C# ListBox.Items.Add in other class

Question

I've just recently started with C# and I know this has been kind of asked here before but all with complex examples which i could not really grasp yet. So here is my simple example.

I'm trying to add a string to a ListBox from an added class. It compiles fine but the "TEST" doesn't get added to the ListBox

Form1.cs

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            ListBoxAdder myAdder = new ListBoxAdder();
            myAdder.Testing();
        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

        }
    }
}

ListBoxAdder.cs

namespace WindowsFormsApplication1
{
    class ListBoxAdder : Form1
    {
        public void Testing()
        {
            listBox1.Items.Add("TEST");
        }
    }
}

I assume nothing is happening because of creating another instance of "ListBoxAdder"? But I couldn't make it static because I wouldn't make Testing() static or I would have no access to listBox1.

Why not pass listbox1 to the Testing function? That way you will not have to inherit from Form1. — user2773289
– user2773289, Commented Mar 2, 2017 at 18:21
Name your controls better. And I assume you have a listbox on both forms. They are separate entities and have nothing to do with each other. Why do you need a separate class/form for this? — Sami Kuhmonen
– Sami Kuhmonen, Commented Mar 2, 2017 at 18:22
You don't need to make it static, but you do need to pass a reference to the actual control. Your assumption at the end is correct, you are instantiating a new copy of ListBoxAdder which hasn't been placed anywhere. — Justin C
– Justin C, Commented Mar 2, 2017 at 18:27
You should create instance of ListBoxAdder at the form level and call method on it in button_Click. Creating new instance in every button click will not help you. — Chetan
– Chetan, Commented Mar 2, 2017 at 18:32

apocalypse · Accepted Answer · 2017-03-02 18:43:18Z

1

namespace WindowsFormsApplication1
{
    class ListBoxAdder
    {
        ListBox listBox;

        public ListBoxAdder (ListBox listBox)
        {
            this.listBox = listBox;
        }

        public void Testing()
        {
            listBox.Items.Add("TEST");
        }
    }
}

And:

private void button1_Click(object sender, EventArgs e)
{
    ListBoxAdder myAdder = new ListBoxAdder(listBox1); // pass ListBox instance here
    myAdder.Testing();
}

answered Mar 2, 2017 at 18:43

apocalypse

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