Form form1,form2form1:
  form2 f2 = new form2();
  f2.Owner = this;
  f2.ShowDialog();  public void UpdateList(string fileName){ listBox1.Items.Add(fileName);}form2:
  if(dialogbox.ShowDialog() == DialogResult.OK)
  {
    form2 f2 = (form2)Owner;
    f2.UpdateList(dialogbox.FileName);
  }