How to read the pasted text line by line in RichTextBox?

Hai all,

I am using a RichTextbox to read its content line by line.Here is the code I used for that:

 Private Sub txt_TextChanged(ByVal sender As System.Object, ByVal e As System.Windows.Controls.TextChangedEventArgs) Handles txt.TextChanged
        Dim line As [String]
        Dim trimmedline As String
        Dim rtbContents As String = New TextRange(txt.Document.ContentStart, txt.Document.ContentEnd).Text
        For Each line In rtbContents.Split(ControlChars.Lf)
            trimmedline = line.Trim(" c", ControlChars.Cr)
            If trimmedLine.Length > 0 Then
                lb.Content = trimmedLine.First
            End If
        Next
End Sub

It works fine when I write the text directly in the textbox but if I paste a few lines in it,the label only shows the last line.I don't know how to fix it.Can anyone please help me solve this problem?