Folder changes monitor made in Visual Basic 2010 does not write changes correctly

Question

I've made a program in Visual Basic 2010, that monitors and write down changes in a folder eg. when a file deletes, when a file renames, when a file creates and which files, but it's a problem. I've writed the code to make a new line when another change is made, when a change is made, it writes it down to a file named log.txt, but the log only looks like "File log.txt has been modified" because the program, when it write changes to the log, it changes the log.txt to write down the log, but the strange is, it deletes everything in the document and writes "File log..txt has been modified" even if I have writed in the code to make a new line before writing. Can someone help me with this problem? Here's the code:

Imports System.IO
Imports System.Diagnostics
Public Class Form1
Public watchfolder As FileSystemWatcher

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

    watchfolder = New System.IO.FileSystemWatcher()

    'this is the path we want to monitor
    watchfolder.Path = TextBox1.Text

    'Add a list of Filter we want to specify
    'make sure you use OR for each Filter as we need to
    'all of those 

    watchfolder.NotifyFilter = IO.NotifyFilters.DirectoryName
    watchfolder.NotifyFilter = watchfolder.NotifyFilter Or _
                               IO.NotifyFilters.FileName
    watchfolder.NotifyFilter = watchfolder.NotifyFilter Or _
                               IO.NotifyFilters.Attributes

    ' add the handler to each event
    AddHandler watchfolder.Changed, AddressOf logchange
    AddHandler watchfolder.Created, AddressOf logchange
    AddHandler watchfolder.Deleted, AddressOf logchange

    ' add the rename handler as the signature is different
    AddHandler watchfolder.Renamed, AddressOf logrename

    'Set this property to true to start watching
    watchfolder.EnableRaisingEvents = True

    Button1.Enabled = False
    Button2.Enabled = True

    'End of code for btn_start_click
End Sub
Private Sub logchange(ByVal source As Object, ByVal e As  _
                    System.IO.FileSystemEventArgs)
    If e.ChangeType = IO.WatcherChangeTypes.Changed Then
        Dim writer As New IO.StreamWriter("log.txt")
        writer.WriteLine(Chr(13) & "File" + " " + e.FullPath + " " + "has been modified")
        writer.Close()
    End If
    If e.ChangeType = IO.WatcherChangeTypes.Created Then
        Dim writer As New IO.StreamWriter("log.txt")
        writer.WriteLine(Chr(13) & "File" + " " + e.FullPath + " " + "has been created")
        writer.Close()
    End If
    If e.ChangeType = IO.WatcherChangeTypes.Deleted Then
        Dim writer As New IO.StreamWriter("log.txt")
        writer.WriteLine(Chr(13) & "Filde" + " " + e.FullPath + " " + "has been deleted")
        writer.Close()
    End If
End Sub
Public Sub logrename(ByVal source As Object, ByVal e As  _
                        System.IO.RenamedEventArgs)
    Dim writer As New IO.StreamWriter("log.txt")
    writer.WriteLine(Chr(13) & "File" + " " + e.FullPath + "has been renamed to" + " " + e.Name)
    writer.Close()
End Sub

Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
    ' Stop watching the folder
    watchfolder.EnableRaisingEvents = False
    Button1.Enabled = True
    Button2.Enabled = False
End Sub

End Class

Answer 1

When you open your streamwriter, you are not telling it to append, so it overwrites:

Dim writer As New IO.StreamWriter("log.txt", True)

Also, you dont need a new stream for each activity:

Dim msg as string= Environment.NewLine & "File " & e.FullPath & " "
Select case e.ChangeType
      case IO.WatcherChangeTypes.Created 
         msg &= "has been created"

      case IO.WatcherChangeTypes.Deleted
         msg &= "has been deleted"
      ...etc
 End Select

 Dim writer As New IO.StreamWriter("log.txt", True)
 writer.WriteLine(msg)
 writer.Close()

..you could also leave the stream open until the watcher ends

You probably should exempt logging changes to log.txt, so test e.FullPath:

 If System.Io.Path.GetFileName(e.FullPath).ToLower = "log.text" Then Exit Sub

Answer 2

Now the program in working! Thank you MPelletier and Plutonix for the amazing help! Here is the complete code:

 Imports System.IO
 Imports System.Diagnostics
 Public Class Form1

Public watchfolder As FileSystemWatcher

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

    watchfolder = New System.IO.FileSystemWatcher()
    watchfolder.IncludeSubdirectories = True

    watchfolder.Path = TextBox1.Text



    watchfolder.NotifyFilter = IO.NotifyFilters.DirectoryName
    watchfolder.NotifyFilter = watchfolder.NotifyFilter Or _
                               IO.NotifyFilters.FileName
    watchfolder.NotifyFilter = watchfolder.NotifyFilter Or _
                               IO.NotifyFilters.Attributes


    AddHandler watchfolder.Changed, AddressOf logchange
    AddHandler watchfolder.Created, AddressOf logchange
    AddHandler watchfolder.Deleted, AddressOf logchange


    AddHandler watchfolder.Renamed, AddressOf logrename


    watchfolder.EnableRaisingEvents = True

    Button1.Enabled = False
    Button2.Enabled = True


End Sub
Private Sub logchange(ByVal source As Object, ByVal e As  _
                    System.IO.FileSystemEventArgs)
    If System.IO.Path.GetFileName(e.FullPath).ToLower = "log.txt" Then Exit Sub
    Dim msg As String = Environment.NewLine & "File " & e.FullPath & " "

    Select Case e.ChangeType
        Case IO.WatcherChangeTypes.Created
            msg &= "has been created" + "  " + "Time:" + " " + Format(TimeOfDay)

        Case IO.WatcherChangeTypes.Deleted
            msg &= "has been deleted" + "  " + "Time:" + " " + Format(TimeOfDay)

        Case IO.WatcherChangeTypes.Changed
            msg &= "has been modified" + "  " + "Time:" + " " + Format(TimeOfDay)

    End Select

    Dim writer As New IO.StreamWriter("log.txt", True)
    writer.WriteLine(msg)
    writer.Close()
End Sub
Public Sub logrename(ByVal source As Object, ByVal e As  _
                        System.IO.RenamedEventArgs)
    Select e.ChangeType
        Case IO.WatcherChangeTypes.Created
            Exit Sub
        Case IO.WatcherChangeTypes.Changed
            Exit Sub
        Case IO.WatcherChangeTypes.Deleted
            Exit Sub
        Case Else
            Dim msgrn As String = Environment.NewLine & "File " + e.OldName + " "
            msgrn &= "has been renamed to" + " " + e.Name + "  " + "Time:" + " " + Format(TimeOfDay)
            Dim writer As New IO.StreamWriter("log.txt", True)
            writer.WriteLine(msgrn)
            writer.Close()
    End Select

End Sub

Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click

    watchfolder.EnableRaisingEvents = False
    Button1.Enabled = True
    Button2.Enabled = False
End Sub

Private Sub Button3_Click(sender As System.Object, e As System.EventArgs) Handles Button3.Click
    Me.Hide()
    MsgBox("To close it later, don't open the program again, press CTRL+ALT+DELETE and press Start Task Manager or something like that, and go to processes and kill FolderMonitor.exe or what you have named the file", 0 + 64, "FolderMonitor")
End Sub
End Class