Why the frick am I getting a System.IndexOutOfRangeException or a System.NullReferenceException here?

[Gammer0909]

So I was building my chess bot, everything was going good (if a little stupid), but I noticed about the same spot every game the bots would draw due to repeating a move 3 times. I wanted to stop this, so I built a little system to check if the moves were previously played twice. I added this, then my code gave me a System.NullReferenceException, so I fixed what I thought was the problem, and then it gave me a System.IndexOutOfRangeException. So I scrapped the "stop getting draws" idea to see if it would still work. It did not. I undid EVERYTHING i had done to make the Repetition Draw system, and it continues to flip flop between NullReferenceException and IndexOutOfBoundsException, for seemingly no reason. The line that the error "is on" constantly changes from 92, 101, 101 and then 50 and 106. Please help if you can.

Code:

using ChessChallenge.API;
using System;

public class MyBot : IChessBot
{

    // 0 is empty, 1 is pawn, 2 is knight, 3 is bishop, 4 is rook, 5 is queen, 6 is king
    readonly int[] PEICE_VALUE = { 0, 1, 3, 3, 5, 9, 1000 };

    public Move Think(Board board, Timer timer)
    {

        Console.WriteLine("--- " + board.PlyCount + " ---");

        // Get all the legal moves
        Move[] moves = board.GetLegalMoves();
        Move bestmove = Move.NullMove;
        

        // If there are no legal moves, return null
        if (moves.Length == 0)
        {
            return bestmove;
        }

        // If there is only one legal move, return that move
        if (moves.Length == 1)
        {
            return moves[0];
        }


        // Play an "opening" on either side
        if (board.PlyCount == 0 && board.IsWhiteToMove)
            return new Move(new[] { "e2e4", "g1f3", "d2d4" }[new Random().Next(3)], board);
        else if (board.PlyCount == 1 && !board.IsWhiteToMove)
            return new Move(new[] { "e7e5", "g8f6", "d7d5" }[new Random().Next(3)], board);


        // Loop through all the legal moves
        for (int i = 0; i < moves.Length; i++)
        {
            int prevscore = 0;
            // Make the move
            board.MakeMove(moves[i]);
            Console.WriteLine(moves[i].ToString() + " " + Evaluate(board));

            // Recursively call the search function
            // We negate the score because we want to maximize the score
            int score = -Search(board, 3, -9999, 9999);

            // Undo the move
            board.UndoMove(moves[i]);

            // If the score is better than the best score, update the best score and best move
            if (score > prevscore)
            {
                bestmove = moves[i];
                prevscore = score;
            }
        }


        return bestmove;
    }

    // Minimax algorithm Alpha-Beta pruning
    public int Search(Board board, int depth, int alpha, int beta) 
    {

        // If we are at the end of the search, evaluate the board
        if (depth == 0)
        {
            return Evaluate(board);
        }

        // Get all the legal moves
        Move[] moves = board.GetLegalMoves();

        // If there are no legal moves, return the evaluation of the board
        if (moves.Length == 0)
        {
            return Evaluate(board);
        }

        int score = 0;

        // Loop through all the legal moves
        for (int i = 0; i <= moves.Length; i++)
        {
            // Make the move
            board.MakeMove(moves[i]);

            if (board.IsInCheckmate())
            {
                return 9999;
            }
            
            // Recursively call the search function
            // We negate the score because we want to maximize the score
            score = -Search(board, depth - 1, -beta, -alpha);

            // Undo the move
            board.UndoMove(moves[i]);

            // If the score is better than the best score, update the best score and best move
            if (score > alpha)
            {
                alpha = score;
            }

            // If the score is better than the beta, return the beta
            if (alpha >= beta)
            {
                return beta;
            }
        }

        return score;

    }

    // Evaluate the board for a score
    public int Evaluate(Board board)
    {
        Square square;
        int score = 0;

        // Loop through all the pieces on the board
        for (int i = 0; i < 7; i++)
        {

            for (int j = 0; j < 7; j++) {

                square = new(i, j); // Hopefully this doesn't kill performance :nervous:

                Piece p = board.GetPiece(square);

                // If the piece is white, add the value of the piece to the score
                if (p.IsWhite)
                {
                    
                    score += GetScore(p);
                }
                // If the piece is black, subtract the value of the piece from the score
                else if (!p.IsWhite)
                {
                    score -= GetScore(p);
                }

            }

        }
        return score;
        
    }


    private int GetScore(Piece p) {
        if(p.IsPawn)
        {
            return PEICE_VALUE[1];
        }
        else if(p.IsKnight)
        {
            return PEICE_VALUE[2];
        }
        else if(p.IsBishop)
        {
            return PEICE_VALUE[3];
        }
        else if(p.IsRook)
        {
            return PEICE_VALUE[4];
        }
        else if(p.IsQueen)
        {
            return PEICE_VALUE[5];
        }
        else if(p.IsKing)
        {
            return PEICE_VALUE[6];
        }
        else
        {
            return 0;
        }
    }
}

I really hope this is a dumb problem and not something that is completely wrong with the api or compiler or something big like that.

[ryanheath]

It is probably related to a missing board.UndoMove(moves[i]);

in

if (board.IsInCheckmate())
{
    return 9999;
}

[Gammer0909]

What do you mean?

[ryanheath]

change the relevant piece of code into

if (board.IsInCheckmate())
{
    board.UndoMove(moves[i]);
    return 9999;
}