check the item in listbox - vb.net

I'm trying to do is check first the item in listbox if the value in textbox is already in listbox.

 Private Sub txtSS_PreviewKeydown(ByVal sender As Object, ByVal e As System.Windows.Forms.PreviewKeyDownEventArgs) Handles txtSS.PreviewKeyDown

    If e.KeyCode = Keys.Tab Then

        For Each a As String In String.Join(vbCrLf, ListBox1.Items.Cast(Of String))
            If txtSS.Text = a Then
                MsgBox("It's already barcoded!")
                txtSS.Text = ""
            End If
        Next

        If txtMS.Text = txtSS.Text Then
            MsgBox("This is already MAIN SERIAL! kindly check your barcoding serial", MsgBoxStyle.Exclamation)
            txtSS.Text = ""
            txtSS.Select()
        Else
            ListBox1.Items.Add(txtSS.Text)
            txtSS.Clear()
            txtSS.Select()
        End If
    End If

End Sub

But my code is not working.

the 'a' value of my for each is get only the first char of my listbox.

Think about this expression, used with the For Each loop:

String.Join(vbCrLf, ListBox1.Items.Cast(Of String))

The result of the expression is one string. When you use a string with a For Each loop, it doesn't go line by line. Rather, the loop runs for every character in the string, one character at a time, just as you saw. If you want each item in the ListBox, you don't need to use Join() at all:

For Each a As String In ListBox1.Items.Cast(Of String)()

Or I could refactor the whole method to be MUCH shorter and with less nesting:

Private Sub txtSS_PreviewKeydown(ByVal sender As Object, ByVal e As System.Windows.Forms.PreviewKeyDownEventArgs) Handles txtSS.PreviewKeyDown

    If e.KeyCode <> Keys.Tab Then Exit Sub

    If ListBox1.Items.Any(Function(a) txtSS.Text = DirectCast(a, String)) Then
        MsgBox("It's already barcoded!")
    Else If txtMS.Text = txtSS.Text Then
        MsgBox("This is already MAIN SERIAL! Kindly check your barcoding serial.", MsgBoxStyle.Exclamation)
    Else 
        ListBox1.Items.Add(txtSS.Text)
    End If

    txtSS.Clear()
    txtSS.Select()
End Sub

This also fixes another bug in the original code, where the "It's already barcoded!" result didn't exit the method. The next If check would still have run, where the empty string isn't likely to match the main serial number, and so the method would still likely add a blank line to the bottom of the ListBox. That will no longer happen.

While I'm here, I see you're looking at the Tab key and using bar codes. Most barcode readers by default will send a return key (Cr/Lf) at the end of the scanned text. You can add code to handle this, as well, and possibly make things much easier and faster for you users to do repeated scans.