Design a C# application that interacts with a PostgreSQL database to store and manage a simple inventory system. The application should support adding items, updating item quantities, and displaying the current inventory.
PLAINFORMAT
using System;
using Npgsql;

namespace InventorySystem
{
    class Program
    {
        private const string ConnectionString = "Host=localhost;Username=postgres;Password=your_password;Database=inventory_db";

        static void Main(string[] args)
        {
            AddItem("Laptop", 10);
            AddItem("Monitor", 5);

            DisplayInventory();

            UpdateItemQuantity(1, 8);

            DisplayInventory();
        }

        private static NpgsqlConnection GetConnection()
        {
            return new NpgsqlConnection(ConnectionString);
        }

        private static void AddItem(string name, int quantity)
        {
            using (var connection = GetConnection())
            {
                connection.Open();
                using (var command = new NpgsqlCommand("INSERT INTO items (name, quantity) VALUES (@name, @quantity)", connection))
                {
                    command.Parameters.AddWithValue("name", name);
                    command.Parameters.AddWithValue("quantity", quantity);

                    command.ExecuteNonQuery();
                }
            }
        }

        private static void UpdateItemQuantity(int id, int newQuantity)
        {
            using (var connection = GetConnection())
            {
                connection.Open();
                using (var command = new NpgsqlCommand("UPDATE items SET quantity = @quantity WHERE id = @id", connection))
                {
                    command.Parameters.AddWithValue("id", id);
                    command.Parameters.AddWithValue("quantity", newQuantity);

                    command.ExecuteNonQuery();
                }
            }
        }

        private static void DisplayInventory()
        {
            using (var connection = GetConnection())
            {
                connection.Open();
                using (var command = new NpgsqlCommand("SELECT * FROM items", connection))
                {
                    using (var reader = command.ExecuteReader())
                    {
                        Console.WriteLine("ID\tName\tQuantity");
                        while (reader.Read())
                        {
                            Console.WriteLine($"{reader.GetInt32(0)}\t{reader.GetString(1)}\t{reader.GetInt32(2)}");
                        }
                    }
                }
            }
        }
    }
}