如何实现C#的窗口只出现一次,再按得时候是激活该窗口

不是showdailog()效果.
好像用foreach能实现.用比较简洁点做法.好的再加20分

Form frm = new Form();
private void button3_Click(object sender, EventArgs e)
{
if (frm.IsDisposed)
{
frm = new Form();
frm.Show();
}
else
{
frm.Show();
frm.Activate();
}
}

[dllimport("user32.dll", entrypoint = "getforegroundwindow")] public static extern int getforegroundwindow(); [dllimport("user32.dll", entrypoint = "setforegroundwindow")] public static extern int setforegroundwindow(int hwnd);