Я хочу сохранить файл в VB.net из количества попыток пользователя

Есть много вопросов, но я не хочу их вам портить.

Я определенно рекомендую поставить «Option Strict On» в настройках проекта, чтобы вы сразу видели, где есть ошибки преобразования (строка, присвоенная целому числу и т. д.).

Чтобы сохранить файл, он должен быть примерно таким

If Save = "yes" Then
    File.WriteAllText("C:\X\Code\VB\Challenges\Challenge 1\MasterMind Test", Tries.ToString())
End If

(есть также функции File.Append...).

Класс Random немного сложен, это объект, который предоставляет случайные значения, но сам по себе еще не является случайным значением. Всегда используйте один и тот же случайный объект для всех разных чисел, иначе вы можете получить одно и то же число:

Private Randomizer As New Random(Environment.TickCount)
Private EasyNum As Int32 = Randomizer.Next(0, 10000) '0 to 9999
Private HardNum As Int32 = Randomizer.Next(0, 100000) '0 to 99999

Значение "от" метода Next рандомизатора всегда является включающим, значение "до" исключающим по каким-либо причинам.

Для увеличения переменной используется синтаксис

Tries += 1 

вместо «Попытки = +1»

Чтобы записать число с начальными цифрами, используйте may use

Console.Out.WriteLine($"The correct solution would have been: {EasyNum:0000}")

или

EasyNum.ToString("0000")

Комментарии в очереди. Пожалуйста, прочитайте все комментарии. Эта программа все еще работает не слишком хорошо, но я оставлю ее вам, чтобы привести ее в порядок.

Module Module1
    Dim Tries As Integer = 0
    'Use only a single instance of Random
    Dim EasyNum As New Random
    'Dim HardNum As New Random
    Dim Attempt As String
    Sub Main()
        Dim Difficulty As String
        Console.WriteLine("Welcome to MasterMind")
        Console.WriteLine("Choose between Easy and Hard difficulty")

        Difficulty = Strings.LCase(Console.ReadLine)
        'AndAlso prevents the second condition from executing if the first condition is true
        While Difficulty <> "easy" AndAlso Difficulty <> "hard"
            Console.WriteLine("That's not a correct mode")
            Difficulty = Strings.LCase(Console.ReadLine)
        End While
        If Difficulty = "easy" Then
            Easy()
            'ElseIf Difficulty = "hard" Then
            '    Hard() 'Not implemented
        End If

    End Sub


    'Dim EasyGuess2 As Integer
    'Dim X As String
    'Dim Y As Integer
    Sub Easy()
        Dim EasyGuess1 As Integer
        Console.WriteLine("You have chosen the easy difficulty")
        Console.WriteLine("You have to guess a 4 Digit number between 1000 and 9999")
        Console.WriteLine("Enter your guess")
        'X = EasyNum.Next(1000, 9999)
        'The next method returns a non-negative integer not a string
        'To get 0 to 9999 with leading zeros do the following
        'Returns a non-negative random integer that is less than the specified maximum.
        Dim X = EasyNum.Next(10000).ToString("0000")
        Console.WriteLine(X)
        'Console.ReadLine returns a String. You should Not assign a String to an Integer
        'EasyGuess1 = Console.ReadLine
        'Never trust a user :-) Use .TryParse to set the variable
        Integer.TryParse(Console.ReadLine, EasyGuess1)
        'This just assigns the value of 1 to Tries
        'Tries = +1
        'If you want to increment Tries
        Tries += 1
        'Let's compare apples and apples
        'If you just convert EasyGuess1 To a string and EasyGuess1 is 54
        'the string is "54" You want a 4 character string
        'If Mid(EasyGuess1.ToString("0000"), 1, 1) = Mid(CStr(X), 1, 1) Then
        '    'but you only compared the first character, what about the rest?
        '    'Mid is aroung for backward compatibility. It should not be used for new code.
        '    'The position Integer is one based which is not at all sympatico with .net
        '    Console.WriteLine("You have 1 number correct, try again?")
        '    Attempt = Strings.LCase(Console.ReadLine)
        '    While Attempt <> "yes" And Attempt <> "no"
        '        Console.WriteLine("Enter either yes or no")
        '        Attempt = Strings.LCase(Console.ReadLine)
        '    End While
        'ElseIf Mid(CStr(EasyGuess1), 2, 1) = Mid(CStr(X), 2, 1) Then
        '    Console.WriteLine("You have 1 number correct, try again?")
        '    Attempt = Strings.LCase(Console.ReadLine)
        '    While Attempt <> "yes" And Attempt <> "no"
        '        Console.WriteLine("Enter either yes or no")
        '        Attempt = Strings.LCase(Console.ReadLine)
        '    End While
        'End If
        Dim CorrectCharacters = TestInput(EasyGuess1, X)
        Console.WriteLine($"You have guessed {CorrectCharacters} correctly. Try again?")
        'Use the .net way. The framework is available to all languages in .net
        'so it will be easier to learn other languages.
        Attempt = (Console.ReadLine).ToLower
        If Attempt = "yes" Then
            EasyYes()
        ElseIf Attempt = "no" Then
            EasyNo()
        End If
        Console.WriteLine("incorrect")

        Console.ReadKey()
    End Sub
    Sub EasyYes()
        Dim guess As Integer
        Console.WriteLine("Enter a new guess")
        'You are doing this twice ???
        'EasyGuess1 = Console.ReadLine
        Integer.TryParse(Console.ReadLine, guess)
        'EasyGuess1 will be 0 if the entry is other than an integer
        'Very nice bu this Sub doesn't do anything
    End Sub
    Sub EasyNo()
        Dim Save As String
        'Unused local variable and if you needed this it is a poor choice for a variable name
        'Dim File As System.IO.File
        Console.WriteLine("Do you want to save your tries? Enter Yes or no")
        Save = Console.ReadLine.ToLower
        If Save = "yes" Then
            'Give the file name an extension.
            'When you pass (String, Integer) to the Create method, the Integer is the number of bytes 
            'buffered for reads And writes to the file.
            'Not exactly what you were expecting.
            'System.IO.File.Create("C:\X\Code\VB\Challenges\Challenge 1\MasterMind Test.txt", Tries)
            'Imports System.IO at top of file
            File.WriteAllText("C:\X\Code\VB\Challenges\Challenge 1\MasterMind Test.txt", Tries.ToString)
        End If
    End Sub
    Sub Hard()
        Console.WriteLine("You have chosen the hard difficulty")
    End Sub

    Private Function TestInput(Guessed As Integer, RandString As String) As Integer
        Dim Correct As Integer
        Dim Guess As String = Guessed.ToString("0000")
        'A String can be a Char array
        'Here we check each character in the 2 strings for equality
        'and if true then increment Correct
        For i = 0 To 3
            If Guess(i) = RandString(i) Then
                Correct += 1
            End If
        Next
        Return Correct
    End Function

End Module