Getting an error when following a tutoral, can someone please help me.

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I'm following a tutor to create a game but I'm getting an error in this line

for (int i = 0; i < stars.Length /2; i++)

C# is telling me that stars are null.

The error is System.NullReferenceException: ‘Object reference not set to an instance of an object’.

I checked the code and can see no difference in my code and your code. Maybe there is something I need to do to the form to stop this error.

The code is included.
Note that the code is not complete but this does not stop it from an giving me an error when compiling the code.

Brian_TheLion

I'm using the latest version of Visual Studio and the tutorial is for #7 of Visual Studio so maybe this is the reason for an error or there is something I needed to do that's missing in the tutorial. I've checked my code with the code in the tutorial but it's the same.

Hoping you can help me.

What I have tried:

C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using WMPLib;

namespace SpaceShooter
{
    public partial class Form1 : Form
    {
        WindowsMediaPlayer gameMedia;
        WindowsMediaPlayer shootgMedia;
        
        PictureBox[] stars;
        PictureBox[] munitions;  //gun fire at enemy
        int backgroundspeed;
        int playerSpeed;
        int munitionsSpeed;
       
        
        Random rnd;
        public Form1()
        {
            InitializeComponent();
        }
       //window_Load methoid
        private void Form1_Load(object sender, EventArgs e)
        {
            backgroundspeed = 4;
            playerSpeed = 4;
            munitionsSpeed = 20;
            munitions = new PictureBox[3];
          
            //Load images
            Image munition = Image.FromFile(@"asserts\munitation.png");

            for(int i = 0; i < munitions.Length; i++)
            {
                munitions[i] = new PictureBox();
                munitions[i].Size = new Size(8, 8);
                munitions[i].Image = munition;
                munitions[i].SizeMode = PictureBoxSizeMode.Zoom;
                munitions[i].BorderStyle = BorderStyle.None;
                this.Controls.Add(munitions[i]);
            }

            //Create WMP
            gameMedia = new WindowsMediaPlayer();
            shootgMedia = new WindowsMediaPlayer();
          

            //Load all songs
            gameMedia.URL = "songs\\GameSong.mp3";
            shootgMedia.URL = "songs\\shoot.mp3";

            //Setup Songs settings
            gameMedia.settings.setMode("loop", true);
            gameMedia.settings.volume = 5;
            shootgMedia.settings.volume = 1;

            stars = new PictureBox[10];
            rnd = new Random();

            for (int i = 0; i < stars.Length; i++)
            {
                stars[i] = new PictureBox();
                {
                    stars[i].BorderStyle = BorderStyle.None;
                    stars[i].Location = new Point(rnd.Next(20, 580), rnd.Next(-10, 400));
                }
                if (i % 2 == 1)
                {
                    stars[i].Size = new Size(2, 2);
                    stars[i].BackColor = Color.Wheat;
                }
                else
                {
                    stars[i].Size = new Size(3, 3);
                    stars[i].BackColor = Color.DarkGray;
                }
                this.Controls.Add(stars[i]);
            }

            gameMedia.controls.play();    //might be correct place for this

        }
        //Move Game Timer_Tick
        private void MoveBgTimer_Tick(object sender, EventArgs e)
        {
            for (int i = 0; i < stars.Length /2; i++)   //ERROR on this line. stars = null
            {
                stars[i].Top += backgroundspeed;   

                if (stars[i].Top >= this.Height)
                {
                    stars[i].Top = -stars[i].Height;
                }
            }
            
            for (int i = stars.Length /2; i <stars.Length; i++)
            {
                stars[i].Top += backgroundspeed - 2;
                if (stars[i].Top >= this.Height)
                {
                    stars[i].Top = -stars[i].Height;
                }
            }
        }

        private void LeftMoveTimer_Tick(object sender, EventArgs e)
        {
            if(Player.Left > 10)
            {
                Player.Left -= playerSpeed;
            }
        }

        private void RightMoveTimer_Tick(object sender, EventArgs e)
        {
            if (Player.Right <580)
            {
                Player.Left += playerSpeed;
            }
        }

