public static class Switcher
    {
        public static MainWindow pageSwitcher;

        public static void Switch(Page newPage)
        {
            pageSwitcher.Navigate(newPage);
        }
    }

[ServiceContract]
    public interface IAppManager
    {
        [OperationContract]
        void DoWork();
        [OperationContract]
        void Page1();
        [OperationContract]
        void Page2();
    }
[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall, ConcurrencyMode = ConcurrencyMode.Multiple)]
    public class AppManager : IAppManager
    {

        public void DoWork()
        {
        }
        public void Page1()
        {
            MainWindow.pageSwitcher = new MainWindow();
            MainWindow.Switch(new Page1());
        }
        public void Page2()
        {
            MainWindow.pageSwitcher = new MainWindow();
            MainWindow.Switch(new Page2());
        }
    }