private void Form1_Load(object sender, EventArgs e)
{
   this.FormBorderStyle = FormBorderStyle.None;
   this.Bounds = GetSecondaryScreen().Bounds;
}

private Screen GetSecondaryScreen()
{
   foreach (Screen screen in Screen.AllScreens)
   {
      if (screen != Screen.PrimaryScreen)
         return screen;
   }
   return Screen.PrimaryScreen;
}

protected override void OnStateChanged(EventArgs e)
    {
        if (WindowState == WindowState.Maximized)
        {
            if (WindowStyle.None != WindowStyle)
                WindowStyle = WindowStyle.None;
        }
        else if (WindowStyle != WindowStyle.SingleBorderWindow)
            WindowStyle = WindowStyle.SingleBorderWindow;

        base.OnStateChanged(e);
    }

this.Left = SystemParameters.PrimaryScreenWidth + 100;
this.WindowState = System.Windows.WindowState.Maximized;

using System.Linq;

namespace ExtendedControls
{
    static public class WindowExt
    {

        // NB : Best to call this function from the windows Loaded event or after showing the window
        // (otherwise window is just positioned to fill the secondary monitor rather than being maximised).
        public static void MaximizeToSecondaryMonitor(this Window window)
        {
            var secondaryScreen = System.Windows.Forms.Screen.AllScreens.Where(s => !s.Primary).FirstOrDefault();

            if (secondaryScreen != null)
            {
                if (!window.IsLoaded)
                    window.WindowStartupLocation = WindowStartupLocation.Manual;

                var workingArea = secondaryScreen.WorkingArea;
                window.Left = workingArea.Left;
                window.Top = workingArea.Top;
                window.Width = workingArea.Width;
                window.Height = workingArea.Height;
                // If window isn't loaded then maxmizing will result in the window displaying on the primary monitor
                if ( window.IsLoaded )
                    window.WindowState = WindowState.Maximized;
            }
        }
    }
}