C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		List<Transaction> transactions = new List<Transaction>
        {
            // Generating sample transactions data for the past year
            new Transaction { TransactionID = 1, CustomerID = 101, TransactionDate = DateTime.Today.AddMonths(-11), Amount = 120.00m },
            new Transaction { TransactionID = 2, CustomerID = 102, TransactionDate = DateTime.Today.AddMonths(-10), Amount = 150.00m },
            new Transaction { TransactionID = 3, CustomerID = 103, TransactionDate = DateTime.Today.AddMonths(-9), Amount = 90.00m },
            new Transaction { TransactionID = 4, CustomerID = 104, TransactionDate = DateTime.Today.AddMonths(-8), Amount = 300.00m },
            new Transaction { TransactionID = 5, CustomerID = 105, TransactionDate = DateTime.Today.AddMonths(-7), Amount = 450.00m },
            new Transaction { TransactionID = 6, CustomerID = 106, TransactionDate = DateTime.Today.AddMonths(-6), Amount = 200.00m },
            new Transaction { TransactionID = 7, CustomerID = 107, TransactionDate = DateTime.Today.AddMonths(-5), Amount = 180.00m },
            new Transaction { TransactionID = 8, CustomerID = 108, TransactionDate = DateTime.Today.AddMonths(-4), Amount = 220.00m },
            new Transaction { TransactionID = 9, CustomerID = 109, TransactionDate = DateTime.Today.AddMonths(-3), Amount = 110.00m },
            new Transaction { TransactionID = 10, CustomerID = 110, TransactionDate = DateTime.Today.AddMonths(-2), Amount = 250.00m },
            new Transaction { TransactionID = 11, CustomerID = 111, TransactionDate = DateTime.Today.AddMonths(-1), Amount = 320.00m }
        };
		
		// Assuming the existence of a list: transactions

		DateTime oneYearAgo = DateTime.Today.AddYears(-1);

		// Step 1: Calculate monthly sales totals for the past year
		var monthlySales = transactions
			.Where(t => t.TransactionDate > oneYearAgo)
			.GroupBy(t => new { t.TransactionDate.Year, t.TransactionDate.Month })
			.Select(g => new
			{
				Year = g.Key.Year,
				Month = g.Key.Month,
				TotalSales = g.Sum(t => t.Amount)
			})
			.OrderByDescending(x => x.Year).ThenByDescending(x => x.Month)
			.ToList();

		// Step 2: Identify the month with the highest sales total
		var highestSalesMonth = monthlySales
			.OrderByDescending(ms => ms.TotalSales)
			.FirstOrDefault();

		Console.WriteLine($"Month with Highest Sales: {highestSalesMonth.Year}-{highestSalesMonth.Month}, Total Sales: {highestSalesMonth.TotalSales}");

		// Step 3: For the highest sales month, list all transactions in descending order of amount
		var transactionsInHighestSalesMonth = transactions
			.Where(t => t.TransactionDate.Year == highestSalesMonth.Year && t.TransactionDate.Month == highestSalesMonth.Month)
			.OrderByDescending(t => t.Amount)
			.ToList();

		Console.WriteLine($"Transactions in {highestSalesMonth.Year}-{highestSalesMonth.Month}:");
		foreach (var transaction in transactionsInHighestSalesMonth)
		{
			Console.WriteLine($"TransactionID: {transaction.TransactionID}, Amount: {transaction.Amount}");
		}


	}
}

public class Transaction
{
    public int TransactionID { get; set; }
    public int CustomerID { get; set; }
    public DateTime TransactionDate { get; set; }
    public decimal Amount { get; set; }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
                    
public class Program
{
    public static void Main()
    {
        List<Transaction> transactions = new List<Transaction>
        {
            // Generating sample transactions data for the past year
            new Transaction { TransactionID = 1, CustomerID = 101, TransactionDate = DateTime.Today.AddMonths(-11), Amount = 120.00m },
            new Transaction { TransactionID = 2, CustomerID = 102, TransactionDate = DateTime.Today.AddMonths(-10), Amount = 150.00m },
            new Transaction { TransactionID = 3, CustomerID = 103, TransactionDate = DateTime.Today.AddMonths(-9), Amount = 90.00m },
            new Transaction { TransactionID = 4, CustomerID = 104, TransactionDate = DateTime.Today.AddMonths(-8), Amount = 300.00m },
            new Transaction { TransactionID = 5, CustomerID = 105, TransactionDate = DateTime.Today.AddMonths(-7), Amount = 450.00m },
            new Transaction { TransactionID = 6, CustomerID = 106, TransactionDate = DateTime.Today.AddMonths(-6), Amount = 200.00m },
            new Transaction { TransactionID = 7, CustomerID = 107, TransactionDate = DateTime.Today.AddMonths(-5), Amount = 180.00m },
            new Transaction { TransactionID = 8, CustomerID = 108, TransactionDate = DateTime.Today.AddMonths(-4), Amount = 220.00m },
            new Transaction { TransactionID = 9, CustomerID = 109, TransactionDate = DateTime.Today.AddMonths(-3), Amount = 110.00m },
            new Transaction { TransactionID = 10, CustomerID = 110, TransactionDate = DateTime.Today.AddMonths(-2), Amount = 250.00m },
            new Transaction { TransactionID = 11, CustomerID = 111, TransactionDate = DateTime.Today.AddMonths(-1), Amount = 320.00m }
        };
        

name:Item3,Item1,Item2
letter:A,A,B
number:1,3,2