NullReferenceException in array of objects

Question

I'm working on a program in c# in which I've got an array of objects of a class. I've got a constructor in the class that should give the attached string a value of "" to make sure they're empty so information can be passed in easily. I then have the array created like so:

participants[8][8] grid = new participants[8][];

But I'm being thrown a NullReferenceExcetption error. Here is my code for your reference.

using System;

namespace TreasurehuntCsharp
{
    class square
    {
        public string strX = "";
        public int y = 0;
        public int x = 0;
    }

    class participants
    {
        public string name = "";
        public string contact = "";
        public participants()
        {
            name = "";
            contact = "";
        }
    }

    class Program
    {

        //Method for user to choose a square
        static void Input(square Coord)
        {
            //Variables
            bool correct = false;

            //Inputs

            Console.WriteLine("Please enter the coordinates of the square you would like to select: \r\n");

            do
            {
                //X coordinate
                Console.WriteLine("X: ");
                Coord.strX = Console.ReadLine().ToLower();
                //Convert letter to array coordinate
                switch (Coord.strX)
                {
                    case "a":
                        Coord.x = 0;
                        correct = true;
                        break;
                    case "b":
                        Coord.x = 1;
                        correct = true;
                        break;
                    case "c":
                        Coord.x = 2;
                        correct = true;
                        break;
                    case "d":
                        Coord.x = 3;
                        correct = true;
                        break;
                    case "e":
                        Coord.x = 4;
                        correct = true;
                        break;
                    case "f":
                        Coord.x = 5;
                        correct = true;
                        break;
                    case "g":
                        Coord.x = 6;
                        correct = true;
                        break;
                    case "h":
                        Coord.x = 7;
                        correct = true;
                        break;
                    default:
                        Console.WriteLine("Please enter a letter from A to H");
                        correct = false;
                        break;
                }
            } while (correct != true);

            correct = false;

            do
            {
                //Y coordinate
                Console.WriteLine("Y: ");
                Coord.y = Convert.ToInt32(Console.ReadLine());
                if (Coord.y >= 1 && Coord.y <= 7)
                {
                    correct = true;
                }
                else
                {
                    Console.WriteLine("Please input an integer value from 1 to 7.");
                    correct = false;
                }
            } while (correct != true);
        }

        static void ParticipantDetails(participants User)
        {
            Console.WriteLine("Please input your name and Contact number: ");
            
            //User name input
            Console.WriteLine("Name: ");
            User.name = Console.ReadLine();

            //User contact number input
            Console.WriteLine("Number: ");
            User.contact = Console.ReadLine();
        }

        static void Main(string[] args)
        {
            //Objects
            square Coord = new square();
            participants User = new participants();

            //Initialise 2D array
            participants[][] grid = new participants[8][];

            //Variables
            bool correct = false;

            do
            {
                //Methods
                Input(Coord);
                ParticipantDetails(User);

                //Input data to array
                if (grid[Coord.x][Coord.y].name == "" && grid[Coord.x][Coord.y].contact == "")
                {
                    grid[Coord.x][Coord.y].name = User.name;
                    grid[Coord.x][Coord.y].contact = User.contact;
                    Console.WriteLine(grid[Coord.x][Coord.y]);
                    correct = true;
                }
                else
                {
                    Console.WriteLine("That square is already filled. Please try again.");
                    correct = false;
                }
            } while (correct == false);
        }
    }

Which line throws the exception? Can you remove all the code that doesn't matter and create a [mre]? — gunr2171, Mar 02 '21 at 13:41
`participants[][]` this is called a jagged array (an array of arrays). You only initialize the array of arrays but not the arrays inside it, so those will be null. Also `participants` is a class so it's default value will also be `null` so you'll also need to initialize every `participant` inside it (or change your logic to do that as you need them) — Knoop, Mar 02 '21 at 13:49

Athanasios Kataras · Accepted Answer · 2021-03-02T13:53:44.907

You did not initialize all the dimensions of the array.

//Initialise 2D jagged array
participants[][] grid = new participants[8][];

// You can do that in a for loop
grid[0] = new participants[8];
grid[1] = new participants[8];
grid[2] = new participants[8];
...
grid[7] = new participants[8];

// For loop equivalent

for (int i = 0; i < grid.Length; i++) 
{
    grid[i] = new participant[8];
}

A better way for your case (since all the sub-arrays have the same number of elements), would be to use multidimensional-arrays

participants[,] grid = new participants[8,8];

// and access like this
grid[Coord.x, Coord.y]

score 0 · Answer 2 · answered Mar 02 '21 at 13:46

You created an array of arrays.

 participants[][] grid = new participants[8][];

It is an array with 8 entries. Each of the entries is supposed to be an array of participants, but it hasn't been created.

What you need to do is to create 8 arrays and assign them to the elements of the grid array.

        participant[][] grid = new participant[8][];
        for (int i = 0; i < grid.Length; i++)
            grid[i] = new participant[8];

Here I assume that each element of the grid array is also going to contain 8 elements, but it's up to you.

NullReferenceException in array of objects

2 Answers2