Command Interpreter - working solution

CommandInterpreter.cs

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


class CommandInterpreter
{
    static void Main()
    {
        List<string> items = ReadInputItems();

        while (true)
        {
            var commandLine = Console.ReadLine();
            if (commandLine == "end")
                break;
            ProcessCommand(items, commandLine);
        }

        Console.WriteLine($"[{string.Join(", ", items)}]");
    }

    private static void ProcessCommand(List<string> items, string commandLine)
    {
        var cmdTokens = commandLine.Split(' ');
        var cmdName = cmdTokens[0];
        switch (cmdName)
        {
            case "reverse":
                ReverseList(items, cmdTokens);
                break;
            case "sort":
                SortList(items, cmdTokens);
                break;
            case "rollLeft":
                RollLeftList(items, cmdTokens);
                break;
            case "rollRight":
                RollRightList(items, cmdTokens);
                break;
        }
    }

    static void ReverseList(List<string> items, string[] cmdTokens)
    {
        int startIndex = int.Parse(cmdTokens[2]);
        int count = int.Parse(cmdTokens[4]);
        if (IsValidRange(items, startIndex, count))
            ReverseList(items, startIndex, count);
        else
            Console.WriteLine("Invalid input parameters.");
    }

    static bool IsValidRange(List<string> items, int startIndex, int count)
    {
        if (startIndex < 0)
            return false;
        if (startIndex >= items.Count)
            return false;
        if (count < 0)
            return false;
        if (startIndex + count - 1 >= items.Count)
            return false;
        return true;
    }

    static void ReverseList(List<string> items, int startIndex, int count)
    {
        var endIndex = startIndex + count - 1;

        while (startIndex < endIndex)
        {
            var oldValue = items[startIndex];
            items[startIndex] = items[endIndex];
            items[endIndex] = oldValue;
            endIndex--;
            startIndex++;
        }
    }

    static void SortList(List<string> items, string[] cmdTokens)
    {
        int startIndex = int.Parse(cmdTokens[2]);
        int count = int.Parse(cmdTokens[4]);
        if (IsValidRange(items, startIndex, count))
            SortList(items, startIndex, count);
        else
            Console.WriteLine("Invalid input parameters.");
    }

    static void SortList(List<string> items, int startIndex, int count)
    {
        // 10 | 20 30 | 40 50
        //      1  2
        // startIndex = 1;
        // count = 2;

        var leftList = items.Take(startIndex);
        var middleList = items.Skip(startIndex).Take(count).OrderBy(x => x);
        var rightList = items.Skip(startIndex + count);
        var allItems = leftList.Concat(middleList).Concat(rightList);

        var index = 0;
        foreach (var item in allItems.ToList())
        {
            items[index] = item;
            index++;
        }
    }

    static void RollLeftList(List<string> items, string[] cmdTokens)
    {
        int count = int.Parse(cmdTokens[1]);
        if (count >= 0)
            RollListList(items, -count);
        else
            Console.WriteLine("Invalid input parameters.");
    }

    static void RollListList(List<string> items, int count)
    {
        count = count % items.Count;
        var result = new string[items.Count];

        for (int i = 0; i < items.Count; i++)
        {
            var newPos = (i + count) % items.Count;
            if (newPos < 0)
                newPos += items.Count;
            result[newPos] = items[i];
        }

        var index = 0;
        foreach (var item in result.ToList())
        {
            items[index] = item;
            index++;
        }
    }

    static void RollRightList(List<string> items, string[] cmdTokens)
    {
        int count = int.Parse(cmdTokens[1]);
        if (count >= 0)
            RollListList(items, count);
        else
            Console.WriteLine("Invalid input parameters.");
    }

    static List<string> ReadInputItems()
    {
        List<string> items =
            Console.ReadLine()
            .Split(' ')
            .Where(i => i.Length > 0)
            .ToList();
        return items;
    }
}