Null Reference Exception was unhandled C# console application

Question

I know this is probably similar to some other posts, but I'm not quite sure what I'm doing wrong here. As an FYI, I'm new to programming and still trying to learn proper flow.

Here is the code, the exception occurs at "MyFriends[i].Name = friendName".

using System;
using System.Collections;

namespace FriendList
{
    class FriendList
    {
        static public Friend[] MyFriends = new Friend[2];
        public static void Main()
        {
            string friendName;
            string friendPhone, friendMonth, friendDay, friendYear;
            int intMonth, intDay, intYear;

            for (int i = 0; i < 2; ++i)
            {
                Console.Write("enter name");
                friendName = Console.ReadLine();
                MyFriends[i].Name = friendName;

                Console.Write("phone");
                friendPhone = Console.ReadLine();
                MyFriends[i].Phone = friendPhone;

                Console.WriteLine("Enter Month: ");
                friendMonth = Console.ReadLine();
                intMonth = Convert.ToInt32(friendMonth);
                MyFriends[i].Month = intMonth;

                Console.WriteLine("Enter Day: ");
                friendDay = Console.ReadLine();
                intDay = Convert.ToInt32(friendDay);
                MyFriends[i].Day = intDay;

                Console.WriteLine("Entery Year: ");
                friendYear = Console.ReadLine();
                intYear = Convert.ToInt32(friendYear);
                MyFriends[i].Year = intYear;
            }

            for (int i = 0; i < 2; ++i)
            {
                string information = string.Format("first name: {0}, phone {1}", MyFriends[i].Name, MyFriends[i].Phone);
                Console.WriteLine(information);
            }
            Console.Read(); 
        }
    }

    class Friend
    {
        string _Name = string.Empty, _Phone = string.Empty;
        int _Day = 0, _Month = 0, _Year = 0;

        public string Name
        {
            get { return _Name; }
            set { _Name = value; }
        }

        public string Phone
        {
            get { return _Phone; }
            set { _Phone = value; }
        }

        public int Month
        {
            get { return _Month; }
            set { _Month = value; }
        }

        public int Day
        {
            get{ return _Day; }
            set{ _Day = value ; }
        }

        public int Year
        {
            get { return _Year;}
            set { _Year = value; }
        }

        public Friend()
        { }

    }
}

Thank you for your guidance!

Welcome to SO. Can you add the exception with the stack trace to your question? — Ryan Gates
– Ryan Gates, Commented Jan 29, 2014 at 18:11
What iteration of the loop does this occur on? I.e., what is the value of i? — pamphlet
– pamphlet, Commented Jan 29, 2014 at 18:32

Geeky Guy · Accepted Answer · 2014-01-29 18:13:14Z

5

Your friend array is initialized empty. So MyFriends[i] will hit a null reference, which is another way to say you are trying to access something that doesn't exist.

In other words, you have an Array with slots for two friends, but both slots are empty. You still need to have a friend in each slot before you can use their properties, such as Name, Phone etc.

Simply start the for loop like this:

for (int i = 0; i < 2; ++i)
{
    MyFriend[i] = new Friend(); //or pass parameters as required by the constructor
    // rest of your code goes here
}

And things will be fine. This way, you are adding a friend to the slot you will be using.

answered Jan 29, 2014 at 18:13

Geeky Guy

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Comments

Daniel A.A. Pelsmaeker · Accepted Answer · 2014-01-29 18:14:13Z

1

You've created an array with two elements, but you have set the elements to any value. They are both null:

static public Friend[] MyFriends = new Friend[2];

So, when you try to use MyFriends[i] from the array, you're actually getting null.

MyFriends[i].Name = friendName;

That where your NullReferenceException came from.

You'll have to initialize the members of the array. For example:

for (int i = 0; i < MyFriends.Length; i++)
{
    // Put a new Friend object in the array.
    MyFriends[i] = new Friend();

    // ...

answered Jan 29, 2014 at 18:14

Daniel A.A. Pelsmaeker

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Comments

Yurii · Accepted Answer · 2014-01-29 18:14:20Z

1

When creating collections, they're populated with default values of the target type, and the default value for any reference type if null. So to solve your problem you'd have to initialize items in the array before accessing them:

....
for (int i = 0; i < 2; ++i)
{
    MyFriends[i] = new Friend();
    ...

answered Jan 29, 2014 at 18:14

Yurii

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Comments

Tal · Accepted Answer · 2014-01-29 18:14:20Z

0

MyFriends is an array of Friend class. each element in the array needs to be initialized with a friend constructor so it will be allocated with a memory.

answered Jan 29, 2014 at 18:14

Tal

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