Garbace Collection questo sconosciuto

Public class Persona : IDisposable
{
     public void Cammina()
     {
            Console.WriteLine("Cammino");
     }

     public void Dispose()
     {
            //LIbero tutte le risorse dell'instanza
     }
}


public class Test
{
      public static void CreaPersonaEFallaCamminare()
     {
            using(Persona IO = new Persona())
            {
                   IO.Cammina();
            }
     }
}

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using Interop.ClearImage;
using System;
using System.Drawing;
using System.Drawing.Imaging;

namespace Inlite.ClearImageNet
{
    public class ImageIO
    {
        public ImageIO();

        public EComprBitonal compressionBitonal { get; set; }
        public EComprColor compressionColor { get; set; }
        public int jpegQuality { get; set; }
        public double pdfMinImageHeight { get; set; }
        public double pdfMinImageWidth { get; set; }
        public EPdfRasterColorMode pdfRasterColorMode { get; set; }
        public int pdfRasterDpiBitonal { get; set; }
        public int pdfRasterDpiColor { get; set; }
        public int pdfRasterDpiGrayscale { get; set; }
        public EPdfReadMode pdfReadMode { get; set; }

        public void Append(Bitmap bmp, string fileName);
        public static void CopyToClipboard(Bitmap bmp);
        public static Bitmap Create(int width, int height);
        public static Bitmap Create(int width, int height, PixelFormat fmt);
        public ImageInfo Info();
        public ImageInfo Info(string fileName);
        public ImageInfo Info(string fileName, int pageNumber);
        public Bitmap Open(string fileName);
        public Bitmap Open(string fileName, int pageNumber);
        public void SaveAs(Bitmap bmp, string fileName);
    }
}

ImageIO img = new ImageIO();
            
int pages = img.Info(fileInput).PageCount;
int j = 0;
Bitmap pic;
for (int i = 0; i < pages; i++)
{
     pic = img.Open(fileInput, i+1);

     pic.Dispose();
     pic = null;

     GC.Collect();
}
     img = null;

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            TestObject a = new TestObject();
            a.Dispose();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            GC.Collect();
        }
    }

    public class TestObject : IDisposable
    {
        public TestObject() //Costruttore
        {
            Console.WriteLine("Costruttore");
        }

        ~TestObject() //Finalize
        {
            Dispose(false);
            Console.WriteLine("Distruttore");
        }

        #region IDisposable

        protected bool _disposed;

        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected void Dispose(bool disposing)
        {
            if (!_disposed)
            {
                if (disposing)
                {
                    Console.WriteLine("Dispose");
                }
                _disposed = true;
            }
        }

        #endregion
    }