private void button1_Click(object sender, EventArgs e)
        {
            listBox1.Items.Add(textBox1.Text);
            textBox1.Text = "";
        }

private void button2_Click(object sender, EventArgs e)
        {
            string s = "";
            foreach(var c in listBox1.SelectedItems)
            {
                if(int.TryParse(c.ToString(), out int o))
                {
                    if(o%2 == 0)
                    {
                         s += $"{o.ToString()}\n";
                    }
                }
            }
            MessageBox.Show(s);
        }