[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SetWindowPos(IntPtr hWnd, int hWndInsertAfter, int x, int y, int Width, int Height, int flags);
/// <summary>
/// 得到當(dāng)前活動的窗口
/// </summary>
/// <returns></returns>
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern System.IntPtr GetForegroundWindow();

Form2 frm = new Form2();
frm.Show();
SetWindowPos(GetForegroundWindow(), -1, 0, 0, 0, 0, 1 | 2);