WPF XamlReader Load xaml and parse

//The xaml material named MainWin.xaml

<Window
        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:WpfApp268" Topmost="True"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindowXamlReader.Load()" Height="450" Width="800">
    <StackPanel>
        <Button Content="Load" Width="200" Height="100" Background="Black"/>
        <Button Content="Load" Width="200" Height="100" Background="Red"/>
        <Button Content="Load" Width="200" Height="100" Background="Green"/>
        <Button Content="Load" Width="200" Height="100" Background="Blue"/>
        <Button Content="Load" Width="200" Height="100" Background="Cyan"/>
    </StackPanel>
</Window>

C# read the xaml file via FileStream and parse it

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Markup;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp268
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Window win = null;
            using (FileStream fs = new FileStream("MainWin.xaml", FileMode.Open, FileAccess.Read))
            {
                win = (Window)XamlReader.Load(fs);
                if (win != null)
                {
                    StackPanel stkPanel=win.Content as StackPanel;
                    if (stkPanel != null)
                    {
                        Button cyanBtn = stkPanel.Children[4] as Button;
                        if(cyanBtn != null)
                        {
                            cyanBtn.Click += CyanBtn_Click;
                        }
                    }
                }
                win.ShowDialog();

            }
        }

        private void CyanBtn_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("This is Cyan Button!");
        }
    }
}