Fullscreen problem with resolution

Starting resolution of my game is 1200x800. In the windows standalone player I press alt + enter and the game is set in fullscreen. At the same time changing the resolution to 1280x800. By clicking the button I call something like this

if (Screen.fullScreen) {
                     Screen.SetResolution (1200, 800, true);

                 else {
                     Screen.SetResolution (1200, 800, false);

But the resolution does not change. How can I keep the resolution (1200x800) at the transition in fullscreen?

You might not be able to due to aspect ratio.