winform只显示一个窗体

private void button1_Click(object sender, EventArgs e)
        {
            //点击按钮的时候只会显示一个窗体
            bool opened = false;
            foreach (Form f1 in Application.OpenForms)
            {
                if (f1 is Form2)
                {
                    opened = true;
                    break;
                }
            }
            if (!opened)
            {
                Form2 f2 = new Form2();
                f2.Show();
            }
        }