Threading doesn't complete the task before ending the thread loop - vb.net

Question

So I did something similar a while ago, but this is essentially a username checker for a (specific) website, they can load in usernames via text file and it will put it into a list box, now I have the start button and it's meant to check each username. Before, however, it froze the program when they checked, but it worked. I tried making it "threaded" so it didn't freeze.
The problem now is that it doesn't check them all, and finishes instantly.
CODE:
Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
Button6.Enabled = False
Dim goodUsers As New SaveFileDialog()
goodUsers.Filter = "TXT file (*.txt)|*.txt"
Dim flag As Boolean
Dim Incomplete As Integer = 0
Dim Taken As Integer = 0
Dim sb As New StringBuilder
If goodUsers.ShowDialog() = DialogResult.OK Then
Dim checkerMT As New Thread(
Sub()
For Each i As String In UsernameList.Items
WebRequest.Create("http://yatobooter.cf/other/checkusr.php?username=" + i.ToString)
Dim cResult As String = New System.Net.WebClient().DownloadString("http://yatobooter.cf/other/checkusr.php?username=" + i.ToString).ToString
If cResult = "taken" Then
flag = False
ElseIf cResult = "nottaken" Then
flag = True
End If
If flag = True Then
sb.Append(i & vbNewLine)
Else
Incomplete = Incomplete + 1
Taken = UsernameList.Items.Count - Incomplete
End If
Next
End Sub
)
checkerMT.Start()
Try
File.WriteAllText(goodUsers.FileName, sb.ToString)
Catch ex As Exception
Exit Sub
End Try
End If
MessageBox.Show("Checking available usernames, complete!", "NameSniper Pro")
Button6.Enabled = True
End Sub

Answer 1

You cannot access UI elements (UsernameList.Items) from a thread other than the UI. Instead add a background worker to your form to handle the basic threading stuff (progress reporting, finish reporting, exception handling). Pass into this an object that contains the settings your job needs to do its work without interacting with the ui.
Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
Button6.Enabled = False
Dim goodUsers As New SaveFileDialog()
goodUsers.Filter = "TXT file (*.txt)|*.txt"
If goodUsers.ShowDialog() = DialogResult.OK Then
'Note: You'll need to add the filenames
BackgroundWorker1.RunWorkerAsync(New State() With {.Names = {}, .FileName = goodUsers.FileName})
End If
End Sub
Class State
Public Names As List(Of String)
Public StringBuilder As New System.Text.StringBuilder
Public Incomplete As Integer
Public Taken As Integer
Public FileName As String
End Class
Private Sub BackgroundWorker1_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
Dim state = CType(e.Argument, State)
For Each i As String In state.Names
Using cli = New System.Net.WebClient()
Dim cResult = cli.DownloadString("http://yatobooter.cf/other/checkusr.php?username=" + i.ToString).ToString
If cResult = "nottaken" Then
state.StringBuilder.Append(i & vbNewLine)
Else
state.Incomplete = state.Incomplete + 1
state.Taken = state.Names.Count - state.Incomplete
End If
End Using
Next
IO.File.WriteAllText(state.FileName, state.StringBuilder.ToString)
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
If e.Error IsNot Nothing Then
MessageBox.Show(e.Error.ToString(), "Error")
Else
MessageBox.Show("Checking available usernames, complete!", "NameSniper Pro")
End If
Button6.Enabled = True
End Sub

Answer 2

The SaveFileDialog can be used as "Using directive"
Why do you create a webrequest on the one hand, and on the other hand use a webclient? That does not make sense at all.
Instead of your strange if condition, write:
flag = (cResult.Equals("nottaken"))
All code you want to run after the actions you are currently running in your thread have to be in your thread as well, because it is async.
You have to invoke if you use user controls within a thread
and so on..
Please turn Option Strict On and Option Infer Off
There are lots of other things you could do better.
Please remove the webrequest and webclient combination yourself, that does not make sense at all.
Take a look at this, I cleared it a little bit:
Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
Dim Incomplete As Integer = 0
Dim Taken As Integer = 0
Dim sb As New StringBuilder
Using goodUsers As SaveFileDialog = new SaveFileDialog()
goodUsers.Filter = "TXT file (*.txt)|*.txt"
If not goodUsers.ShowDialog() = DialogResult.OK Then Exit Sub
Dim checkerMT As New Thread(
Sub()
Me.invoke(sub()
Button6.Enabled = False
For Each i As String In UsernameList.Items
WebRequest.Create("http://yatobooter.cf/other/checkusr.php?username=" + i.ToString)
Dim cResult As String = New System.Net.WebClient().DownloadString("http://yatobooter.cf/other/checkusr.php?username=" + i.ToString).ToString
If (cResult.toLower().Equals("nottaken")) Then
sb.Append(String.Concat(i , Environment.NewLine)
Else
Incomplete += 1
Taken = UsernameList.Items.Count - Incomplete
End If
Next
File.WriteAllText(goodUsers.FileName, sb.ToString)
Button6.Enabled = True
MessageBox.Show("Checking available usernames, complete!", "NameSniper Pro")
End Sub)
End Sub
)
checkerMT.IsBackground = True;
checkerMT.Start()
End Sub