如何在主窗口中获得当前渲染的用户控件作为子控件

我是MVVM和使用MVVMLight工具包的新手。因此，我正在研究一个项目，在这个项目中，导航发生在基于Uri的地方，该Uri设置在视图上暴露为x:Name的框架上。这是我的代码…

public  virtual void SetFrame(Uri userControll, string FrameElementName)
{
     var mainWindow = Application.Current.MainWindow;
     var frame = mainWindow.FindName(FrameElementName) as Frame;
     if (frame != null)
     {
          frame.Source = userControll;
     }
 }

如果我们想让导航功能在当前渲染的usercontrol上工作，它是主窗口的子控件?我们需要执行哪些步骤?像

public  virtual void SetFrame(Uri anotherUserControll, string FrameElementName)
{
     var userControl= // what to do to get Currently rendered Usercontroll as child of MainWindow?
     var frame = userControl.FindName(FrameElementName) as Frame;
     if (frame != null)
     {
        frame.Source = anotherUserControll;      
     }
 }

    public  virtual void SetFrame(Uri anotherUserControll, string FrameElementName)
     {
         var mainFrame =(Frame)Application.Current.MainWindow.FindName("MainFrame");
         var userControll = mainFrame.Content as UserControl;
         var frame = userControll.FindName(contentControll) as Frame;
         if (frame != null)
         {
             frame.Source = anotherUserControll;
         }           
     }