VB.et FileSystemWatcher Event Handler - Calls of Handler Repeats (Unwanted) - vb.net

Question

I'm trying to log file activity in a directory with the FileSystemWatch class by adding the file to a custom class within a list, unfortunately, it seems that when copying a file or adding a new file to the target directory, it's running 4 times instead of once.
When adding a file to the directory, the AddressOf Sub will write 3 to 4 times instead of just once. I'm not sure why.
Here's the code snippet.
Public fileTypefilter As String = "*.xlsx"
Public DesDir As String = "C:\"
Public EventLog As List(Of FileEvent)
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim watchDir As String = DesDir
Dim watch As New FileSystemWatcher(watchDir)
Me.Text = "Monitoring " & watch.Path
watch.IncludeSubdirectories = false
watch.Filter = "*.xlsx"
AddHandler watch.Changed, AddressOf watch_Changed
watch.EnableRaisingEvents = True
End Sub
Private Sub watch_Changed(ByVal sender As Object, ByVal e As FileSystemEventArgs)
Try
EventLog.Add(New FileEvent(e.FullPath))
Exit Sub
Catch ex As NullReferenceException
EventLog = New List(Of FileEvent)({New FileEvent(e.FullPath)})
Exit Sub
End Try
End Sub

Answer 1

The Changed event handler is executed multiple times because multiple changes occur. As is ALWAYS the case, you should have read the relevant documentation for yourself first, which says:
The change of a file or folder. The types of changes include: changes to size, attributes, security settings, last write, and last access time.
If you don't want to log each individual change then you need to filter them somehow. For instance, you might want to record the time a change occurred for a particular path and then not log a change if some minimum time has not elapsed since the last change for the same path.
There would also be the option of using a HashSet instead of a List. That could be configured to use an appropriate equality comparer and then you could call Add as many times as you like for the same path and it will only be added once. That would mean that, were a set of changes to occur later for the same path, those would not be logged. I'm not sure whether that would be an issue for you or not.