uses group by to partition a list of each customer's orders, first by year, and then by month. : GroupBy « LINQ « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. LINQ
3. GroupBy

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class MainClass {
    public static void Main() {

        List<Customer> customers = GetCustomerList();

        var customerOrderGroups =
            from c in customers
            select
                new {
                    c.CompanyName,
                    YearGroups =
                        from o in c.Orders
                        group o by o.OrderDate.Year into yg
                        select
                            new {
                                Year = yg.Key,
                                MonthGroups =
                                    from o in yg
                                    group o by o.OrderDate.Month into mg
                                    select new { Month = mg.Key, Orders = mg }
                            }
                };

        Console.Write(customerOrderGroups);
    }
    static List<Customer> GetCustomerList() {
        List<Product> empTree = new List<Product>();
        empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });

        List<Customer> l = new List<Customer>();
        l.Add(new Customer { CompanyName = "A", Region = "R1", UnitsInStock = 1, Orders = empTree, CustomerId = 0 });
        l.Add(new Customer { CompanyName = "B", Region = "R2", UnitsInStock = 2, Orders = empTree, CustomerId = 1 });
        l.Add(new Customer { CompanyName = "C", Region = "R3", UnitsInStock = 3, Orders = empTree, CustomerId = 2 });
        l.Add(new Customer { CompanyName = "D", Region = "R4", UnitsInStock = 4, Orders = empTree, CustomerId = 3 });
        l.Add(new Customer { CompanyName = "E", Region = "R5", UnitsInStock = 5, Orders = empTree, CustomerId = 4 });
        return l;
    }
}
class Customer : IComparable<Customer> {
    public string CompanyName { get; set; }
    public string Region { get; set; }
    public List<Product> Orders { get; set; }
    public int UnitsInStock { get; set; }
    public int CustomerId { get; set; }

    public override string ToString() {
        return String.Format("Id: {0}, Name: {1}, Region: {3}", this.CustomerId, this.CompanyName, this.Region);
    }
    int IComparable<Customer>.CompareTo(Customer other) {
        if (other == null)
            return 1;

        if (this.CustomerId > other.CustomerId)
            return 1;

        if (this.CustomerId < other.CustomerId)
            return -1;

        return 0;
    }
}
class Product : IComparable<Product> {
    public string ProductName { get; set; }
    public string Category { get; set; }
    public int UnitPrice { get; set; }
    public int UnitsInStock { get; set; }
    public int Total { get; set; }
    public DateTime OrderDate { get; set; }
    public int Id { get; set; }

    public override string ToString() {
        return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
    }
    int IComparable<Product>.CompareTo(Product other) {
        if (other == null)
            return 1;
        if (this.Id > other.Id)
            return 1;

        if (this.Id < other.Id)
            return -1;

        return 0;
    }
}