请用C#给出在运用MVVM模式设计WPF时，如何实现指定初始窗口或页面，给出具体完整的实例代码

在MVVM模式下，我们可以使用ViewModel First的方式来指定初始窗口或页面。具体实现步骤如下：

1. 在App.xaml中指定默认的StartupUri为一个空白窗口，如下所示：

<Application x:Class="MVVMStartupUri.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MVVMStartupUri"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

2. 在MainWindow.xaml中定义一个ContentControl，并将其Content属性绑定到一个ViewModel的实例，如下所示：

<Window x:Class="MVVMStartupUri.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MVVMStartupUri"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ContentControl Content="{Binding CurrentViewModel}" />
    </Grid>
</Window>

3. 在MainWindow.xaml.cs中定义一个MainViewModel的实例，并将其赋值给MainWindow的DataContext属性，如下所示：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        // Create and set the DataContext
        DataContext = new MainViewModel();
    }
}

4. 在MainViewModel中定义一个CurrentViewModel属性，用于表示初始窗口或页面对应的ViewModel，如下所示：

public class MainViewModel : INotifyPropertyChanged
{
    private ViewModelBase _currentViewModel;

    public MainViewModel()
    {
        // Set the initial view model
        CurrentViewModel = new InitialViewModel();
    }

    public ViewModelBase CurrentViewModel
    {
        get { return _currentViewModel; }
        set
        {
            _currentViewModel = value;
            OnPropertyChanged(nameof(CurrentViewModel));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

5. 在InitialViewModel中定义初始窗口或页面的逻辑，如下所示：

public class InitialViewModel : ViewModelBase
{
    // Define the initial window or page
}

6. 在ViewModelBase中实现INotifyPropertyChanged接口，如下所示：

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

完整的实例代码如下：

App.xaml

<Application x:Class="MVVMStartupUri.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MVVMStartupUri"
             StartupUri="MainWindow.xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

MainWindow.xaml

<Window x:Class="MVVMStartupUri.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:MVVMStartupUri"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ContentControl Content="{Binding CurrentViewModel}" />
    </Grid>
</Window>

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        // Create and set the DataContext
        DataContext = new MainViewModel();
    }
}

MainViewModel.cs

public class MainViewModel : INotifyPropertyChanged
{
    private ViewModelBase _currentViewModel;

    public MainViewModel()
    {
        // Set the initial view model
        CurrentViewModel = new InitialViewModel();
    }

    public ViewModelBase CurrentViewModel
    {
        get { return _currentViewModel; }
        set
        {
            _currentViewModel = value;
            OnPropertyChanged(nameof(CurrentViewModel));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

InitialViewModel.cs

public class InitialViewModel : ViewModelBase
{
    // Define the initial window or page
}

ViewModelBase.cs

public abstract class ViewModelBase : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}