Trying to keep a total coin variable located in random device location Unity2D

So I have an infinite platformer runner game where I keep values of total travelled distance and total collected coins during the game. From this video, my boy Brackeys taught how to save and load critical data by formatting the data using Binary Formatter. I used his code to create my own high score saving system. I can save highest travelled distance. Check above code:

HighScoreData.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class HighScoreData
{
    public float bestDistanceCount;

    public HighScoreData(Player player){
        bestDistanceCount = player.distanceCount;
    }
}

HighScoreSaveSystem.cs

using UnityEngine;
using System.IO;
using System.Runtime.Serialization.Formatters.Binary;

public static class HighScoreSaveSystem
{
    public static void SaveHighScore(Player player){
        BinaryFormatter formatter = new BinaryFormatter();
        string path = Application.persistentDataPath + "/highscore.highscorefile";
        FileStream stream = new FileStream(path,FileMode.Create);

        HighScoreData data = new HighScoreData(player);
        formatter.Serialize(stream,data);
        stream.Close();
    }
    public static HighScoreData LoadHighScore(){
        string path = Application.persistentDataPath + "/highscore.highscorefile";
        if(File.Exists(path)){
            BinaryFormatter formatter = new BinaryFormatter();
            FileStream stream = new FileStream(path,FileMode.Open);
            HighScoreData data = formatter.Deserialize(stream) as HighScoreData;
            stream.Close();
            return data;
        }
        else{
            Debug.LogError("Save file not found!");
            return null;
        }
    }
}

And by calling these methods everytime my player die in KillPlayer() method,

public void KillPlayer(){
        isDead = true;
        HighScoreData data = HighScoreSaveSystem.LoadHighScore();
        if(distanceCount > data.bestDistanceCount){
            HighScoreSaveSystem.SaveHighScore(this);
        }
        Time.timeScale = 0f;
    }

This is working just fine. But when it comes to saving coins, I couldn't figure it out.

I have to create a variable in this binary file which will take 0 when player installs the game. And everytime player die, coins collected in that level should be added to the one I keep in my binary file. But I don't know how to implement it.

What did I try?

I tried adding totalCoins variable to HighScoreData.cs:

HighScoreData.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[System.Serializable]
public class HighScoreData
{
    public float bestDistanceCount;
    public int totalCoins;

    public HighScoreData(Player player){
        bestDistanceCount = player.distanceCount;
        totalCoins += player.coinCount;
    }
}

And in my KillPlayer() method, I tried to store that totalCoins data in a temp variable, add current coin count to that temp variable and update data.totalCoins with the temp variable, which named totalCoinRef.

public void KillPlayer(){
        isDead = true;
        HighScoreData data = HighScoreSaveSystem.LoadHighScore();
        int totalCoinRef = data.totalCoins;
        if(distanceCount > data.bestDistanceCount){
            totalCoinRef += coinCount;
            data.totalCoins = totalCoinRef;
            HighScoreSaveSystem.SaveHighScore(this);
        }
        totalCoinRef += coinCount;
        data.totalCoins = totalCoinRef;
        HighScoreSaveSystem.SaveHighScore(this);
        Time.timeScale = 0f;
    }

Result:

This solution only keeps the count of coins collected each level. It is not keeping a sum of total coins. For example if I collect 5 coins, data.TotalCoins will return 5. If i collect 6 coins, data.TotalCoins will return 6. I need it to return 11.

I hope this is clear. Thanks a lot for your time.

In general STOP USING BinaryFormatter!

Use other formats e.g. (JSON, raw text file, etc) or a custom binary serialization.

Then in SaveHighScore you do

HighScoreData data = new HighScoreData(player);

completely ignoring any previous existing data!

You seem to believe that

int totalCoinRef = data.totalCoins;

creates a reference so editing totalCoinRef will also automatically edit data.totalCoins .. it does not! int is a Value Type and therefore you create a COPY of the value which is in no way connected to the data anymore at all!

I would always rather directly pass in a HighScoreData and go (will simply demo using JSON for now)

[System.Serializable]
public class HighScoreData
{
    public float bestDistanceCount;
    public int totalCoins;
}

and

private static readonly string path = Path.Combine(Application.persistentDataPath, "highscore.highscorefile");

public static void SaveHighScore(HighScoreData data)
{  
    var json = JsonUtility.ToJson(data, true);  
    
    File.WriteAllText(path, json);
}

public static HighScoreData LoadHighScore()
{       
    if(!File.Exists(path)) return new HighScoreData();

    var json = File.ReadAllText(path);
      
    return JsonUtility.FromJson<HighScoreData>(json);
}

And then finally in your player

public void KillPlayer()
{
    isDead = true;

    var data = HighScoreSaveSystem.LoadHighScore();

    data.totalCoins += coinCount;
    data.bestDistanceCount = Mathf.Max(data.bestDistanceCount, distanceCount);    
    
    HighScoreSaveSystem.SaveHighScore(data);
    
    Time.timeScale = 0f;
}

If you want to stay binary you can also look into BinaryWriter and BinaryReader .. in your case JSON might be a bit overkill since you anyway only really need 8 bytes .. 4 for the float bestDistanceCount and 4 for the int totalCoins. JSON of course adds quite some overhead and string parsing is always more expensive than simple byte conversions

Might look like e.g.

public static void SaveHighScore(HighScoreData data)
{  
    using(var stream = File.Open(path, FileMode.OpenOrCreate, FileAccess.Write, FileShare.Write)
    {
        using(var writer = new BinaryWriter(stream))
        {
            writer.Write(data.bestDistanceCount);
            writer.Write(data.totalCoins);      
        }    
    }       
}

public static HighScoreData LoadHighScore()
{ 
    var data = new HighScoreData();      
    if(File.Exists(path))
    {
        using(var stream = File.Open(path, FileMode.Open, FileAccess.Read, FileShare.Read)
        {
            using(var reader = new BinaryReader(stream))
            {           
                data.bestDistanceCount = reader.ReadSingle();
                data.totalCoins = reader.ReadInt32();      
            }    
        }  
    }     
      
    return data;
}

Either way (even - and maybe especially - using BinaryFormatter) have in mind that a user has full access to the persistent data and can just modify their values there, you can use some encryption to make it harder .. but in order to fully prevent cheating you will need a server solution