Private ReadOnly _strPath As String = "c:\_temp"
Private ReadOnly _strFileName As String = "test.txt"

Private _textReader As StreamReader
Private _fileLength As Integer

Private Sub btnStartMonitoring_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnStartMonitoring.Click
    listBox1.Items.Add($"Start monitoring {_strFileName} at {DateTime.Now.ToLongTimeString()}")

    'load the current file
    FullLoad()

    'initialize the file system watcher and start monitoring
    fileSystemWatcher1.Path = _strPath
    fileSystemWatcher1.Filter = _strFileName
    fileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite
    fileSystemWatcher1.EnableRaisingEvents = True
End Sub

Private Sub FullLoad()
    Try
        Dim fs = New FileStream(Path.Combine(_strPath, _strFileName), FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
        _textReader = New StreamReader(fs)

        Do
            Dim line As String = _textReader.ReadLine()
            If line Is Nothing Then Exit Do
            listBox1.Items.Add(line)
        Loop

        listBox1.SelectedIndex = listBox1.Items.Count - 1
        _fileLength = CInt(_textReader.BaseStream.Length)
    Catch ex As FileNotFoundException
        MessageBox.Show("You need to create your test file first")
    End Try
End Sub

private readonly string _strPath = @"c:\_temp";
private readonly string _strFileName = "test.txt";

private StreamReader _textReader;
private int _fileLength;

private void btnStartMonitoring_Click(object sender, EventArgs e)
{
    if (!string.IsNullOrWhiteSpace(fileSystemWatcher1.Path))
    {
        fileSystemWatcher1.Changed -= fileSystemWatcher_Changed;
    }

    //load the current file
    listBox1.Items.Add($"Start monitoring {_strFileName} at {DateTime.Now.ToLongTimeString()}");
    FullLoad();

    //initialize the file system watcher and start monitoring
    fileSystemWatcher1.Path = _strPath;
    fileSystemWatcher1.Filter = _strFileName;
    fileSystemWatcher1.NotifyFilter = NotifyFilters.LastWrite;
    fileSystemWatcher1.EnableRaisingEvents = true;

    fileSystemWatcher1.Changed += fileSystemWatcher_Changed;
}

private void FullLoad()
{
    try
    {
        string line;
        var fs = new FileStream(Path.Combine(_strPath, _strFileName), FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
        _textReader = new StreamReader(fs);

        while ((line = _textReader.ReadLine()) != null)
        {
            listBox1.Items.Add(line);
        }

        listBox1.SelectedIndex = listBox1.Items.Count - 1;
        _fileLength = (int)_textReader.BaseStream.Length;
    }
    catch (FileNotFoundException )
    {
        MessageBox.Show("You need to create your test file first");
    }
}

Private Sub fileSystemWatcher_Changed(ByVal sender As Object, ByVal e As FileSystemEventArgs) Handles fileSystemWatcher1.Changed
    'sometimes the event is triggered with an invalid stream
    If _textReader.BaseStream.Length = 0 Then Return

    listBox1.Items.Add(New String("-"c, 40))
    listBox1.Items.Add($"{e.ChangeType} - Previous length={_fileLength} / Current length={_textReader.BaseStream.Length}")

    If _textReader.BaseStream.Length > _fileLength Then
        listBox1.Items.Add(New String("-"c, 40))
        listBox1.Items.Add($"Adding new items at {DateTime.Now.ToLongTimeString()}")
        Dim strEndOfFile = _textReader.ReadToEnd()

        For Each strNewItem As String In strEndOfFile.Split({vbCrLf}, StringSplitOptions.None)
            listBox1.Items.Add(strNewItem)
        Next

        listBox1.SelectedIndex = listBox1.Items.Count - 1
        _fileLength = CInt(_textReader.BaseStream.Length)
    ElseIf _textReader.BaseStream.Length < _fileLength Then
        'file is shorter, just reload to start fresh
        listBox1.Items.Add($"Stream is shorter than the previous one. Fully reloading the file at {DateTime.Now.ToLongTimeString()}")
        FullLoad()
    End If
End Sub

private void fileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
    //sometimes the event is triggered with an invalid stream
    if (_textReader.BaseStream.Length == 0) return;

    listBox1.Items.Add(new string('-', 40));
    listBox1.Items.Add($"{e.ChangeType} - Previous length={_fileLength} / Current length={_textReader.BaseStream.Length}");

    if (_textReader.BaseStream.Length > _fileLength)
    {
        listBox1.Items.Add(new string('-', 40));
        listBox1.Items.Add($"Adding new items at {DateTime.Now.ToLongTimeString()}");

        var strEndOfFile = _textReader.ReadToEnd();

        foreach (string strNewItem in strEndOfFile.Split(new[] { "\r\n" }, StringSplitOptions.None))
        {
            listBox1.Items.Add(strNewItem);
        }

        listBox1.SelectedIndex = listBox1.Items.Count - 1;

        _fileLength = (int)_textReader.BaseStream.Length;
    }
    else if (_textReader.BaseStream.Length < _fileLength)
    {
        //file is shorter, just reload to start fresh
        listBox1.Items.Add($"Stream is shorter than the previous one. Fully reloading the file at {DateTime.Now.ToLongTimeString()}");
        FullLoad();
    }
}