Question

I’m trying to add items to a listbox from another form. Form1 has a listbox with a “Dummy” item, and when I try to add more items from that form, everything works fine. However, when I try to add items from a different form (AddContact.cs), no items are being added. I will supply code from both forms.

PS: The listbox is set to public, to be able to access it from outside Form1.

Form1:

    public Form1()
    {
        InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        list_names.Items.Add("Dummy");
    }

    private void btn_check_Click(object sender, EventArgs e)
    {
        if (list_names.SelectedItem == null)
        {
            MessageBox.Show("No item has been selected.", "Error",
                MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
        else if (list_names.SelectedItem.ToString() == "Dummy")
        {
            //Dummy code for testing
            MessageBox.Show("Dummy has been selected!");
        }
    }

    private void btn_add_Click(object sender, EventArgs e)
    {
        new AddContact().Show();
    }

    private void btn_remove_Click(object sender, EventArgs e)
    {
        //TODO: Remove items from listbox
    }

AddContact:

    Form1 form;

    public AddContact()
    {
        InitializeComponent();
        form = new Form1();
    }

    private void btn_add_Click(object sender, EventArgs e)
    {
        if (textBox1.Text == string.Empty)
        {
            MessageBox.Show("No input has been given.");
        }
        else
        {
            //This doesn't work
            string s = textBox1.Text;
            form.list_names.Items.Add(s);
            textBox1.Text = "";
        }
    }

Answer 1

The problem is that you creating new form in AddContact, You need the reference to Form1.

Form1 form;

public AddContact(Form1 frm)
{
    InitializeComponent();
    form = frm;
}

and also

private void btn_add_Click(object sender, EventArgs e)
{
    new AddContact(this).Show();
}