替换字符串中的单个字符

Question

So, I'm using unity and I have this string that contains the current state of a tic-tac-toe board. It looks like that:

XOX
ONX
OXX

"N" being a neutral space.

The thing is I want to replace one of those "N" with either an "X" or an "O". I tried using :

board[n] = "X";

but i get

"property or indexer cannot be assigned, it is read only"

So I'm trying to figure a way to change one of those "N". Here is my code

if (manager.turn == 1 && !isUsed)
{
    xSprite.enabled = true;
    manager.board[int.Parse(gameObject.name)-1] = 'X';
    manager.GameEndingDraw();
    isUsed = true;
    manager.turn = 2;
}
else if (manager.turn == 2 && !isUsed)
{
    oSprite.enabled = true;
    manager.board[int.Parse(gameObject.name)-1] = 'O';
    manager.GameEndingDraw();
    isUsed = true;
    manager.turn = 1;
}

Answer 1

A Tic-Tac-Toe board is not a string, it is a 3×3 grid of markers such as chars. You should represent it as an array, not a string:

var board = new char[3, 3];
for (int i = 0; i < board.GetLength(0); i++)
    for (int j = 0; j < board.GetLength(1); j++)
        board[i, j] = 'N';

Then you can set each marker by its coordinates:

board[0, 0] = 'X';

Answer 2

Here are 4 methods you can use:

ToCharArray

var array = board.ToCharArray();
array[indexOfNew] = newChar;
board = new string(array);

StringBuilder

var sb = new StringBuilder(board);
sb[indexOfNew] = newChar;
board = sb.ToString();

String.Remove and String.Insert

board = board.Remove(indexOfNew, 1)
    .Insert(indexOfNew, newChar.ToString());

SubString and String Interpolation

board = $"{board.Substring(0, indexOfNew)}{newChar}{board.Substring(indexOfNew+1, board.Length-indexOfNew-1)}";

Assuming board contains the original string, newChar contains the newly placed "piece" and indexOfNew contains the index in the string which you need to replace.

And you could of course wrap the option you choose in an extension method:

public static string ReplaceAt(this string inputString, int newCharIndex, char newChar)
{
    var newString = // Insert your choice of replacement from above here
    return newString;
}