List.Add() changes everything in List

Question

This is surely a simple question and because I am too tired I don't see the answer myself, but my question is:

I have a for loop, and in it a List<T>.Add(), so that at each turn of the loop, it adds something to the list. But I checked and I see that at each turn, the adding adds the right element, but changes every other element of the list to the new one ! Would you know what could cause this? Here is my code:

private static List<Niveau> computeNiveaux(int[,] tri) {
    List<Lot> lotsNiveau = new List<Lot>();
    List<Niveau> niveaux = new List<Niveau>();
    Lot lot;
    bool isNiveau = false; 

    for (int row = 0; row < tri.GetLength(0); row++)
    {
        for (int column = 0; column < tri.GetLength(1); column++)
        {
            if (tri[row, column] == 1) { 
                lot = lots.ElementAt(column);
                lotsNiveau.Add(lot);
                isNiveau = true;
            }
        }
        if (isNiveau)
        {
            Niveau niv = new Niveau(lotsNiveau.First().niveau, lotsNiveau.First().etage, lotsNiveau);
            niveaux.Add(niv);

        lotsNiveau.Clear();
        isNiveau = false;
    }
    return niveaux;
}

• lots is a class variable (don't know what is the right name in english ^^') List<Lot>

• A Niveau is :

  public class Niveau {
      public int numero;
      public string etage;
      public List<Lot> lots;
  
      public Niveau(int numero, string etage, List<Lot> lots) {
          this.numero = numero;
          this.etage = etage;
          this.lots = lots;
      }
  }
  

• What I get for example when I want to see what are every niveaux.ElementAt(i).lots.Count (for i=0,...,niveaux.Count) : 16 16 16 ... Even if I know that in the first turn of the loop it was 7, then 4, then 10, etc. and the last turn is 16. (that's not really clear, I hope that you understand x') )

• At the next step of the loop, if the value added is for example 23, niveaux will become : 23 23 23 23 ...

• If you need any more information, tell me !

• Sorry for my english ! :(

Answer 1

Because you are using same instance of the lot all times, So what ever changes you applied to this instance(through assignment lots.ElementAt(column)) will affect all previously added elements as well. So try using like the following:

if (tri[row, column] == 1) 
{ 
    Lot lot = lots.ElementAt(column); // change is here
    lotsNiveau.Add(lot);
    isNiveau = true;
}

Answer 2

You are using the same list lotsNiveau over and over again. Change:

Niveau niv = new Niveau(lotsNiveau.First().niveau, lotsNiveau.First().etage, lotsNiveau.ToList());//Create new list instance

Answer 3

You need to create new instance each time inside the if condition. But currently you are just adding the same instance defined earlier to the list instead of creating new instance and adding to the list.

So Modify inside your if condition like below

private static List<Niveau> computeNiveaux(int[,] tri) {
    List<Lot> lotsNiveau = new List<Lot>();
    List<Niveau> niveaux = new List<Niveau>();

    bool isNiveau = false; 

    for (int row = 0; row < tri.GetLength(0); row++)
    {
        for (int column = 0; column < tri.GetLength(1); column++)
        {
            if (tri[row, column] == 1) { 
                Lot lot = lots.ElementAt(column);
                lotsNiveau.Add(lot);
                isNiveau = true;
            }
        }
        if (isNiveau)
        {
            Niveau niv = new Niveau(lotsNiveau.First().niveau, lotsNiveau.First().etage, lotsNiveau);
            niveaux.Add(niv);

        lotsNiveau.Clear();
        isNiveau = false;
    }
    return niveaux;
}

Answer 4

Move List< Lot> lotsNiveau = new List< Lot>(); Inside the for loop. Other same instance will be referred and pointed by the member variable causing this error.