Timer event is not triggering button click

I'm trying transfer Image file for each 3 seconds from source to destination directory, when I click on the "Start" button. And "Stop" button is for stopping the file transfer.

If I use the Start button for the third time, the event is not firing; so, the files are not transferring.

The progress I've done is in the below:

1. Start --- File Transferring; Pressed STOP Button // It Works as well.
2. Start --- File Transferring; Pressed STOP Button // It works as well.
3. Start ----- File is not transferring, Event is not firing // It not works!

What I've done for solving the problem:

• I put a break-point in the Tick event and the tick event is not firing.
• I Checked that SourceFiles.Count is greater than the TransferCount (SourceFiles.Count > TransferCount).
• I noticed that during the file transfer, if I click Stop button during a file transfer that particular file remains un-transferred.

How can i fix this? Thank you in advance.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Threading;

namespace GridTest
{
    /// <summary>
    /// Interaction logic for Window3.xaml
    /// </summary>
    public partial class Window3 : Window
    {
        DispatcherTimer dispatcherTimer = new DispatcherTimer();
        private static List<string> SourceFiles = new List<string>();
        private static readonly string SourceDir = @"C:\TestFiles\Images\";
        private static readonly string DestinationDir = @"C:\Files\Images\3_5x5\";
        private static int TransferCount = 0;
        public Window3()
        {
            InitializeComponent();
            this.Loaded += Window3_Loaded;
        }
        void Window3_Loaded(object sender, RoutedEventArgs e)
        {
            dispatcherTimer.Interval = new TimeSpan(0, 0, 3);
            dispatcherTimer.Tick += dt_Tick;
        }
        void dt_Tick(object sender, EventArgs e)
        {
            if (TransferCount < SourceFiles.Count)
            {
                var fileName = System.IO.Path.GetFileName(SourceFiles[TransferCount]);
                var destFile = System.IO.Path.Combine(DestinationDir, fileName);
                System.IO.File.Copy(SourceFiles[TransferCount], destFile,true);
                System.IO.File.Delete(SourceFiles[TransferCount]);
                Console.WriteLine(string.Format("Total Files: {0} Number of files transferred: {1}", SourceFiles.Count, TransferCount + 1));
                TransferCount += 1;
            }
            else
            {
                Console.WriteLine(string.Format("Total number of files transferred: {0}. Transfer Completed", TransferCount + 1));
                (sender as DispatcherTimer).Stop();
                (sender as DispatcherTimer).Tick -= dt_Tick;
            }
        }

        private void Start_Click(object sender, RoutedEventArgs e)
        {
            if (!System.IO.Directory.Exists(DestinationDir))
            {
                System.IO.Directory.CreateDirectory(DestinationDir);
            }

            if (System.IO.Directory.Exists(SourceDir))
            {
                SourceFiles = Directory.GetFiles(SourceDir).ToList();
            }
        }

        private void Stop_Click(object sender, RoutedEventArgs e)
        {
            dispatcherTimer.Stop();
        }
    }
}