[DllImport("user32.dll")] static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags); IntPtr HWND_BOTTOM = (IntPtr)1; uint SWP_NOSIZE = 0x0001; uint SWP_NOMOVE = 0x0002; private void BaseForm_Activated(object sender, EventArgs e) { this.TopMost = true; } private void BaseForm_Deactivate(object sender, EventArgs e) { //this.TopMost = false; SetWindowPos(this.Handle, HWND_BOTTOM, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE); }