Simple working example of working with Npgsql.

npgsql_example.cs

using System;
using Npgsql;

namespace learningcsharp
{
    class Program
    {
        static void Main(string[] args)
        {
            var connString = "Host=localhost;Username=arete;Database=arete";

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

                using (var cmd = new NpgsqlCommand())
                {
                    cmd.Connection = conn;
                    cmd.CommandText = "INSERT INTO customers (first_name, last_name) VALUES (@first_name, @last_name)";
                    cmd.Parameters.AddWithValue("first_name", "Warren");
                    cmd.Parameters.AddWithValue("last_name", "Henning");
                    cmd.Prepare();
                    cmd.ExecuteNonQuery();
                }

                DateTime start = DateTime.Now;

                using (var cmd = new NpgsqlCommand("SELECT id, first_name, last_name FROM customers order by id desc LIMIT 5", conn))
                {
                    cmd.Prepare();
                    using (var reader = cmd.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            Console.WriteLine("{0}: {1}, {2}", reader.GetInt32(0), reader.GetString(1), reader.GetString(2));
                        }
                            
                    }                        
                }
                DateTime end = DateTime.Now;
                Console.WriteLine(end - start);
                Console.ReadLine();                
            }
        }
    }
}