internal class Program
{
    private static void Main(string[] args)
    {
        var db = new DataBase { DBName = "mydb"};

        // 쿼리식 표현
        var v = from ord in db.Orders
                where ord.Customer_ID == "FRANS"
                orderby ord.Order_Date ascending
                select ord;

        // 메서드식 표현1
        var w = db.Orders.Where(order => order.Customer_ID == "FRANS").Select(o => o);

        // 메서드식 표현2
        // 위의 메서드식 표현1의 Select()메서드는 동일데이타를
        // 변형없이 리턴하므로 생략가능
        var x = db.Orders.Where(order => order.Customer_ID == "FRANS");

        x.Select((item, index) => new { IDX = index, ITEM = item });

        //기타 확장메서드
        x.First();
        x.FirstOrDefault();
        x.Last();
        var l = x.ToList();
        l.ForEach(item => Console.WriteLine(item.Customer_ID));
        x.Select(item => item.Order_Date).Distinct();
        
    }
}

class DataBase
{
    public string DBName = string.Empty;
    public List<Order> Orders = new();
}

class Order
{
    public string Customer_ID = string.Empty;
    public DateTime Order_Date;
    public List<string> Items = new();
}