VB.net multithreading for loop, parallel threads - vb.net

Question

I have a simple form with 2 RichTextBoxes and 1 button, the code grabs the url address from RichTextBox1 and phrases the page for the title field using regex and appends it to RichTextBox2. I want to multithread everything in such way that none of the url's are skipped and the thread numbers can be set ( according to the system free resources ) For example, let's say 10 threads to run in parallel. I searched everything and the best that I managed to do is run everything in a background worker and keep the GUI from freezing while working. A short code sample will be of much help, I am a beginner in VB.net.
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
For i = 0 To RichTextBox1.Lines.Length - 1
If RichTextBox1.Lines(i).Contains("http://") Then
Dim html As String = New System.Net.WebClient() _
.DownloadString(RichTextBox1.Lines(i))
Dim pattern As String = "(?<=\<title\>)([^<]+?)(?=\</title\>)"
Dim match As System.Text.RegularExpressions.Match = _
System.Text.RegularExpressions.Regex.Match(html, pattern)
Dim title As String = match.Value
RichTextBox2.AppendText(title & vbCrLf)
End If
Next
End Sub
End Class
Updated code ( throwing "Index was outside the bounds of the array." errors. )
Imports System
Imports System.Threading
Public Class Form1
Public Sub test(ByVal val1 As String, ByVal val2 As String)
Dim zrow As String
zrow = RichTextBox1.Lines(val1)
If zrow.Contains("http://") Then
Dim html As String = New System.Net.WebClient().DownloadString(zrow)
Dim pattern As String = "(?<=\<title\>)([^<]+?)(?=\</title\>)"
Dim match As System.Text.RegularExpressions.Match = System.Text.RegularExpressions.Regex.Match(html, pattern)
Dim title As String = match.Value
RichTextBox2.AppendText(val2 & title & vbCrLf)
End If
End Sub
Public Sub lastfor(ByVal number)
Dim start As Integer = number - 100
For x = start To number - 1
Try
test(x, x)
RichTextBox2.AppendText(x & RichTextBox1.Lines(x).Trim & vbCrLf)
Catch ex As Exception
'MsgBox(ex.Message)
RichTextBox3.AppendText(ex.Message & vbCrLf & vbCrLf)
End Try
Next
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Control.CheckForIllegalCrossThreadCalls = False
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim TotalLines As String = RichTextBox1.Lines.Length - 1
Dim TotalThreads As Integer = 10
Dim LinesPerThread As Integer = TotalLines / TotalThreads
Dim increment As String = LinesPerThread
Dim zdata(TotalThreads) As String
For i = 0 To TotalThreads - 1
zdata(i) = increment
increment = increment + LinesPerThread
Next
Dim lst As New List(Of Threading.Thread)
For Each bump As String In zdata
Dim t As New Threading.Thread(Function(l As String)
'Do something with l
'Update GUI like this:
If bump = String.Empty Or bump Is Nothing Then
Else
lastfor(l)
'MsgBox(l)
End If
End Function)
lst.Add(t)
t.Start(bump)
Next
'test(1)
End Sub
End Class

Answer 1

There are two ways two achieve this:
First, if you are using .NET 4.0, you could use a Parallel.ForEach loop:
Parallel.ForEach(RichTextBox1.Lines, Function(line As String)
' Do something here
' To update the GUI use:
Me.Invoke(Sub()
' Update GUI like this...
End Sub)
Return Nothing
End Function)
The other way is to do this manually (and you will have slightly more control):
Dim lst As New List(Of Threading.Thread)
For Each line In RichTextBox1.Lines
Dim t As New Threading.Thread(Function(l As String)
'Do something with l
'Update GUI like this:
Me.Invoke(Sub()
'Update Gui...
End Sub)
End Function)
lst.Add(t)
t.Start(line)
Next
Both of these are very crude, but will get the job done.
EDIT:
Here is a sample code that will control the number of threads:
Dim lst As New List(Of Threading.Thread)
Dim n As Integer = 1 ' Number of threads.
Dim npl As Integer = RichTextBox1.Lines / n
Dim seg As New List(Of String)
For Each line In RichTextBox1.Lines
For i = npl - n To npl
seg.Add(RichTextBox1.Lines.Item(i))
Next
Dim t As New Threading.Thread(Function(l As String())
For Each lin In l
' TO-DO...
Next
'Do something with l
'Update GUI like this:
Me.Invoke(Sub()
'Update Gui...
End Sub)
End Function)
lst.Add(t)
t.Start(seg.ToArray())
Next
*The above code might have bugs.