Format Multi-Font in System.Windows.Forms.RichTextBox with FlowDocument

Question

Hi,

I want to use Richtextbox to store text with format in SQL database. I love the way System.Windows.Forms.RichTextBox handle binding RTF format but i hate the way it change multi-font format. So I use flowdocument to handle multi-font format. Below is my code

Imports System.Windows.Forms

Imports System.Windows.Documents

Imports System.IO

Imports System.Text

.......

    Private Sub chkbBold_Click(sender As Object, e As EventArgs) Handles chkbBold.Click

            Try

                Dim SelStart As Integer = rtbOtherInfo.SelectionStart

                Dim SelLength As Integer = rtbOtherInfo.SelectionLength

                Dim fdTmp As FlowDocument = New FlowDocument()

                Dim inStream As MemoryStream = New MemoryStream(Encoding.ASCII.GetBytes(rtbOtherInfo.SelectedRtf))

                Dim outStream As New MemoryStream

                Dim textRangeTmp As TextRange = New TextRange(fdTmp.ContentStart, fdTmp.ContentEnd)

                textRangeTmp.Load(inStream, DataFormats.Rtf)

                'Toggle chkbBold state

                If chkbItalic.Checked = True Then

                    textRangeTmp.ApplyPropertyValue(TextElement.FontWeightProperty, Windows.FontWeights.Bold)

                Else

                    textRangeTmp.ApplyPropertyValue(TextElement.FontWeightProperty, Windows.FontWeights.Normal)

                End If

                textRangeTmp.Save(outStream, DataFormats.Rtf)

                rtbOtherInfo.SelectedRtf = System.Text.ASCIIEncoding.Default.GetString(outStream.ToArray())

                rtbOtherInfo.SelectionStart = SelStart

                rtbOtherInfo.SelectionLength = SelLength

                rtbOtherInfo.Focus()

            Catch ex As Exception

                MsgBox(ex.Message)

            End Try

    End Sub

But i see some schange behavior of richtextbox:

- in some selected text in richtextbox, this code work well, but in some case, there's nothing change in richtextbox after chkbBold_Click.

- some time I can set selected text to Bold but cannot set the selected text back to normal (with the same selected text)

Please kindly help to show me what im I wrong?

Please also kindly share you ideas.

Thank you all