine(reader.GetString(0));
}