unity - How to save/persist a GameObject with incremental levels? Dictionary? List?

I create buildings and they have child Text objects that will reflect the building level.

The Problem:

What can I use to store the buildings a player has built so the data can be persisted?

I've looked at Lists, but when I upgrade a building, how would I update the list so that particular building has its new level reflected?

I'm working on a dictionary so I can update the key with the new value (level) but running into a problem with the keys and names. If I have lets say 10 "Farms" and try to add them to the dictionary, even though they have different properties, I get an error that I have a duplicate.

Currently I am learning how to persist that data through scenes;

GameManager.CS

public class GameManager : MonoBehaviour
{

public static Dictionary playerBuilt = new Dictionary();

public void SaveState()
    {
        BinaryFormatter bf = new BinaryFormatter();
        FileStream file = File.Open(Application.persistentDataPath + "/EmpireGame", FileMode.Open);


        SaveClass sc = new SaveClass()
        {
            food = Player.food.amount,
            buildingList = playerBuilt
        };

        bf.Serialize(file, sc);
        file.Close();
    }
}


[Serializable]
class SaveClass
    {
        public int food, wood, iron, stone;
        public Dictionary buildingList = new Dictionary();
    }

Building.CS

public abstract class Building : MonoBehaviour, IUpgradable {


    public int ID;

    //level this building is
    public Text levelText;
    public int level = 0;

    //starting cost to amplify the upgrade cost
    public int foodBase, woodBase, ironBase, stoneBase;


    //Upgrade cost
    public int foodCost, woodCost, ironCost, stoneCost;
    public float BuildingCostPercentage = 5f;

    public void UpdateDictionary()
    {
        if (GameManager.playerBuilt.ContainsKey(this))
            GameManager.playerBuilt[this] = level;
        else
            GameManager.playerBuilt.Add(this, level);

    }
}

Note: I have not tested the save function so I expect errors.

Answer

Myself, I'd pull out just the parts of the building I really want to save - ie. those that are unique to this instance of the building. Eg..

[System.Serializable]
public struct BuildingRecord {
    public BuildingType type;
    public Vector2 position;

    public int level;
}

Here BuildingType could be an enumeration, or a reference/index to a Type Object, or even just a string key - whatever will help you look up the appropriate source assets like a prefab when you want to spawn this building back into the game when you load from your save file.

Your Building can have a method like so...

public BuildingRecord GetRecord() {
    return new BuildingRecord(){
       type = this.type,
       position = (Vector2)transform.position,
       level = this.level

    };
}

Then your save function can simply iterate over a list of Building component instances that you've maintained all along, or fetched on demand with FindObjectsOfType() and make a simple serializable array of them:

BuildingRecord[] SaveBuildings(IList buildings) {
    var records = new BuildingRecord[buildings.Count];
    for(int i = 0; i < records.Length; i++)
        records[i] = buildings[i].GetRecord();
    return records;
}

On deserialization, you can pass each record to a factory method to look up the right prefab using the type information, spawn it at the given position, and initialize it to the given level.