using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Order> orders = new List<Order>

        {

            new Order { ID = 1, CustomerID = 101, OrderDate = new DateTime(2023, 01, 15), TotalAmount = 150 },

            new Order { ID = 2, CustomerID = 102, OrderDate = new DateTime(2023, 03, 10), TotalAmount = 250 },

            new Order { ID = 3, CustomerID = 101, OrderDate = new DateTime(2023, 04, 05), TotalAmount = 100 },

            new Order { ID = 4, CustomerID = 103, OrderDate = new DateTime(2023, 05, 20), TotalAmount = 200 },

            new Order { ID = 5, CustomerID = 101, OrderDate = new DateTime(2023, 07, 25), TotalAmount = 300 },

            new Order { ID = 6, CustomerID = 101, OrderDate = new DateTime(2023, 12, 30), TotalAmount = 450 },

            new Order { ID = 7, CustomerID = 102, OrderDate = new DateTime(2023, 02, 17), TotalAmount = 150 }

        };

        var frequentCustomers = orders

            .GroupBy(o => new { o.CustomerID, Year = o.OrderDate.Year })

            .Where(g => g.Count() > 3)

            .Select(g => new { g.Key.CustomerID, g.Key.Year })

            .ToList();

​

        foreach (var customer in frequentCustomers)

        {

            Console.WriteLine($"CustomerID: {customer.CustomerID}, Year: {customer.Year}");

        }

​

    }

}

​

public class Order

{

    public int ID { get; set; }

    public int CustomerID { get; set; }

    public DateTime OrderDate { get; set; }

    public decimal TotalAmount { get; set; }

}