C# List Reported as Empty when called in method

Questions & Answers
1

I’m using some dictionaries to put monsters into my grid map. I’m trying to call a method that will choose a random key from the dictionary of monster types (AvailableMonsters) by converting my dictionary keys to a list (MonsterKeys) and choosing by random index. However, whenever I reference the list of keys in the method that places the objects, I get an index out of range error on the MonsterKeys list. I have tried hard-coding the value to 0 or 1 and still get the same error, or a NullReferenceException on the list. I added a debug.log to the start() method and it reports that MonsterKeys.Count == 2, then I Debug.Log the same thing from inside the PlaceItem function and the result is 0. Code for the entire class is below:

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

public class MonsterManager : MonoBehaviour {

    private List<RogueMonster> Monsters;
    private List<RogueMonster> ActiveMonsters;
    private List<string> MonsterKeys;

    public GameObject Slime;
    public GameObject Alien;
    private Dictionary<string, GameObject> AvailableMonsters;
    private Dictionary<string, Dictionary<string, int>> MonsterStats;

    private void Awake()
    {
        DontDestroyOnLoad(gameObject);
        Monsters = new List<RogueMonster>();
        AvailableMonsters = GetAvailableMonsters();
        MonsterKeys = new List<string>();
        foreach (KeyValuePair<string, GameObject> kvp in AvailableMonsters)
        {
            MonsterKeys.Add(kvp.Key);
        }
        Debug.Log(MonsterKeys.Count.ToString());
        Debug.Log(MonsterKeys.Count.ToString());
        MonsterStats = SetMonsterStats();        
    }

    private void Update()
    {
        Debug.Log(MonsterKeys.Count.ToString());
    }

    private Dictionary<string, GameObject> GetAvailableMonsters()
    {
        return new Dictionary<string, GameObject>
        {
            { "Alien", Alien },
            { "Slime", Slime }            
        };
    }

    private Dictionary<string, Dictionary<string, int>> SetMonsterStats()
    {
        return new Dictionary<string, Dictionary<string, int>>
        {
            { "Slime", new Dictionary<string,int>
                {
                    {"hp", 2 },
                    {"melee", 1 },
                    {"magic", 0 },
                    {"ranged", 0 },
                    {"def", 0 }
                }
            },
            { "Alien", new Dictionary<string, int>
                {
                    {"hp", 2 },
                    {"melee", 1 },
                    {"magic", 0 },
                    {"ranged", 0 },
                    {"def", 0 }
                }
            },
        };
    }

    public void GenerateMonsters(List<Rect> rooms, int mCount)
    {
        foreach(Rect r in rooms)
        {
            PlaceMonsters(r, mCount);
        }
    }

    private void PlaceMonsters(Rect r, int mCount)
    {
        int monsterCount = Random.Range(1, mCount);
        for(int m = 0; m < monsterCount; m++)
        {
            bool placed = false;
            while (!placed)
            {
                int x = Random.Range(r.x1, r.x2);
                int y = Random.Range(r.y1, r.y2);
                int i = Random.Range(0, MonsterKeys.Count);
                string mType = MonsterKeys*;*

GameObject monster = Instantiate(AvailableMonsters[mType], new Vector3(x, y, 3), Quaternion.identity);
RogueMonster rm = monster.GetComponent();
Combatant c = monster.GetComponent();
rm.SetCombatValues(MonsterStats[mType]);
rm.Name = mType;
Monsters.Add(rm);
CombatManager.AddCombatantToList(c);
}
}
}
}
EDIT: When I move my Dictionary and List initialization the GenerateMonsters method, everything works fine. For some reason, initializing in the Awake method isn’t persisting.

2

You receive “Index out of range” so where you’re trying to access MonsterKeys*, i is actually out of the range of the list.*
Over here you’re missing index 0, so if you only have 1 monster you will already trigger this error.
At:
Random.Range(1, MonsterKeys.Count);
In the case you have more than one monster and receive this error, your list is simply not initialized before the method is called.
----------
On a possible fix, you could lazy load it (which isn’t as great for performance because this means it will load your data during the first time it accesses the property, but it should fix this issue.).
I would advise to rather find the core problem where the list is not being initialized, and fix it there.
private List _monsterKeys;
private List MonsterKeys
{
get { return _monsterKeys ?? (_monsterKeys = AvailableMonsters.Keys.ToList()); }
}