Search and highlight text in Silverlight RichTextBox control

<TextBox Name="txtFind" Grid.Column="0" Grid.Row="0" Width="100" Height="20" HorizontalAlignment="Right" SelectionChanged="txtFind_SelectionChanged"></TextBox>
<Button Name="btnFind" HorizontalAlignment="Right" Grid.Column="1" Grid.Row="0" Content="Search" Width="70" Height="20" Click="btnFind_Click"    VerticalAlignment="Center"></Button>
<RichTextBox x:Name="rtb"  Height="295"  VerticalAlignment="Top"   BorderBrush="{x:Null}"  AcceptsReturn="True"   TextInputStart="rtb_TextInputStart"   TextWrapping="Wrap"  FontSize="12" DataContext="{Binding}" Cursor="Arrow" Foreground="Black"  /> 

 
 ''' <summary>
    ''' This method will search the text in Richtextbox control and highlight it with Red color
    ''' </summary>
    ''' <remarks></remarks>
     Private Sub btnFind_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
        ' First select entire content of text control
        rtb.SelectAll()

        Dim runTemp As Run
        Dim pointerStart As TextPointer = Nothing
        Dim pointerEnd As TextPointer = Nothing
        Dim i As Integer = 0
        Dim length As Integer = 0
        Dim InlineCnt As Integer = 0
        Dim TotalBlocksCount As Integer = 0
        Dim FindOccurance As Boolean = False
        Dim TotalOccurance As Integer = 0
        ' calculate how many times serach text appear in line
        TotalOccurance = UBound(Split(UCase(rtb.Selection.Text.ToUpper.ToString()), UCase(Me.txtFind.Text.Trim.ToUpper.ToString())))

        If TotalOccurance <> 0 Then
            Do
                For Each block As Paragraph In rtb.Blocks
                    If (TotalBlocksCount = rtb.Blocks.Count And TotalOccurance = 1) Then
                        TotalBlocksCount = 0
                        InlineCount = 0
                        BlockCount = 0
                        MessageBox.Show("No Further Match Found.")
                        Exit Do
                    End If
                    TotalBlocksCount = TotalBlocksCount + 1
                    For Each inline In block.Inlines
                        If (inline.GetType() Is GetType(Run)) Then
                            runTemp = New Run()
                            runTemp = DirectCast(inline, Run)
                            Dim brush1 As New SolidColorBrush(Colors.Black)
                            rtb.Selection.ApplyPropertyValue(Run.ForegroundProperty, brush1)
                            If (Not runTemp.Text.ToUpper.Equals(String.Empty)) Then
                                If runTemp.Text.ToUpper.Contains(Me.txtFind.Text.Trim.ToUpper.ToString()) Then
                                    If InlineCnt = InlineCount Then
                                        If BlockCount = 0 Then
                                            i = InStr(runTemp.Text.ToUpper.ToString(), Me.txtFind.Text.Trim.ToUpper.ToString())
                                        Else
                                            i = InStr(BlockCount, runTemp.Text.ToUpper.ToString(), Me.txtFind.Text.Trim.ToUpper.ToString())
                                        End If
                                        FindCount = i
                                        length = Len(Me.txtFind.Text.ToUpper.ToString())
                                        BlockCount = i + length
                                        InlineCnt = InlineCnt + 1
                                        If i <> 0 Then
                                            pointerStart = runTemp.ElementStart
                                            pointerEnd = runTemp.ElementStart
                                            pointerStart = pointerStart.GetPositionAtOffset(i, LogicalDirection.Forward)
                                            pointerEnd = pointerEnd.GetPositionAtOffset((i + length), LogicalDirection.Backward)
                                            rtb.Selection.Select(pointerStart, pointerEnd)
                                            Dim brush As New SolidColorBrush(Colors.Red)
                                            rtb.Selection.ApplyPropertyValue(Run.ForegroundProperty, brush)
                                            rtb.Focus()
                                            FindOccurance = True
                                            Exit Do
                                        Else
                                            BlockCount = 1
                                            InlineCount = InlineCount + 1
                                        End If
                                    Else
                                        InlineCnt = InlineCnt + 1
                                    End If
                                End If
                            End If
                        End If
                    Next
                Next
            Loop
        Else
            MessageBox.Show("No Match Found.")
        End If
    End Sub