c# - AlphaBeta Pruning TicTacToe is not blocking, is it eval problem?

Question

I'm debuging this for days and i don't know what I made wrong in this piece of code for Tic-Tac-Toe game and AI (well I know its not real AI but...) I have choosen for this was Alpha-Beta pruning. Its 7x7 board so it would be too heavy for pure minimax implementation.

Problem I have is that i can't figure out why Alpha-Beta is not blocking player to stall the game and wait for player move and use proper move in his favour, or just simply tie the game.

I've decided that center of board will have more points(for end score) than the one at the edges of the board. I belive that moves more to the center will have more chance to success than the ones in the edges, thats why i made AddScoreToMove function that evaluates that move. To Ensure that eval function will check EVERY possible moves at board I didn't made that function to work as find first xxx (for example at row0 and col0,col1,col2) and return (cause maybe there is 4X's or 4O's). Also 4X or 4O gives substantially more score than other ones and should be considered as win.
At this moment my PCplayer( O's) play like this

Can anyone tell me what i made wrong? It's my second program with AI, and first was with minimax on 3x3 board whitch worked fine.

C# code is below

VB.NET CODE:

Public Class Form1
    Dim board As Char(,) = {
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "},
            {" ", " ", " ", " ", " ", " ", " "}}
    Class Move
        Public row, col As Integer
    End Class
    
    Dim BestMoveRow As Integer = 0
    Dim BestMoveCol As Integer = 0
    Dim BestMoveScore As Integer = 0

    Shared player As Char = "X", opponent As Char = "O"

    Shared Function AddScoreToMove(thatMove As Move) As Integer
        Dim row As Integer = thatMove.row
        Dim col As Integer = thatMove.col
        '0 score, move is at border 
        If ((row >= 1 And row <= 5) And col = 0) Then
            Return 0
        ElseIf ((row >= 1 And row <= 5) And col = 6) Then
            Return 0
        ElseIf (row = 0 And (col >= 0 And col <= 6)) Then
            Return 0
        ElseIf (row = 6 And (col >= 0 And col <= 6)) Then
            Return 0
        End If

        '1 score, thatMove is at border +1
        If ((row >= 2 And row <= 4) And col = 1) Then
            Return 1
        ElseIf ((row >= 2 And row <= 4) And col = 5) Then
            Return 1
        ElseIf (row = 1 And (col >= 1 And col <= 5)) Then
            Return 1
        ElseIf (row = 5 And (col >= 1 And col <= 5)) Then
            Return 1
        End If

        '2 score, thatMove is at border +2
        If (row = 2 And col = 2) Then
            Return 2
        ElseIf (row = 2 And col = 4) Then
            Return 2
        ElseIf (row = 2 And (col >= 2 And col <= 4)) Then
            Return 2
        ElseIf (row = 4 And (col >= 2 And col <= 4)) Then
            Return 2
        End If

        '3 Center thatMove 
        If (row = 3 And col <= 3) Then
            Return 3
        End If

        Return 0 'error not added lane

    End Function

    Private Shared Function eval(ByVal b As Char(,)) As Integer



        Dim playerScorerow As Integer = 0
        Dim playerScorecol As Integer = 0
        Dim playerScorecross As Integer = 0
        Dim pcScorerow As Integer = 0
        Dim pcScorecol As Integer = 0
        Dim pcScorecross As Integer = 0
        
        ''EVALUATE rows 
        For row As Integer = 0 To 3
            For col As Integer = 0 To 6
            'initialize moves to evaluate
                Dim move3 As New Move With {
                                        .row = row + 3,
                                        .col = col
                                    }
                Dim move2 As New Move With {
                                        .row = row + 2,
                                        .col = col
                                    }
                Dim move1 As New Move With {
                                        .row = row + 1,
                                        .col = col
                                    }
                Dim move0 As New Move With {
                                        .row = row,
                                        .col = col
                                    }
                                    
