Wpf get value to Form1 from mainWindow

Question

I have the MainWindow and Form1. I want to get the value from MainWindow to Form1. When i click on the button in Form1 the result is 0. Where do i do a mistake? Would you help me please?

<Window x:Class="WpfNewForm.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfNewForm"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Grid>
<Button Content="Button1" HorizontalAlignment="Left" Margin="233,116,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
<Button Content="Button2" HorizontalAlignment="Left" Margin="233,141,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click_1"/>
</Grid>
</Window>

namespace WpfNewForm
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public int indexMax;
public int[] indexy = new int[1000];
public MainWindow()
{
InitializeComponent();
}
private void Button_Click(object sender, RoutedEventArgs e)
{
for (int i = 1; i < 100; i++)
{
indexy[i] = i;
indexMax = i;
}
MessageBox.Show(indexMax.ToString());
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
Form1 form1 = new Form1();
form1.Show();
}
}
}

namespace WpfNewForm
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
MainWindow mainWindow = new MainWindow();
MessageBox.Show(mainWindow.indexMax.ToString());
}
}
}

Answer 1

Try something like this:

private void Button_Click_1(object sender, RoutedEventArgs e)
{
   Form1 form1 = new Form1();
   form1.mainWindow = this;
   form1.Show();
}
. . .
public partial class Form1 : Form
{
   public MainWindow mainWindow;

   public Form1()
   {
      InitializeComponent();
   }
   private void button1_Click(object sender, EventArgs e)
   {
      MessageBox.Show(mainWindow.indexMax.ToString());
   }
}