C# / Unity - Need help debugging my Chess program - trouble creating instance of class within another instance of the same class

Question

I'm creating a Chess program. I've decided to make it so that, when a piece is selected, all of its possible legal moves are displayed. In order for that to work, I have to have a mechanism to predict the outcomes of moves (because you can't make a legal move which puts your own king in check, for example). This will also be useful when I start working on computer opponents, since the AI will need to be able to see several turns in advance in order to make intelligent decisions.

To this end, I've made it so that, at the start of each turn, all possible moves are calculated for the player's pieces, and then each of those possible moves is simulated to see whether or not it leaves the King in check. If so, that option is removed. I have a main class called BoardManager which controls the visuals and contains the actual chess board layout (in a ChessBoardSim), and I have a class ChessBoardSim, whose objects each contain one possible board state. ChessBoardSim can create other instances of ChessBoardSim, allowing the state of the board any number of turns in advance to be simulated.

I have a bug which I've been struggling with for over 5 hours, which appeared after I tried to restructure a lot of the code. I'm at my wit's end, and I just need a fresh pair of eyes. The issue is making it so that pieces aren't being removed from their old positions when moved, and the locations of some of the pieces on the board seem to be shifted two tiles up. I believe, based on the debugging, that the problem is appearing in CalculateAllMovementOptions() in the ChessBoardSim class - for some reason, when a ChessBoardSim has a ChessBoardSim child and the CalculateAllMovementOptions() function is called in the child, it changes the data in the parent. I've included the full code below, and I'll describe the specific issue as best I can.

The flow of the problem is:

1: private void Start() in BoardManager is called.

2: The program draws and populates the board properly. A ChessBoardSim (called ChessPieces) is created to contain the data of the current board state.

3: StartTurn() is called in BoardManager, in order to begin the game.

4: StartTurn() calls ChessPieces.CalculateAllMovementOptions(). The intended function of CalculateAllMovementOptions() is to get back an array of Lists. Each List in the array contains the legal moves of one of the player's pieces. However, this actually seems to be changing the actual board data, and I can't work out why.

5. When a player tries to move a piece, the game breaks because the pieces aren't where they're supposed to be and the software can't cope with it. The piece which is moved doesn't remove itself from its old location (despite my best efforts), and this causes there to be two instances of that object, leading to an indexoutofrange exception when a piece of the code tries to access the 17th element of an array with 16 elements.

I would really appreciate it if anyone could help me, I feel like I've wasted an entire day on this already and I'm sure I'm missing something simple.

The link to my code is https://github.com/FC123321/Chess

Answer 1

Array.Clone doesn't create a deep copy of an array, so when you call boardLayout.Clone() in SimulateBoard (and again in the ChessBoardSim constructor), you are copying the references in boardLayout to the new array. This means that the pieces in the new array are the same object in the old one.

This means that when you're in MovePiece in SimulateBoard and you call piece.SetPosition and such on members of that copied array, you're setting the position of the pieces in the source of the copy as well.

Instead of using boardLayout.Clone() , you need to loop through the whole source array, and do newBoardLayout[x,y] = new ChessPiece(); and then copy over the values to the new ChessPiece . Alternatively, you could create a new ChessPiece constructor that takes another chess piece and copies the values there:

// ChessPiece copy constructor
public ChessPiece(ChessPiece other) {
    this.Position = new int[2] { other.Position[0], other.Position[1]};
    this.isWhite = other.isWhite;
    this.movementType = other.movementType;
    this.hasMoved = other.hasMoved;
    this.turnDoubleMoved = other.turnDobleMoved;
}

// instead of boardLayout.Clone() in the ChessBoardSim constructor:

for (int x=0 ; x<8;x++){
    for(int y=0; y<8;y++){
        if (boardLayout[x,y] != null)
            this.boardLayout[x,y] = new ChessPiece(boardLayout[x,y]);
    }
}


// In SimulateBoard, take out the redundant Clone call
ChessBoardSim simBoard = new ChessBoardSim(boardLayout, turnNumber);