        private void DownMoveTimer_Tick(object sender, EventArgs e)
        {
            if (Player.Top <400)
            {
                Player.Top += playerSpeed;
            }
        }

        private void UpMoveTimer_Tick(object sender, EventArgs e)
        {
            if (Player.Top >10)
            {
                Player.Top -= playerSpeed;
            }
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Right)
            {
                RightMoveTimer.Start();
            }
            if (e.KeyCode == Keys.Left)
            {
                LeftMoveTimer.Start();
            }
            if (e.KeyCode == Keys.Down)
            {
                DownMoveTimer.Start();
            }
            if (e.KeyCode == Keys.Up)
            {
                UpMoveTimer.Start();
            }
        }

        private void Form1_KeyUp(object sender, KeyEventArgs e)
        {
           RightMoveTimer.Stop();
           LeftMoveTimer.Stop();
           DownMoveTimer.Stop();
           UpMoveTimer.Stop();
        }

        private void MoveMunitionTimer_Tick(object sender, EventArgs e)
        {
            for (int i = 0; i < munitions.Length; i++)
            {
                if (munitions[i].Top > 0)
                {
                    munitions[i].Visible = true;
                    munitions[i].Top -= munitionsSpeed;
                }
                else
                {
                    munitions[i].Visible = false;
                    munitions[i].Location = new Point(Player.Location.X + 20, Player.Location.Y + 30);
                }
            }
        }
    }
}

Posted 19-Nov-19 13:37pm

Brian_TheLion

Updated 19-Nov-19 22:26pm

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TheRealSteveJudge · Answer 1 · 2019-11-19T22:27:00

The problem ist your Timer.

MoveBgTimer_Tick

This timer seems to be configured as Enabled = True in the designer.
Therefore it is immediately active after the application has started.
The event handler then accesses the stars array before it is initialized.
You should set Enabled = False in the designer and the set Enabled = True at the end of the

Form1_Load

event handler.

Afzaal Ahmad Zeeshan · Answer 2 · 2019-11-19T17:10:00

Quote:
The error is System.NullReferenceException: ‘Object reference not set to an instance of an object’.

That is one of the most basic problems developers encounter. It means that you forgot to do new for your data. It might be your code, or the code in the library and you forgot to call some sort of initialization method.

Inside your code, the problem is at,

for (int i = 0; i < stars.Length /2; i++)

Thus it means that you forgot to initialize the value for stars. If you look at the code, you see that you left the field uninitialized,

public partial class Form1 : Form
{
    WindowsMediaPlayer gameMedia;
    WindowsMediaPlayer shootgMedia;
    
    PictureBox[] stars; // <-- here

There is a simple way to solve this, just initialize the value for the stars either in the constructor, or you can do that in a separate method when you know the value for the array.

C#

public Form1()
{
    InitializeComponent();

    // Here
    stars = new PictureBox[10]; // 10 hardcoded elements, each null.
}

This brings us to another problem in C#, that the arrays are of fixed sizes. And they cannot accommodate more than their size, and will always consume the exact size in memory. This is where List<PictureBox> types come into handy.

If you can change the code, I highly recommend using a List<T> instead of an array.
.net - Array versus List<T>: When to use which? - Stack Overflow[^]
List<T> Class (System.Collections.Generic) | Microsoft Docs[^]

One more thing, I noticed that you are doing the same thing I recommended in Form1_Load method,

stars = new PictureBox[10];
rnd = new Random();

for (int i = 0; i < stars.Length; i++)
{

It would be great if you can safely continue with the execution of code. Like this,

C#

if(stars != null) {
    for (int i = 0; i < stars.Length; i++) {
        // Code here.
    }
}

This can prevent the null reference exceptions in the code.

Edit
If you want to show the stars even when they are null, just do:

C#

if(stars == null) {
   stars = new PictureBox[10];
}

// Your code here...

Getting an error when following a tutoral, can someone please help me.

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