ParkingValidation

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

namespace ParkingValidation
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            var userBook = new Dictionary<string, string>();

            for (int i = 0; i < n; i++)
            {
                string[] text = Console.ReadLine().Split().ToArray();
                string instruction = text[0];
                string userName = text[1];
                if (instruction == "register")
                {

                    string licensePlate = text[2];

                    if (userBook.ContainsKey(userName) == false)
                    {
                        if (LicenseValidation(licensePlate) == false)
                        {
                            Console.WriteLine($"ERROR: invalid license plate {licensePlate}");

                        }
                        else if (userBook.ContainsValue(licensePlate))
                        {
                            Console.WriteLine($"ERROR: license plate {licensePlate} is busy");

                        }
                        else
                        {
                            userBook.Add(userName, licensePlate);
                            Console.WriteLine($"{userName} registered {licensePlate} successfully");
                        }
                    }
                    else
                    {
                        Console.WriteLine($"ERROR: already registered with plate number {userBook[userName]}");

                    }
                }
                else if (instruction == "unregister")
                {
                    if (userBook.ContainsKey(userName) == false)
                    {
                        Console.WriteLine($"ERROR: user {userName} not found");
                    }
                    else
                    {
                        userBook.Remove(userName);
                        Console.WriteLine($"user {userName} unregistered successfully");
                    }
                }
            }
            foreach (var pair in userBook)
            {
                Console.WriteLine($"{pair.Key} => {pair.Value}");
            }
        }

        static bool LicenseValidation(string licensePlate)
        {

            var firstLast = licensePlate.Substring(0, 2) + licensePlate.Substring(6, 2);
            bool firstLastValidation = false;

            foreach (var symbol in firstLast)
            {
                if (symbol >= 'A' && symbol <= 'Z')
                {
                    firstLastValidation = true;
                }
                else
                {
                    firstLastValidation = false;
                    break;
                }
            }


            var middleNumbers = licensePlate.Substring(2, 4);
            bool middleValidation = false;

            foreach (var number in middleNumbers)
            {
                if (number >= '1' && number <= '9')
                {
                    middleValidation = true;
                }
                else
                {
                    middleValidation = false;
                    break;
                }
            }

            if (middleValidation && firstLastValidation == true)
            {
                return true;
            }
            else
            {
                return false;
            }

        }


    }

}