C#: Random Number Guessing Game

using System;
using System.Collections.Generic;
using System.Text;

namespace GuessNumberGame {
    public class Program {
        int numberOfAttempts = 5;
        public class Result {
            public int Index { get; set; }
            public bool Flag { get; set; }
        }

        static void Main() {
            Program game = new Program();
            game.Run();
        }

        void Run() {
            // initialize the number of attempts

            Console.WriteLine("\nWelcome to Random Number Guessing Game.");
            Console.WriteLine("\n\nGuess the 4 digit random number XXXX.");
            Console.WriteLine("\nFor each digit, the number is chosen from 1 to 5  \nNumbers can repeat.");
            Console.WriteLine(string.Format("\nYou have {0} attempts to win the game.", numberOfAttempts));

            // Call the method to Generate the Random Number
            var randomNumber = GenerateRandomNumber();
            var correctInputTillNow = new StringBuilder("xxxx");
            for (int i = 1; i <= numberOfAttempts; i++) {
                // Call the method to get the user input
                var userInput = new StringBuilder(GetUserInput(i, correctInputTillNow));

                // Get the result - Collection containing the result of all 4 digits
                int flagCount = 0;
                List<Result> result = GetResult(randomNumber, userInput, out flagCount);                
                if (flagCount > 0) {
                    foreach (var r in result) {
                        if (r.Flag) {
                            int index = r.Index;
                            correctInputTillNow[index] = randomNumber[index];
                        }
                    }
                }
                              

                // check the flag count and display appropriate message
                if (flagCount == 4) {
                    Console.WriteLine("Your Input:{0}, Random Number:{1}", userInput, randomNumber);
                    Console.WriteLine("You guess is correct! Game Won..hurray...:)");
                    break;
                } else if (i == numberOfAttempts) {
                    Console.WriteLine("sorry, You missed it! Game Lost..:(");
                    Console.WriteLine("Random Number is {0}", randomNumber);
                } else {
                    // Get the place(s)/index of digits that are correct
                    Console.WriteLine(
                        string.Format(
                            "Correct input till now: {0}", 
                            correctInputTillNow
                        )
                    );
                }
            }

            Console.ReadLine();
        }

        public List<Result> GetResult(StringBuilder randomNumber, StringBuilder userInput, out int flagCount) {
            List<Result> results = new List<Result>(4);
            flagCount = 0;
            for (int index = 0; index < randomNumber.Length; index++) {
                Result result = new Result();
                result.Index = index;
                bool flag = randomNumber[index] == userInput[index];
                if(flag){
                    flagCount++;
                }
                result.Flag = flag;
                results.Add(result);
            }

            return results;
        }

        public string GetUserInput(int attempt, StringBuilder correctInputTillNow) {
            int inputNumber;

            if (attempt == 1) {
                Console.WriteLine(
                    string.Format(
                        "\nGuess the 4-digit number. Remaining attempts: {0}",
                        numberOfAttempts - (attempt)
                    )
                );
            } else {
                Console.WriteLine(
                    string.Format(
                        "\nGuess the 4-digit number. Guess x: {0}, Remaining attempts: {1}",
                        correctInputTillNow,
                        numberOfAttempts - (attempt)
                    )
                );
            }

            if (
                int.TryParse(Console.ReadLine(), out inputNumber)
                && inputNumber.ToString().Length == 4
            ) {
                return inputNumber.ToString();
            } else {
                Console.WriteLine("You have entered a invalid input.");
                return "0000";
            }
        }

        public StringBuilder GenerateRandomNumber() {
            Random random = new Random();
            StringBuilder number = new StringBuilder(7);
            int length = 4;

            for (int i = 0; i < length; i++) {
                number.Append(random.Next(1, 6));
            }

            return number;
        }
    }
}

List<result>

List<Result>

List<T>