                If Not b(row, col) = " " Then 'ITS NOT EMPTY - PLAYER OR PC MOVED HERE
                    Dim moveScore As Integer = AddScoreToMove(move0) 'EVALUATE THAT MOVE 
                    If b(row, col) = b(row + 1, col) Then 'THERE IS 2 X or 2 O
                        Dim move1Score As Integer = AddScoreToMove(move1)
                        If b(row + 1, col) = b(row + 2, col) Then 'THERE IS 3x or 3O
                            Dim move2Score As Integer = AddScoreToMove(move2)
                            If b(row + 2, col) = b(row + 3, col) Then 'THERE IS 4X or 4O
                                Dim move3Score As Integer = AddScoreToMove(move3)
                                If b(row, col) = player Then 'PLAYER HAVE 4X HERE
                                    playerScorerow = Math.Max(playerScorerow, 100 + move3Score + move2Score + move1Score + moveScore) 'GET HIGHEST OF ALL EVALUATIONS OF THAT FOR LOOPS
                                ElseIf b(row, col) = opponent Then 'PC HAVE 4O HERE
                                    pcScorerow = Math.Min(pcScorerow, -100 - move3Score - move2Score - move1Score - moveScore)
                                End If
                            End If
                            If b(row, col) = player Then
                                playerScorerow = Math.Max(playerScorerow, 5 + move2Score + move1Score + moveScore)
                            ElseIf b(row, col) = opponent Then
                                pcScorerow = Math.Min(pcScorerow, -5 - move2Score - move1Score - moveScore)
                            End If
                        End If
                        If b(row, col) = player Then
                            playerScorerow = Math.Max(playerScorerow, 2 + move1Score + moveScore)
                        ElseIf b(row, col) = opponent Then
                            pcScorerow = Math.Min(pcScorerow, -2 - move1Score - moveScore)
                        End If
                    End If
                    If b(row, col) = player Then
                        playerScorerow = Math.Max(playerScorerow, moveScore)
                    ElseIf b(row, col) = opponent Then
                        pcScorerow = Math.Min(pcScorerow, -moveScore)
                    End If
                End If
            Next
        Next

        ''col win
        For row As Integer = 0 To 6
            For col As Integer = 0 To 3
                Dim move3 As New Move With {
                                        .row = row + 3,
                                        .col = col
                                    }
                Dim move2 As New Move With {
                                        .row = row + 2,
                                        .col = col
                                    }
                Dim move1 As New Move With {
                                        .row = row + 1,
                                        .col = col
                                    }
                Dim move0 As New Move With {
                                        .row = row,
                                        .col = col
                                    }
                If Not b(row, col) = " " Then
                    Dim moveScore As Integer = AddScoreToMove(move0)
                    If b(row, col) = b(row, col + 1) Then
                        Dim moveScore1 As Integer = AddScoreToMove(move1)
                        If b(row, col + 1) = b(row, col + 2) Then
                            Dim moveScore2 As Integer = AddScoreToMove(move2)
                            If b(row, col + 2) = b(row, col + 3) Then
                                Dim moveScore3 As Integer = AddScoreToMove(move3)
                                If b(row, col) = player Then
                                    playerScorerow = Math.Max(playerScorerow, 100 + moveScore3 + moveScore2 + moveScore1 + moveScore)
                                ElseIf b(row, col) = opponent Then
                                    pcScorerow = Math.Min(pcScorerow, -100 - moveScore3 - moveScore2 - moveScore1 - moveScore)
                                End If
                            End If
                            If b(row, col) = player Then
                                playerScorerow = Math.Max(playerScorerow, 5 + moveScore2 + moveScore1 + moveScore)
                            ElseIf b(row, col) = opponent Then
                                pcScorerow = Math.Min(pcScorerow, -5 - moveScore2 - moveScore1 - moveScore)
                            End If
                        End If
                        If b(row, col) = player Then
                            playerScorerow = Math.Max(playerScorerow, 2 + moveScore1 + moveScore)
                        ElseIf b(row, col) = opponent Then
                            pcScorerow = Math.Min(pcScorerow, -2 - moveScore1 - moveScore)
                        End If
                    End If
                    If b(row, col) = player Then
                        playerScorerow = Math.Max(playerScorerow, moveScore)
                    ElseIf b(row, col) = opponent Then
                        pcScorerow = Math.Min(pcScorerow, -moveScore)
                    End If
                End If
            Next
        Next

        'NOT FULLY IMPLEMENTED
        'cross win
        For row As Integer = 0 To 3
            For col As Integer = 0 To 3
                If Not b(row, col) = " " Then
                    If (b(row, col) = b(row + 1, col + 1) AndAlso b(row + 1, col + 1) = b(row + 2, col + 2) AndAlso b(row + 2, col + 2) = b(row + 3, col + 3)) Then
                        If b(row, col) = player Then
                            Return +10
                        ElseIf b(row, col) = opponent Then
                            Return -10
                        End If
                    End If
                End If
            Next
        Next

