using System;

using System.Linq;

using System.Collections.Generic;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        var monthNames = Enumerable.Range(1, 12).Select((index) => DateTimeFormatInfo.CurrentInfo.GetMonthName(index)).ToList();

        Index indexer = new Index(monthNames.FindIndex(item => item.Equals("april", StringComparison.OrdinalIgnoreCase)));

        var fourMonths = monthNames.ByIndex(indexer.Value, 4);

        Console.WriteLine(string.Join(",", fourMonths));

        Console.WriteLine("");

        List<int> list = new()

        {1, 2, 3, 4, 5};

        Console.WriteLine($"list[^1] => {list[^1]}");

        Console.WriteLine("");

        var monthNamesArray = Enumerable.Range(1, 12).Select((index) => DateTimeFormatInfo.CurrentInfo.GetMonthName(index)).ToArray();

        Range firstFourRange = ..4;

        Console.WriteLine(string.Join(",", monthNamesArray[firstFourRange]));

    }

}

​

static class Extensions

{

    public static List<T> ByIndex<T>(this List<T> value, int startIndex, int endIndex) => value.GetRange(startIndex, endIndex);

}