Object reference not set to an instance of an object run-time error

Lali

I am new to C# but experienced in C++. I have been strugling with the "Object reference not set to an instance of an object" run-time error when accessing array elements in the example below. The error is reported in Main when assigning 100 to myItemList.item[0].somedata (shown in red). Any help would be appreciated. Thx.

namespace ConsoleApplication1
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class Item
    {
        public Item()
        {
            someData = 0;
        }

        public int someData;
    }

    public class ItemList
    {
        public ItemList()
        {
            itemCount = 0;
        }

        public ItemList(int count)
        {
            itemCount = count;
            items = new Item[count];
        }

        public int itemCount;
        public Item[] items;
    }

    class Program
    {
        static void Main(string[] args)
        {
            ItemList myItemList = new ItemList(2);
            myItemList.items[0].someData = 100;
        }
    }

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Kirtan Patel16y ago
Accepted answer
Ok Friend,

Now check it out I corrected the code

if my answer helps you then Check "Do you like this answer" check box please :)

   public class Item

    {

        public int someData;

        public Item(int value)

        {

            someData = value;

        }

    }

    public class ItemList

    {

        public int itemCount;

        public Item[] items;

        public ItemList()

        {

            itemCount = 0;

        }

        public ItemList(int count)

        {

            itemCount = count;

            items = new Item[count];

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            ItemList x = new ItemList(2);

            x.items[0] = new Item(50);

            x.items[1] = new Item(100);

            Console.WriteLine("Element 1 Data : " + x.items[0].someData.ToString());

            Console.WriteLine("Element 2 Data : " + x.items[1].someData.ToString());

            Console.ReadKey();

        }

    }

0
Lali16y ago
Sory no offence. both answers were correct. I accepted both, but the system seems to be only accepting the last one selected.
0

Lali16y ago

Thank you both for providing solutions. I have decided on the following implementation of ItemList constructor based on your answers. move the responsibility of Item instantiation to ItemList constructor. Any feedback is welcome.

    public class ItemList
    {
        //public int itemCount;
        //public Item[] items;

        public ItemList()
        {
            itemCount = 0;
        }

        public ItemList(int count)
        {
            itemCount = count;
            items = new Item[count]; // Allocates array pointers
            for (int i = 0; i < count; i++)
            {
                items[i] = new Item(); // Allocate and assign Item instance
            }
        }

        public int itemCount;
        public Item[] items;
    }

Kirtan Patel16y ago
friend what is problem in my answer???
you have unselected my answer ...
0

theLizard16y ago

after your declaration

ItemList myItemList = new ItemList(2);

you need to do this myItemList.items[0] = new item();
myItemList.items[1] = new item();

then you can do myItemList.items[0].someData = 100;

Lali16y ago
Thanks for the quick reply. I made the change as you suggested, but the problem remains the same. If the public members of the class is not in scope as you indicated, wouldn't it be a compile error?
0

Kirtan Patel16y ago

Error Was Because you have Used the variable before declaring it

namespace ConsoleApplication1
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class Item
    {
        public Item()
        {
            someData = 0;
        }

        public int someData;
    }

    public class ItemList
    {
        // It should be Here you have declared it after using it
         
        public int itemCount;
        public Item[] items;

        public ItemList()
        {
            itemCount = 0;
        }

        public ItemList(int count)
        {
            itemCount = count;
            items = new Item[count];
        }

    class Program
    {
        static void Main(string[] args)
        {
            ItemList myItemList = new ItemList(2);
            myItemList.items[0].someData = 100;
        }
    }