        'NOT FULLY IMPLEMENTED
        'cross win
        For row As Integer = 0 To 3
            For col As Integer = 3 To 6
                If Not b(row, col) = " " Then
                    If (b(row, col) = b(row + 1, col - 1) AndAlso b(row + 1, col - 1) = b(row + 2, col - 2) AndAlso b(row + 2, col - 2) = b(row + 3, col - 3)) Then
                        If b(row, col) = player Then
                            Return +10
                        ElseIf b(row, col) = opponent Then
                 

Answer 1

After another day of debuging this I'm kinda lost, but i figured out how to bypass this which might be a real solution to this problem. AlphaBeta concentrates only on winning. That's related actually to eval function, and if we take more factor's to eval function to consider then better that function is. That's why we have

1. Basic Win factor -> that is evaluation of moves to win the game
2. Blocking enemy factor -> to stall the game
3. There's also Forking factor that i have not implemented yet. Info for that is here: https://en.wikipedia.org/wiki/Tic-tac-toe#Strategy and from https://stackoverflow.com/a/19406575/9283016

In nutshell - Simple AlphaBeta Function with simple evaluation of (only) winning does not take into consideration to play time. We have to write proper function of blocking and forking.

Private Shared Function evalBlock(ByVal b As Char(,)) As Move
    Dim blockingMove As New Move With {
         .row = -1,
        .col = -1
        }

    ''row block
    For row As Integer = 0 To 4
        For col As Integer = 0 To 6
            If Not b(row, col) = " " Then
                If b(row, col) = b(row + 1, col) Then '2 X or 2 O
                    If b(row, col) = player Then
                        If b(row + 2, col) = " " Then
                            blockingMove.row = row + 2
                            blockingMove.col = col
                            Return blockingMove
                        End If
                        If row > 0 Then
                            If b(row - 1, col) = " " Then
                                blockingMove.row = row - 1
                                blockingMove.col = col
                                Return blockingMove
                            End If
                        End If

                    End If
                End If
            End If
        Next
    Next

    ''col block
    For row As Integer = 0 To 6
        For col As Integer = 0 To 4
            If Not b(row, col) = " " Then
                If b(row, col) = b(row, col + 1) Then '2 X or 2 O
                    If b(row, col) = player Then
                        If b(row, col + 2) = " " Then
                            blockingMove.row = row
                            blockingMove.col = col + 2
                            Return blockingMove
                        End If
                        If col > 1 Then
                            If b(row, col - 1) = " " Then
                                blockingMove.row = row
                                blockingMove.col = col - 1
                                Return blockingMove
                            End If
                        End If
                    End If
                End If
            End If
        Next
    Next


    ' cross block
    For row As Integer = 0 To 4
        For col As Integer = 0 To 4
            If Not b(row, col) = " " Then
                If (b(row, col) = b(row + 1, col + 1)) Then
                    If b(row, col) = player Then
                        If b(row + 2, col + 2) = " " Then
                            blockingMove.row = row + 2
                            blockingMove.col = col + 2
                        End If

                        If (row > 0 And col > 0) Then
                            If b(row - 1, col - 1) = " " Then
                                blockingMove.row = row - 1
                                blockingMove.col = col - 1
                            End If
                        End If
                    End If
                End If
            End If
        Next
    Next


    '/ cross block
    For row As Integer = 0 To 4
        For col As Integer = 2 To 6
            If Not b(row, col) = " " Then
                If (b(row, col) = b(row + 1, col - 1)) Then
                    If b(row, col) = player Then
                        If b(row, col) = " " Then
                            blockingMove.row = row + 2
                            blockingMove.col = col - 2
                        End If
                    End If
                End If
            End If
        Next
    Next

    blockingMove.row = -1
    blockingMove.col = -1
    Return blockingMove

End Function

Of course returning blocking move don't do anything. In AlphaBeta function we have to assign proper value to this. So i figured out that our AI will block player if he made 2 winning moves and blocking is preferable than winning. Also blocking after 2 player moves makes more sense than blocking after 3 cause it might be to late if we play using rules that 4 moves win.

Something like this:

                Dim score As Integer = Math.Min(AlphaBetaPruning(board, Not machineMove, depth - 1, beta, alpha), eval(board) + depth)
                Dim BlockingMove As Move = evalBlock(board)
                If BlockingMove.row <> -1 Then
                    Dim blockingMoveScore As Integer = 5000
                    If score < blockingMoveScore Then
                        Form1.BestMoveRow = BlockingMove.row
                        Form1.BestMoveCol = BlockingMove.col
                        Form1.BestMoveScore = score
                    End If
                End If

And that's how i lost after first game with my AI.