Declare and Initialising array of Checkboxes in other class (not Form1)

Question

I have a WinForm with only 2 CheckBoxes on it, and firing an event, when CheckedChanged. Now....this Event calls a Method from an other Class, where i try to assign the value of the event-firing method to an checkboxarray with index of [0].

But i get always a "NullReferenceException" when the new initialized CheckBoxArray gets a Value.....why ist that so ??? Here is my Code:

public partial class Form1 : Form
{   
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this.checkBox1.CheckedChanged += 
            new EventHandler(new DoSomething().FromForm1Class);
    }
}

class DoSomething
{
    CheckBox[] mycheckboxes = new CheckBox[2];

    public void FromForm1Class(object sender, EventArgs e)
    {
        CheckBox n = sender as CheckBox;

        mycheckboxes[0].Checked = n.Checked;
    }
}

What is your actual requirement? I think you are complicating things — sujith karivelil, Aug 17 '17 at 07:31

score 3 · Answer 1 · answered Aug 17 '17 at 07:41

In class DoSomething you are declaring an Array of CheckBoxes, but you do not initialise each element in the Array. That is why you get a NullReferenceException, because the 0th element is null - it is not a Checkbox! So you need:

class DoSomething
{
    CheckBox[] mycheckboxes = new CheckBox[2];

    public DoSomething()
    {
         mycheckboxes[0] = new CheckBox();
         mycheckboxes[1] = new CheckBox();
    }

    public void FromForm1Class(object sender, EventArgs e)
    {
        CheckBox n = sender as CheckBox;

        mycheckboxes[0].Checked = n.Checked;
    }
}

score 0 · Accepted Answer · answered Aug 17 '17 at 07:47

U are missing initialization

public partial class Form1 : Form
{   
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this.checkBox1.CheckedChanged += new EventHandler(new DoSomething().FromForm1Class);
    }
}

class DoSomething
{
    CheckBox[] mycheckboxes = new CheckBox[2]{new CheckBox(), new CheckBox()};

    public void FromForm1Class(object sender, EventArgs e)
    {
        CheckBox n = sender as CheckBox;

        mycheckboxes[0].Checked = n.Checked;
    }
}

Declare and Initialising array of Checkboxes in other class (not Form1)

2 Answers2