Multiple threads in a for-loop using a parameterized function(x, y, z) - vb.net

Question

I have a list which contains folder ID's and folder paths. I would like to pass some of these folders to a function which zips them. What I want is to have three threads run in parallel and zip three different paths at a time. What happens now is each thread waits until the next one has finished in order to process the next. Any ideas?
Dim SelectedRange = From folders In listFolders Where folders.FolderID >= 150101
For Each item In SelectedRange
Dim t As New Thread(
Sub()
Me.BeginInvoke(DirectCast(Sub() ZipFolder(sInclearDestination, item.FolderID.ToString, item.FolderPath), MethodInvoker))
End Sub)
t.Start()
t.Join()
Next
Public Function ZipFolder(ByVal sFolderPathDestination As String, ByVal folderID As String, ByVal folderPath As String) As Boolean
Try
Using zip = New Ionic.Zip.ZipFile()
'If the zip file does not exist then get the folder and zip it to the destination
If Not File.Exists(Path.Combine(sFolderPathDestination, folderID & ".zip")) Then
zip.AddDirectory(folderPath)
zip.Save(Path.Combine(sFolderPathDestination, CType(folderID, String) & ".zip"))
Return True
Else
Logging.Log("Aborting zipping: " & Path.Combine(sFolderPathDestination, folderID & ".zip") & ". The zip file already exists!")
Return False
End If
End Using
Catch ex As Exception
Logging.Log("Error in zipping: " & Path.Combine(sFolderPathDestination, folderID & ".zip") & " Error: " & ex.Message)
Return False
End Try
End Function

Answer 1

There are two problems with your code.
The first problem is the call to Me.BeginInvoke. Presumably you are creating a WinForm application and Me is a reference to the current Form. The Form.BeginInvoke (inherited from the base Control class) causes the given delegate to be executed on the UI thread. So, all you are doing is creating three separate threads which all immediately invoke back to the UI thread to do all of their work. You obviously can't do that and still expect the tasks to be processed in parallel. You need to remove the call to BeginInvoke. If you need to call BeginInvoke in-order to update the display of some data on the form, you need to do it as late as possible and do as little work as possible within that UI-invoked code so that the majority of the work is still being done in the worker threads.
The second problem is the call to Thread.Join. You are calling Join inside your For loop right after starting the thread. That means that it will sit there and wait, at that call to Join, until the worker thread is complete. Therefore, your loop waits for each thread to complete before starting the next one, in essence, making it single threaded. You should just remove the call to Join. If you need the calling method to wait for all the threads to complete, just wait to call Join on the threads until all of them have been started (i.e. after the For loop).