Getting state updates from a listbox on a seperate form.

Hello,

In my main form, form1, i have an instance of my secondary form, form2.

 public partial class Form1 : Form
{
    Form2 f2;
    public Form1(Form2 frm2)
    {
       f2 = frm2;
       int count = f2.lstListBox.Items.Count;
       lblCount.Text = count.ToString();
    }

    private void Button_Click(object sender, EventArgs e)
    {
       Form2 popup = new Form2();
       popup.show();
    }
}
       
              

In Form2,

I have some simple commands to add and remove items from the ListBox. How do I keep Form1 updated with these changes? I've tried a million and one things all weekend but none of them seemed to work. does anyone have any examples they'd be willing to share?

Thanks

Malik