Requesting source code from a webpage with Visual Basic, and returning matched text in a ListBox, using regular expressions - error message

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VB

Visual-Studio

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I have written the following code, which is supposed to request source code from a webpage with Visual Basic, and return matched text in a ListBox, using regular expressions. But I am getting the following error message:

"An unhandled exception of type 'System.ArgumentException' occurred in System.Windows.Forms.dll

Additional information: Items collection cannot be modified when the DataSource property is set."

It relates to this line of code:

ListBox1.Items.Add(itemcode.Value.Split("><").GetValue(0))

The entire code is as follows:

Imports System.Text.RegularExpressions
Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim request As System.Net.HttpWebRequest = System.Net.HttpWebRequest.Create("http://oxford.craigslist.co.uk/search/hhh")
        Dim response As System.Net.HttpWebResponse = request.GetResponse

        Dim sr As System.IO.StreamReader = New System.IO.StreamReader(response.GetResponseStream())

        Dim rssourcecode As String = sr.ReadToEnd
        Dim r As New System.Text.RegularExpressions.Regex("class=""hdrlnk"">.*?<")
        Dim matches As MatchCollection = r.Matches(rssourcecode)

        For Each itemcode As Match In matches
            ListBox1.Items.Add(itemcode.Value.Split("><").GetValue(0))

        Next


    End Sub
End Class

As suggestions as to what may be going wrong here is much appreciated. (Of course, it could be that the regular expression does not match - suggestions for how to quickly test regular expressions against text, would also be much appreciated)