Task: 21 game
C#: 

using System;

namespace _21Game
{
    public class Program
    {
        private const string computerPlayer = "Computer";
        private const string humanPlayer = "Player 1";

        public static string SwapPlayer(string currentPlayer)
        {
            if (currentPlayer == computerPlayer)
            {
                currentPlayer = humanPlayer;
            }
            else
            {
                currentPlayer = computerPlayer;
            }

            return currentPlayer;
        }

        public static void PlayGame()
        {
            bool playAnother = true;
            int total = 0;
            int final = 21;
            int roundChoice = 0;
            string currentPlayer = RandomPLayerSelect();
            int compWins = 0;
            int humanWins = 0;

            while (playAnother)
            {
                Console.WriteLine($"Now playing: {currentPlayer}");
                try
                {
                    if (currentPlayer == computerPlayer)
                    {
                       roundChoice =  CompMove(total);
                    }
                    else
                    {
                        roundChoice = int.Parse(Console.ReadLine());
                    }
                    

                    if (roundChoice != 1 && roundChoice != 2 && roundChoice != 3)
                    {
                        throw new Exception();
                    }

                    total += roundChoice;
                }
                catch (Exception)
                {
                    Console.WriteLine("Invalid choice! Choose from numbers: 1, 2, 3.");
                    continue;
                }

                Console.WriteLine(total);

                if (total == final)
                {
                    if (currentPlayer == computerPlayer)
                    {
                        compWins++;
                    }
                    if (currentPlayer == humanPlayer)
                    {
                        humanWins++;
                    }
                    Console.WriteLine($"Winner: {currentPlayer}");
                    Console.WriteLine($"Comp wins: {compWins}. Human wins: {humanWins}");
                    Console.WriteLine($"do you wan to play another round? y/n");
                    var choice = Console.ReadLine();
                    if (choice == "y")
                    {
                        total = 0;
                    }
                    else if (choice == "n")
                    {
                        break;
                    }
                    else
                    {
                        Console.WriteLine("Invalid choice! Choose from y or n");
                        continue;
                    }
                }

                else if (total > 21)
                {
                    Console.WriteLine("Not the right time to play this game :)");
                    break;
                }

                currentPlayer = SwapPlayer(currentPlayer);
            }
        }

        public static bool CheckIfCanWin(int total)
        {
            bool result = false;
            if (total == 18)
            {
                result = true;
            }
            return result;
        }

        public static int CompMove(int total)
        {
            int choice = 0;

            if (CheckIfCanWin(total))
            {
                choice = 21 - total;
            }
            else
            {
                choice = new Random().Next(1,4);
            }

            return choice;
        }

        public static string RandomPLayerSelect()
        {
            string[] players = new string[] { computerPlayer, humanPlayer };
            var random = new Random().Next(0,2);
            return players[random];
        }

        public static void Main(string[] args)
        {
            
            Console.WriteLine("Welcome to 21 game \n");
            Console.WriteLine(@"21 is a two player game. 
The game is played by choosing a number.
1, 2, or 3 to be added a total sum. \n
The game is won by the player reaches exactly 21. \n" );            
            Console.WriteLine("Choose your number: (1, 2 or 3)");

            PlayGame();
        }
    }
}

VB: 

Class MainWindow
    Private Const GOAL As Integer = 21
    Private total As Integer = 0
    Private random As New Random

    Private Sub Update(box As TextBox, player As String, move As Integer)
        total = total + move
        box.Text = move
        boxTotal.Text = total
        If total + 1 > GOAL Then button1.IsEnabled = False
        If total + 2 > GOAL Then button2.IsEnabled = False
        If total + 3 > GOAL Then button3.IsEnabled = False
        If total = GOAL Then
            winner.Content = $"The winner is {player}."
        End If
    End Sub

    Private Sub Ai()
        Dim move As Integer = 1
        For i = 1 To 3
            If (total + i - 1) Mod 4 = 0 Then move = i
        Next i
        For i = 1 To 3
            If total + i = GOAL Then move = i
        Next i
        Update(boxAI, "AI", move)
    End Sub

    Private Sub Choice(sender As Object, e As RoutedEventArgs) _
            Handles button1.Click, button2.Click, button3.Click
        Update(boxHuman, "human", sender.Content)
        If total < GOAL Then Ai()
    End Sub

    
    
    
    Private Sub StartGame(sender As Object, e As RoutedEventArgs) Handles restart.Click

        total = 0

        boxAI.Text = ""
        boxHuman.Text = ""
        boxTotal.Text = ""
        
        winner.Content = ""

        button1.IsEnabled = True
        button2.IsEnabled = True
        button3.IsEnabled = True

        
        
        
        If random.Next(2) = 0 Then
            first.Content = "First player is AI player."
            Ai()
        Else
            first.Content = "First player is human player."
        End If
    End Sub
End Class