C# Object reference not defined for an object instance

Question

Who can help me and explain this problem I will be grateful

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace BuscaLargura
{
   partial class Program
    {
        static void Main(string[] args)
        {

            /* Node grafos = Grafo();
             Console.WriteLine(" ");
             bfs_transversal(grafos);*/
            List<Node> listanos = new List<Node>();
            List<Node> conectados = new List<Node>();
            String[] text = System.IO.File.ReadAllLines(@"C:\Users\Pedro Pacheco\Desktop\Mapa.txt");
            for (int i = 0; i < text.Length; i++)
            {
                if (text[i].Contains("city"))
                {
                    //Console.WriteLine(text[i][5]);
                    listanos.Add(new Node(text[i][5]));//adiciona um Nó com o nome da 5 posição da linha do Mapa.txt
                }
            }
            for (int i = 0; i < text.Length; i++)
            {
                if (text[i].Contains("route"))
                {
                    Console.WriteLine(text[i][6]);
                    for (int nopai = 0; nopai < listanos.Count; nopai++)//varrendo os nós sem conexão
                    {
                        if (listanos[nopai].data == text[i][6])//verificando se o Nó.data é igual a  text[i][6] ( exemplo= route=A)
                        {
                            for (int nofilho = 0; nofilho < listanos.Count; nofilho++)
                            {                                
                                if (listanos[nofilho].data == text[i][8])
                                {
                                    Console.WriteLine(text[i][8]);
                                    //listanos[nopai].conectados.Add(new Node(listanos[nofilho]));
                                    conectados.Add(listanos[nofilho]);
                                }

                            }
                                listanos[nopai].conectados.AddRange(conectados);  
                                conectados.Clear();
                        }                      
                    }
                }
            }

        }
    }
}

---

Node class:

using System; 
using System.Collections.Generic; 
using System.Linq; using System.Text; 
using System.Threading.Tasks;

namespace BuscaLargura { 

  public class Node { 

    public List<Node> conectados; 

    public char data;

    public Node(char data,List<Node> conectados)
    {
        this.conectados = conectados;
        this.data = data;
    }
    public Node(char data)
    {
        this.data = data;
    }
    public Node (List<Node> filhos)
    {
        this.conectados.AddRange(filhos);
    }
  }
}

Answer 1

What’s the matter?

The problem is that the field public List<Node> conectados; is not being initialized next to the object Node.

Your program does all route calculation but at the time of passing the results it finds a null instead of the list conectados.

'Cause this is happening?

Although you have three constructors for class Node only two make the field initialization public List<Node> conectados; and just the only constructor you use in the code is the constructor that doesn’t initialize the field conectados.

Solution:

There are two solutions to your problem:

1 Within the class Node in the constructor whose signature is public Node(char data) make the following change:

        public Node(char data){
            this.data = data;

            //Adicione está linha para corrigir o problema.
            this.conectados = new List<Node>();
        }

2 Another solution would be to change, within the function Main(), the constructor used in your program which would look like this:

      for (int i = 0; i < text.Length; i++)
           {
                if (text[i].Contains("city"))
                {
                    // Essa era a linha antiga:
                    // listanos.Add(new Node(text[i][5]));
                    // que substitui por essa:
                    listanos.Add(new Node(text[i][5], new List<Node>()));
                }
            }

Answer 2

It is not a direct answer to the question, but with Ingles you can simplify your code:

static void Main(string[] args)
{
    String[] text = System.IO.File.ReadAllLines(@"C:\Users\Pedro Pacheco\Desktop\Mapa.txt");

    var result = text.Where(x => x.Contains("route"))
        .GroupBy(y => y[6])
        .Select(z => new Node(z.Key, z.Select(nf => new Node(nf[8])).ToList()))
        .ToList();

    // Adiciona existentes em "city" que não existem em "route" (neste caso 'H')
    result.AddRange(text.Where(x => x.Contains("city") && !result.Any(y => y.data == x[5])).Select(x => new Node(x[5])));

    // Ordena pelos nodes pai
    result = result.OrderBy(x => x.data).ToList();

    Console.ReadKey();
}

public class Node
{
    public List<Node> conectados;

    public char data;

    public Node(char data, List<Node> conectados)
    {
        this.conectados = conectados;
        this.data = data;
    }

    public Node(char data)
    {
        this.data = data;
    }
}

Answer 3

city=A(100,80);
city=B(160,70);
city=C(110,50);
city=D(140,120);
city=F(155,40);
city=G(210,60);
city=H(190,10);
city=I(170,110);
route=A-C;140;
route=A-D;155;
route=C-F;125;
route=D-B;115;
route=D-I;152;
route=B-F;119;
route=B-G;136;
route=G-F;133;
route=F-H;163;
route=I-H;197;