Trouble with fieldInfo.SetValue() and arrays

Hmmm... Again having trouble with SetValue... Well, since I didn't found anything about this in the web I've decided to ask it here:

I have this void, witch is:

 public void AddItem(Collider2D other, int amount)
 {
     int[] itemArray = (int[])GetType().GetField(other.GetComponent<Drops>().ItemType.ToString(), BindingFlags.Public | BindingFlags.Instance).GetValue(this);
     int id = other.GetComponent<Drops>().id;

     if (itemArray.Length > id)
         GetType().GetField(other.GetComponent<Drops>().ItemType.ToString()).SetValue(this, itemArray[id] + amount);
     else Debug.LogWarning("Help here! Not working at all! " + other.GetComponent<Drops>().id);
 }

It was supposed to detect two things when a player triggers with an item:

  public enum ItemType {basicMaterial, advancedMaterials};
  public class Drops : MonoBehaviour
  {
       int id; //If ItemType is equals to basicMaterial: 0 = Carbon, 1 = Copper...
  }

My only problem is after that. Once it is done detecting those things, it needs to ADD, lets say one, to an array. How does it knows where to add it? Easy. Using:

  GetType().GetField(Collider2D other.GetComponent<Drops>().ItemType.ToString());

I can easely find the array (since its name is equals to the ItemType (ex. array, ItemType: basicMaterials)), but I can't apply the value to it, sice I can't define at what point of the array's length I must add it. Any help would be amazing!

PS: After all that I still place that array inside another script, and then I save it to a .json file, but this I've already made and mastered.

I'm using fieldInfo.SetValue because I want my scripts to be as short as possible, and as readable as possible.

Another thing is that I call AddItem() inside of OnTriggerEnter2D() and set the Collider2D from AddItem() as the Collider2D from OnTriggerEnter2D().

Yes, I'm aware that I could just change the values individually for each array instead of trying to find it depending of witch ItemType is assigned and witch id is assigned, but I don't want my scripts to get overpopulated, so...