I was chatting with Ken Getz the other day about the RichTextBox. Ken pointed out to me that the SelectionFont can sometimes return Nothing.  The documentation isn’t specific on when exactly this occurs, but a quick bit of disassembly and Win32 Api cross referencing, and the case is pretty clear.  It occurs when the selection has multiple font faces in it.  The problem is the font type can’t represent that. In fact you can’t have a faceless Font which is probably what you’d have to have.  This is classic example of the “leaky abstraction” 😉

So latter on this was bothering me a bit.  Imagine you have a toolbar that displays whether or not the current selection is Bold.  If your selection spans font faces, you can’t get that information.  And what’s worse is you can’t set the entire selection to Bold.  These are simple operations we expect in any word processor.

So what I ended up doing was to extend the RichTextbox and give it properties such as SelectionFontBold. These properties are a tri-state, Off, On, and Indeterminate. The Indeterminate state is for when different parts of the selection have differing Bold state for example.
To use the property you might write code like:
 
     If Me.RichTextBox1.SelectionFontBold = RichTextBoxEx.FontStyleState.On Then
         Me.RichTextBox1.SelectionFontBold = RichTextBoxEx.FontStyleState.Off
      Else
         Me.RichTextBox1.SelectionFontBold = RichTextBoxEx.FontStyleState.On
      End If

Oh, and you can happily cast the FontStyleState to a CheckState.

Anyway, here’s the hack code. Enjoy 🙂

Imports System.Runtime.InteropServices
Imports System.ComponentModel

Public Class RichTextBoxEx
   Inherits RichTextBox

   <Browsable(False), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
   Public Property SelectionFontBold() As FontStyleState
      Get
         Return GetSelectionFontStyle(FontStyle.Bold)
      End Get
      Set(ByVal value As FontStyleState)
         If value = FontStyleState.Indeterminate Then Return
         SetSelectionFontStyle(FontStyle.Bold, (value = FontStyleState.On))
      End Set
   End Property

   <Browsable(False), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
     Public Property SelectionFontItalic() As FontStyleState
      Get
         Return GetSelectionFontStyle(FontStyle.Italic)
      End Get
      Set(ByVal value As FontStyleState)
         If value = FontStyleState.Indeterminate Then Return
         SetSelectionFontStyle(FontStyle.Italic, (value = FontStyleState.On))
      End Set
   End Property

   <Browsable(False), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
       Public Property SelectionFontUnderline() As FontStyleState
      Get
         Return GetSelectionFontStyle(FontStyle.Underline)
      End Get
      Set(ByVal value As FontStyleState)
         If value = FontStyleState.Indeterminate Then Return
         SetSelectionFontStyle(FontStyle.Underline, (value = FontStyleState.On))
      End Set
   End Property

   <Browsable(False), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)> _
    Public Property SelectionFontStrikeOut() As FontStyleState
      Get
         Return GetSelectionFontStyle(FontStyle.Strikeout)
      End Get
      Set(ByVal value As FontStyleState)
         If value = FontStyleState.Indeterminate Then Return
         SetSelectionFontStyle(FontStyle.Strikeout, (value = FontStyleState.On))
      End Set
   End Property

   Public Enum FontStyleState
      Off = 0
      [On] = 1
      Indeterminate = 2
   End Enum

#Region “win32 API”

   Private Sub SetSelectionFontStyle(ByVal style As FontStyle, ByVal apply As Boolean)
      Dim charInfo As CHARFORMAT = New CHARFORMAT
      charInfo.dwMask = style
      If apply Then charInfo.dwEffects = style
      SendMessage(New HandleRef(Me, Me.Handle), EM_SETCHARFORMAT, SCF_SELECTION, charInfo)
   End Sub

   Private Function GetSelectionFontStyle(ByVal style As FontStyle) As FontStyleState
      Dim charInfo As New CHARFORMAT
      charInfo.dwMask = style
      SendMessage(New HandleRef(Me, Me.Handle), EM_GETCHARFORMAT, SCF_SELECTION, charInfo)
      If (charInfo.dwMask And style) = style Then
         If (charInfo.dwEffects And style) = style Then
            Return FontStyleState.On
         Else
            Return FontStyleState.Off
         End If
      Else
         Return FontStyleState.Indeterminate
      End If
   End Function

   <StructLayout(LayoutKind.Sequential, Pack:=4)> _
   Private Class CHARFORMAT
      Public cbSize As Integer
      Public dwMask As Integer
      Public dwEffects As Integer
      Public yHeight As Integer
      Public yOffset As Integer
      Public crTextColor As Integer
      Public bCharSet As Byte
      Public bPitchAndFamily As Byte
      <MarshalAs(UnmanagedType.ByValArray, SizeConst:=32)> _
      Public szFaceName As Byte()

      Public Sub New()
         Me.cbSize = Marshal.SizeOf(GetType(CHARFORMAT))
         ReDim Me.szFaceName(31)
      End Sub
   End Class

   Private Declare Auto Function SendMessage Lib “user32” (ByVal hWnd As HandleRef, ByVal msg As Int32, ByVal wParam As Int32, <[In](), Out()> ByVal lParam As CHARFORMAT) As IntPtr

   Private Const WM_USER As Int32 = &H400
   Private Const EM_GETCHARFORMAT As Int32 = (WM_USER + 58)
   Private Const EM_SETCHARFORMAT As Int32 = (WM_USER + 68)
   Private Const SCF_SELECTION As Int32 = 1

#End Region

End Class