用C#在mvvm设计模式下，用IHostedService设计登陆跳转窗口程序，请给出完整实例代码

以下是一个简单的基于MVVM设计模式和IHostedService的登录跳转窗口程序的示例代码：

MainWindow.xaml：

<Window x:Class="LoginRedirectApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Login Redirect App" Height="350" Width="525">
    <Grid>
        <ContentControl Content="{Binding CurrentViewModel}" />
    </Grid>
</Window>

App.xaml.cs：

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Threading.Tasks;
using System.Windows;

namespace LoginRedirectApp
{
    public partial class App : Application
    {
        private readonly IHost _host;

        public App()
        {
            _host = Host.CreateDefaultBuilder()
                .ConfigureServices((context, services) =>
                {
                    services.AddSingleton<MainWindow>();
                    services.AddSingleton<LoginViewModel>();
                    services.AddSingleton<HomeViewModel>();
                    services.AddSingleton<IHostedService, LoginRedirectService>();
                })
                .Build();
        }

        protected override async void OnStartup(StartupEventArgs e)
        {
            await _host.StartAsync();

            var mainWindow = _host.Services.GetService<MainWindow>();
            mainWindow.Show();

            base.OnStartup(e);
        }

        protected override async void OnExit(ExitEventArgs e)
        {
            using var scope = _host.Services.CreateScope();
            var hostedServices = scope.ServiceProvider.GetServices<IHostedService>();

            foreach (var service in hostedServices)
            {
                await service.StopAsync();
            }

            await _host.StopAsync();

            base.OnExit(e);
        }
    }
}

LoginViewModel.cs：

using System;
using System.Windows.Input;

namespace LoginRedirectApp
{
    public class LoginViewModel : ViewModelBase
    {
        private readonly ILoginService _loginService;
        private readonly IRedirectService _redirectService;

        private string _username;
        public string Username
        {
            get => _username;
            set => SetProperty(ref _username, value);
        }

        private string _password;
        public string Password
        {
            get => _password;
            set => SetProperty(ref _password, value);
        }

        public ICommand LoginCommand { get; }

        public LoginViewModel(ILoginService loginService, IRedirectService redirectService)
        {
            _loginService = loginService;
            _redirectService = redirectService;

            LoginCommand = new RelayCommand(Login, CanLogin);
        }

        private bool CanLogin()
        {
            return !string.IsNullOrEmpty(Username) && !string.IsNullOrEmpty(Password);
        }

        private async void Login()
        {
            try
            {
                var result = await _loginService.LoginAsync(Username, Password);

                if (result)
                {
                    _redirectService.RedirectToHome();
                }
                else
                {
                    // handle login failure
                }
            }
            catch (Exception ex)
            {
                // handle login exception
            }
        }
    }
}

HomeViewModel.cs：

namespace LoginRedirectApp
{
    public class HomeViewModel : ViewModelBase
    {
        // add properties and methods for home view model here
    }
}

LoginRedirectService.cs：

using Microsoft.Extensions.Hosting;
using System;
using System.Threading;
using System.Threading.Tasks;

namespace LoginRedirectApp
{
    public class LoginRedirectService : IHostedService
    {
        private readonly IRedirectService _redirectService;
        private readonly ILoginService _loginService;
        private readonly MainWindow _mainWindow;
        private readonly LoginViewModel _loginViewModel;
        private readonly HomeViewModel _homeViewModel;

        public LoginRedirectService(IRedirectService redirectService,
            ILoginService loginService,
            MainWindow mainWindow,
            LoginViewModel loginViewModel,
            HomeViewModel homeViewModel)
        {
            _redirectService = redirectService;
            _loginService = loginService;
            _mainWindow = mainWindow;
            _loginViewModel = loginViewModel;
            _homeViewModel = homeViewModel;
        }

        public Task StartAsync(CancellationToken cancellationToken)
        {
            _redirectService.RedirectToLogin();
            _mainWindow.DataContext = _loginViewModel;

            return Task.CompletedTask;
        }

        public Task StopAsync(CancellationToken cancellationToken)
        {
            return Task.CompletedTask;
        }
    }

    public interface IRedirectService
    {
        void RedirectToLogin();
        void RedirectToHome();
    }

    public class RedirectService : IRedirectService
    {
        private readonly MainWindow _mainWindow;
        private readonly LoginViewModel _loginViewModel;
        private readonly HomeViewModel _homeViewModel;

        public RedirectService(MainWindow mainWindow,
            LoginViewModel loginViewModel,
            HomeViewModel homeViewModel)
        {
            _mainWindow = mainWindow;
            _loginViewModel = loginViewModel;
            _homeViewModel = homeViewModel;
        }

        public void RedirectToLogin()
        {
            _mainWindow.DataContext = _loginViewModel;
        }

        public void RedirectToHome()
        {
            _mainWindow.DataContext = _homeViewModel;
        }
    }

    public interface ILoginService
    {
        Task<bool> LoginAsync(string username, string password);
    }

    public class LoginService : ILoginService
    {
        public async Task<bool> LoginAsync(string username, string password)
        {
            // add login logic here, return true if login is successful
            return await Task.FromResult(true);
        }
    }
}

ViewModelBase.cs：

using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace LoginRedirectApp
{
    public class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void SetProperty<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
        {
            if (!Equals(field, value))
            {
                field = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }
}

RelayCommand.cs：

using System;
using System.Windows.Input;

namespace LoginRedirectApp
{
    public class RelayCommand : ICommand
    {
        private readonly Action _execute;
        private readonly Func<bool> _canExecute;

        public RelayCommand(Action execute, Func<bool> canExecute = null)
        {
            _execute = execute;
            _canExecute = canExecute ?? (() => true);
        }

        public event EventHandler CanExecuteChanged
        {
            add => CommandManager.RequerySuggested += value;
            remove => CommandManager.RequerySuggested -= value;
        }

        public bool CanExecute(object parameter)
        {
            return _canExecute();
        }

        public void Execute(object parameter)
        {
            _execute();
        }
    }
}

在这个示例中，我们使用了三个视图模型：`LoginViewModel`，`HomeViewModel`和`ViewModelBase`，其中`LoginViewModel`和`HomeViewModel`继承自`ViewModelBase`。`LoginViewModel`用于处理登录逻辑，`HomeViewModel`用于处理主页逻辑。`ViewModelBase`实现了`INotifyPropertyChanged`接口，用于在属性更改时通知视图。

我们还使用了三个服务：`ILoginService`，`IRedirectService`和`LoginRedirectService`。`ILoginService`用于处理登录逻辑，`IRedirectService`用于处理跳转逻辑，`LoginRedirectService`实现了`IHostedService`接口，用于在应用程序启动时将用户重定向到登录界面。

最后，我们使用了`RelayCommand`类实现了`ICommand`接口，以便我们可以在视图中使用命令。