Error: The object reference was not defined as an instance of a C#object

Question

I wanted to make a table in console but all ran the program appears error in the method AlunoLista(), I’ve tried to make changes but I can’t get there.

     static void Main(string[] args)
        {
            var client = new string[7, 7];
            InsertData<ClientHeader>(client);

            Console.Clear();
            InsertData<ClientHeader>(client);
            AlunoLista(client);
            Console.ReadKey();
        }
        static int getInsertIndex(string[,] matrix)
        {
            for (int j = 0; j < matrix.GetLength(0); j++)
            {
                if (string.IsNullOrEmpty(matrix[j, 0])) return j;
            }

            return -1;
        }
        private static void InsertData<T>(string[,] matrix)
        {

            int n = getInsertIndex(matrix), id = 1;

            matrix[n, 0] = Convert.ToString(id++);
            int x = matrix.GetLength(1) - 1;
            matrix[n, x] = "true";

            for (var j = 1; j < matrix.GetLength(1); j++)
            {
                do
                {
                    Console.Write($"\nInsert {GetHeader<T>(j)}: ");
                    matrix[n, j] = Console.ReadLine();
                } while (string.IsNullOrEmpty(matrix[n, j]));
            }
        }

        private static string GetHeader<T>(int i) => Enum.GetName(typeof(T), i);


        static void AlunoLista(string[,] lista)
        {
            Console.Clear();
            string linha = new String('-', 49);
            int[] tamanho = new int[] { 4, 10, 10, 20,10,10,10 };


            for (int i = 0; i < lista.GetLength(1); i++)
            {
                Console.WriteLine(linha);
                Console.Write("|");
                for (int j = 0; j < lista.GetLength(0); j++)
                {
                    if (lista[j, i] != null) lista[j, i] = "";
                    string espaço =new string (' ', tamanho[j] - lista[j, i].Length);

                    Console.Write($"{lista[j, i]}{espaço}");
                    Console.Write("|");
                }
                Console.WriteLine();
            }
            Console.WriteLine(linha);
        }


        enum ClientHeader { Id, Name, Surname, Addres, CodPostal, Telephone, Email, State };


    }
}

Answer 1

In the line before the error is transforming into string empty if what you find nay is null, which makes no sense. It makes sense to make that transformation if it is null. Changing this solves the problem. I improved some things, but I think I can improve more. There is something that is really not ideal, but it can be a requirement.

using static System.Console;

public class Program {
     public static void Main(string[] args) {
         var client = new string[7, 7];
         InsertData<ClientHeader>(client);
         InsertData<ClientHeader>(client);
         AlunoLista(client);
     }
    static int getInsertIndex(string[,] matrix) {
        for (int j = 0; j < matrix.GetLength(0); j++) if (string.IsNullOrEmpty(matrix[j, 0])) return j;
        return -1;
    }
    private static void InsertData<T>(string[,] matrix) {
        int n = getInsertIndex(matrix), id = 1;
        matrix[n, 0] = (id++).ToString();
        int x = matrix.GetLength(1) - 1;
        matrix[n, x] = "true";
        for (var j = 1; j < matrix.GetLength(1); j++) {
            do {
                Write($"\nInsert {GetHeader<T>(j)}: ");
                matrix[n, j] = ReadLine();
            } while (string.IsNullOrEmpty(matrix[n, j]));
        }
    }
    
    private static string GetHeader<T>(int i) => System.Enum.GetName(typeof(T), i);
    
    static void AlunoLista(string[,] lista) {
        var linha = new string('-', 49);
        int[] tamanho = new int[] { 4, 10, 10, 20, 10, 10, 10 };
        for (int i = 0; i < lista.GetLength(1); i++) {
            WriteLine($"{linha}|");
            for (int j = 0; j < lista.GetLength(0); j++) {
                lista[j, i] = lista[j, i] ?? "";  // <===================== mudei aqui
                var espaço = new string (' ', tamanho[j] - lista[j, i].Length);
                Write($"{lista[j, i]}{espaço}|");
            }
            WriteLine();
        }
        WriteLine(linha);
    }
    enum ClientHeader { Id, Name, Surname, Addres, CodPostal, Telephone, Email, State };
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.