How to access a class declared in MainWindow from another window?

Hi all,

I'm new to wpf. I added a new class to my project to hold global data (globalData.cs) using the Solution Explorer (add class). I'm using it in my MainWindow to store some info. I added another two windows (stage_1 and Stage_2) to my project as I want to move from one to the next like a wizard (but simpler) and save data to this globalData (variable/instance?) as I progress through the program. How do I access the stageData (variable/members?) from the other windows? Thanks.

Here is the declaration of stageData in MainWindow:

public partial class MainWindow : Window
{
  GlobalData stageData = new GlobalData { currentStage = 0 };   //init to fist stage

  public MainWindow()
  {
      InitializeComponent();
  }

  private void ButtonBrowse_Click(object sender, RoutedEventArgs e)
  {
     OpenFileDialog openDialog = new OpenFileDialog();
     
     // .....etc, etc
  }
  //etc, etc
}

This is what I want to do but Stage_1 can't see stageData:

  public partial class stage_1 : Window
    {
        //Need something to access stageData.currentStage !! 

        public stage_1()
        {
            InitializeComponent();
        }

        private void ButtonNext_Click(object sender, RoutedEventArgs e)
        {
            //I want to access stageData.currentStage here!
        }
    }

This only shows one variable (stageData.currentStage) but they'll be many more.Thanks