The C#setwindowpos window is sticky

[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>
Get the currently active window
</summary>
<returns></returns>
[DllImport ("user32.dll", CharSet = CharSet.Auto)]
private static extern system.intptr GetForegroundWindow ();

Which form wants to be top, add in Form_Load

SetWindowPos (this. Handle,-1, 0, 0, 0, 0, 1 | 2); Last parameter also useful 1 | 4
Description, see API function description
If you click a button to pop up a new form, and the top, then:

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