MainWindow.xaml.cs

using System.IO;
using System.Windows;

namespace ThreadAffinity;

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void findButton_Click(object sender, RoutedEventArgs e)
    {
        SynchronizationContext uiContext = SynchronizationContext.Current!;

        Task.Run(() =>
        {
            string pictures =
                Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
            var folder = new DirectoryInfo(pictures);
            FileInfo[] allFiles =
                folder.GetFiles("*.jpg", SearchOption.AllDirectories);
            FileInfo? largest =
                allFiles.OrderByDescending(f => f.Length).FirstOrDefault();

            if (largest is not null)
            {
                uiContext.Post(_ =>
                {
                    long sizeMB = largest.Length / (1024 * 1024);
                    outputTextBox.Text =
                        $"Largest file ({sizeMB}MB) is {largest.FullName}";
                },
                null);
            }
        });
    }
}