using System;

​

public class GameData {

​

    public static GameData INSTANCE;

    public readonly int PosX;

    public GameData(int posX) {

        this.PosX = posX;   

    }

    public GameData With(int? PosX = null) {

        return new GameData(

            PosX ?? this.PosX

        );

    }

}

​

public class Program {

    public static void Main() {

        GameData.INSTANCE = new GameData(42);

        // ...

        GameData.INSTANCE = GameData.INSTANCE.With(PosX: 34);

        // ...

        GameData.INSTANCE = GameData.INSTANCE.With(PosX: 69);

    }

}