Question

I am attempting to solve an exercise from a book. (with no published answer)

I need to reference a PictureBox object which is present on my form to an Object Array. (I need to assign four of them really)

I initialise the array and assign the variables to it. I then call a method within each item in the array however the PictureBox object is not assigned. (Null Exception)

I’m a little baffled as I’ve found snippets of code publicly on the net which show I’m doing this correctly.

Code Below for pointers please:

Main Class

public partial class Form1 : Form
{
    Greyhound[] greyhoundArray = new Greyhound[4];

    public Form1()
    {
        greyhoundArray[0] = new Greyhound() { Location = 0, MyPictureBox = dog1, RaceTrackLenght = 100, StartingPosition = 0 };
        greyhoundArray[1] = new Greyhound() { Location = 0, MyPictureBox = dog2, RaceTrackLenght = 100, StartingPosition = 0 };
        greyhoundArray[2] = new Greyhound() { Location = 0, MyPictureBox = dog3, RaceTrackLenght = 100, StartingPosition = 0 };
        greyhoundArray[3] = new Greyhound() { Location = 0, MyPictureBox = dog4, RaceTrackLenght = 100, StartingPosition = 0 };

        InitializeComponent();
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        foreach (Greyhound greyhound in greyhoundArray)
        {
            greyhound.Run();
        }
    }
}

Greyhound Class

public class Greyhound
{
    public int StartingPosition;
    public int RaceTrackLenght;
    public PictureBox MyPictureBox;
    public int Location = 0;
    public Random Randomiser;

    public void Run()
    {
        // MessageBox.Show(MyPictureBox.Name + " was called");
        Randomiser = new Random();

        int distance = Randomiser.Next(0, 4);

        Point p = MyPictureBox.Location;
        p.X += distance;
        MyPictureBox.Location = p;
    }

    public void TakeStartingPosition()
    { }
}

Also I can confirm each dog PictureBox does exist on the form:

snippet from Form1.Designer.cs

// 
// dog1
// 
this.dog1.Image = ((System.Drawing.Image)(resources.GetObject("dog1.Image")));
this.dog1.Location = new System.Drawing.Point(17, 21);
this.dog1.Name = "dog1";
this.dog1.Size = new System.Drawing.Size(71, 26);
this.dog1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.CenterImage;
this.dog1.TabIndex = 2;
this.dog1.TabStop = false;

Answer 1

Place the call to InitializeComponent() before the initialization of GreyHound array.

When you call the initialization of the Greyhound array, you have not called yet the this.dog1 = new PictureBox() inside InitializeComponent, so you copy a null into the MyPictureBox property of each Greyhound instances

Also, I think you have a problem with the Randomiser variable in your Greyhound class