Undoing a move that causes a promotion can cause a crash.

[moonheart08]

As it says on the tin, it'll improperly count the number of pieces for whatever reason and overrun the piece array.

[Disservin]

always more helpful if you provide a working example

[moonheart08]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using ChessChallenge.API;

public class MyBot : IChessBot
{
    Board b;
    Random rng = new Random();
    public Move Think(Board board, Timer timer)
    {
        b = board;
        var moves = board.GetLegalMoves();
        var m = JudgeMovesOnce(moves.OrderBy(_=>rng.Next()));
        if (!b.TrySkipTurn())
            return m.First();
        return m.FirstOrDefault(moves[rng.Next(moves.Length)]);
    }
    
    IOrderedEnumerable<Move> JudgeMovesOnce(IEnumerable<Move> moves) => moves.OrderBy(x => TryMove(x, _ =>
    {
        var r= b.GetLegalMoves().OrderByDescending(CalcMovePoints).Select(CalcMovePoints).FirstOrDefault(0);
        Console.WriteLine($"Trying {x}, result is {r}");
        return r;
    }, true)).ThenBy(MoveIsCheckmate).ThenBy(CalcMovePoints);

    int CalcCaptureRisk(Square x, PieceType t)
        => B2I(b.SquareIsAttackedByOpponent(x)) * (int) t * 5;

    int B2I(bool b) => b ? 1 : 0;
    int CalcMovePoints(Move m)
        => (int)Distance(m.TargetSquare,  m.StartSquare) + (int) m.CapturePieceType * 5 + (int)m.PromotionPieceType * -100 + EnemyKingDistanceInverse(m) * 3 - CalcCaptureRisk(m.TargetSquare, m.MovePieceType) + CalcCaptureRisk(m.StartSquare, m.MovePieceType);

    int EnemyKingDistanceInverse(Move m)
        => 8 - (int) Distance(b.GetKingSquare(!b.IsWhiteToMove), m.TargetSquare);

    bool MoveIsCheckmate(Move move)
        => TryMove(move, _ => b.IsInCheckmate());

    T TryMove<T>(Move move, Func<Board, T> judge, bool turnSkip = false)
    {
        if (turnSkip && !b.TrySkipTurn())
            return default!;
        
        b.MakeMove(move);
        var res = judge(b);
        b.UndoMove(move);
        if (turnSkip) b.UndoSkipTurn();
        return res;
    }
    
    Vector2 SquareVec(Square x) => new(x.Rank, x.File);
    float Distance(Square x, Square y) => (SquareVec(x) - SquareVec(y)).Length();
}

is my current (broken, heck) bot, which triggers it after ~30sec against evilbot.