Добавьте ссылки System.Windows.Forms.dll и System.Drawing.dll и попробуйте это:
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
foreach (var screen in System.Windows.Forms.Screen.AllScreens)
{
var window = new Window1
{
WindowStartupLocation = WindowStartupLocation.Manual,
WindowState = WindowState.Maximized,
WindowStyle = WindowStyle.None,
Title = screen.DeviceName,
Width = screen.Bounds.Width,
Height = screen.Bounds.Height,
Left = screen.Bounds.Left,
Top = screen.Bounds.Top,
};
window.Show();
}
}
}
Не забудьте удалить StartupUri = "Window1.xaml" из App.xaml.