Recursively searching for file in task freezes form on Mono - vb.net

Question

I am working on a file browser in VB.Net that will be run on Ubuntu on the Mono framework. Everything was going fine up until I decided to implement the search function. I have it set up so that the search runs in a new task, and the user can cancel it from the form. This works fine on Windows, but when run on Mono, I get weird results:
Sometimes the form freezes (can still be dragged around, but everything inside is unresponsive)
Sometimes a few search results come up, and then the form freezes
Since the form is frozen, I cannot click the cancel button and have to force quit
There are no error messages or exceptions, so I have no idea what is going wrong.
Sometimes the form artifacts if dragged across screen, even though in theory the search code is running in a separate task
I have tried inserting 'Application.DoEvents()' throughout, but that didn't help. I even tried running the code on the UI thread without a task, but that obviously just causes everything to freeze.
Here is the code:
The Search() method is called through a textbox, when it is called a button to cancel is displayed, and if clicked calls tokenSource2.Cancel()
Dim tokenSource2 As New CancellationTokenSource()
Dim ct As CancellationToken = tokenSource2.Token
Private Sub Search(ByVal txt As String, ByVal dir As String)
CancelSearch()
tokenSource2 = New CancellationTokenSource()
ct = tokenSource2.Token
pnl_cancelsearch.Show()
Dim t As Task = Task.Factory.StartNew(Sub()
If ct.IsCancellationRequested Then
Exit Sub
End If
ListView1.Clear()
Dim iscasesensitive As Boolean = ConfigManager.SearchIsCaseSensitive
If Not searchhistory.Contains(txt) Then
searchhistory.Add(txt)
combo_search.Items.Add(txt)
End If
If ct.IsCancellationRequested Then
Exit Sub
End If
If dir = "" Then
For Each item As String In Directory.GetLogicalDrives
If ct.IsCancellationRequested Then
Exit Sub
End If
SearchRec(txt, item, iscasesensitive)
Next
Else
SearchRec(txt, dir)
End If
pnl_cancelsearch.Hide()
End Sub)
End Sub
Private Sub SearchRec(ByVal txt As String, ByVal rootdir As String, Optional ByVal casesensitive As Boolean = True)
For Each item As String In Directory.GetFiles(rootdir)
If ct.IsCancellationRequested Then
Exit Sub
End If
If casesensitive Then
If item.Contains(txt) Then
AddItem(item)
End If
Else
If item.ToLower.Contains(txt.ToLower) Then
AddItem(item)
End If
End If
Next
For Each item As String In Directory.GetDirectories(rootdir)
If ct.IsCancellationRequested Then
Exit Sub
End If
SearchRec(txt, item)
Next
End Sub
How do I fix this? What am I doing wrong? It works perfectly fine on Windows, but not on Mono.

Answer 1

You have here a problem: you're modifying the UI and you want your task to be executed as a new thread (because you're not using await so there's no shared time if it's being excuted on the main thread).
If you force the task to create a new thread (and for that you must explicitly set TaskScheduler.Default as the used task scheduler) then the UI code will lead to a cross threading exception. And if you don't force the task to be run on a new thread then it is executed on the main thread blocking it.
Mono Forms implementation is poor at a minimum, so its a lot slower than on Windows, it can perfectly being that on Windows you don't notice the slowness because it's fast enough and on another OS using Mono it's slower and you notice the UI update.
First of all, if you can, avoid using Forms on Mono, use GTK# or another UI kit.
Also, sepparate the UI logic from the data logic, create a task which retrieves all the data (using a task or the thread pool) and when you got it then update the UI, my bet is you will still find it slow because the UI update.

Answer 2

The solution, thanks to another user, was to use BeginInvoke when updating the UI, which worked perfectly.