DataGridViewで文字単位でフォントを変える （DataGridViewRichTextBoxColumnの実装）

Public Class DataGridViewRichTextBoxColumn
    Inherits DataGridViewColumn

    Public Sub New()
        MyBase.New(New DataGridViewRichTextBoxCell())
    End Sub

    Public Overrides Property CellTemplate() As System.Windows.Forms.DataGridViewCell
        Get
            Return MyBase.CellTemplate
        End Get
        Set(ByVal value As System.Windows.Forms.DataGridViewCell)
            If Not value Is Nothing AndAlso Not value.GetType.IsAssignableFrom(GetType(DataGridViewRichTextBoxCell)) Then
                Throw New InvalidCastException("Must be a DataGridViewRichTextBoxCell")
            End If
            MyBase.CellTemplate = value
        End Set
    End Property
End Class

Public Class DataGridViewRichTextBoxCell
    Inherits DataGridViewCell

    '描画処理
    Protected Overrides Sub Paint(ByVal graphics As System.Drawing.Graphics, _
        ByVal clipBounds As System.Drawing.Rectangle, _
        ByVal cellBounds As System.Drawing.Rectangle, _
        ByVal rowIndex As Integer, _
        ByVal cellState As System.Windows.Forms.DataGridViewElementStates, _
        ByVal value As Object, _
        ByVal formattedValue As Object, _
        ByVal errorText As String, _
        ByVal cellStyle As System.Windows.Forms.DataGridViewCellStyle, _
        ByVal advancedBorderStyle As System.Windows.Forms.DataGridViewAdvancedBorderStyle, _
        ByVal paintParts As System.Windows.Forms.DataGridViewPaintParts)

        '背景の描画
        If (paintParts And DataGridViewPaintParts.Background) = _
            DataGridViewPaintParts.Background Then

            Dim cellBackground As New SolidBrush(cellStyle.BackColor)
            graphics.FillRectangle(cellBackground, cellBounds)
            cellBackground.Dispose()
        End If

        '境界線の描画
        If (paintParts And DataGridViewPaintParts.Border) = _
            DataGridViewPaintParts.Border Then

            PaintBorder(graphics, clipBounds, cellBounds, cellStyle, _
                advancedBorderStyle)
        End If

        '描画エリアを計算
        Dim drawArea As Rectangle = cellBounds
        Dim drawAdjustment As Rectangle = _
            Me.BorderWidths(advancedBorderStyle)
        drawArea.X += drawAdjustment.X
        drawArea.Y += drawAdjustment.Y
        drawArea.Height -= drawAdjustment.Height
        drawArea.Width -= drawAdjustment.Width

        '文字列に変換
        Dim strValue As String = TryCast(value, String)
        If strValue Is Nothing Then
            strValue = String.Empty
        End If

        'こんな風にvalue値（Rtf）をイメージに変換できれば
        Dim img As Image = GetImageFromRtf(strValue)

        '内容を描画
        graphics.DrawImage(img, 0, 0)

    End Sub

    '編集モード開始時、EditingControlのRtfプロパティにセルのValueをコピーする
    Public Overrides Sub InitializeEditingControl(ByVal rowIndex As Integer, ByVal initialFormattedValue As Object, ByVal dataGridViewCellStyle As System.Windows.Forms.DataGridViewCellStyle)
        MyBase.InitializeEditingControl(rowIndex, initialFormattedValue, dataGridViewCellStyle)
        Dim ctrl As RichTextBoxEditingControl = CType(DataGridView.EditingControl, RichTextBoxEditingControl)
        ctrl.Rtf = CType(Me.Value, String)
    End Sub

    '編集モードが終了して、EditingControlが削除される前にRtfプロパティの値をセルのValueにコピーする
    Public Overrides Sub DetachEditingControl()
        Dim ctrl As RichTextBoxEditingControl = CType(DataGridView.EditingControl, RichTextBoxEditingControl)
        Me.Value = ctrl.Rtf
        MyBase.DetachEditingControl()
    End Sub

    Public Overrides Property ValueType() As System.Type
        Get
            Return GetType(String)
        End Get
        Set(ByVal value As System.Type)
            MyBase.ValueType = value
        End Set
    End Property

    Public Overrides ReadOnly Property EditType() As System.Type
        Get
            Return GetType(RichTextBoxEditingControl)
        End Get
    End Property

    'RTF形式の文字列からイメージを作成
    Private Function GetImageFromRtf(ByVal rtf As String) As Image
        'ここの実装が思いつきません
    End Function

End Class

Public Class RichTextBoxEditingControl
    Inherits RichTextBox
    Implements IDataGridViewEditingControl

    Private _dataGridView As DataGridView
    Private _rowIndex As Integer
    Private _valueChanged As Boolean = False
    Public Sub ApplyCellStyleToEditingControl(ByVal dataGridViewCellStyle As System.Windows.Forms.DataGridViewCellStyle) Implements System.Windows.Forms.IDataGridViewEditingControl.ApplyCellStyleToEditingControl
        Me.Font = dataGridViewCellStyle.Font
        Me.ForeColor = dataGridViewCellStyle.ForeColor
        Me.BackColor = dataGridViewCellStyle.BackColor
    End Sub

    Public Property EditingControlDataGridView() As System.Windows.Forms.DataGridView Implements System.Windows.Forms.IDataGridViewEditingControl.EditingControlDataGridView
        Get
            Return _dataGridView
        End Get
        Set(ByVal value As System.Windows.Forms.DataGridView)
            _dataGridView = value
        End Set
    End Property

    'TextではなくRtfをValueとして扱う
    Public Property EditingControlFormattedValue() As Object Implements System.Windows.Forms.IDataGridViewEditingControl.EditingControlFormattedValue
        Get
            Return Me.Rtf
        End Get
        Set(ByVal value As Object)
            If TypeOf value Is [String] Then
                Me.Rtf = value.ToString
            End If
        End Set
    End Property

    Public Property EditingControlRowIndex() As Integer Implements System.Windows.Forms.IDataGridViewEditingControl.EditingControlRowIndex
        Get
            Return _rowIndex
        End Get
        Set(ByVal value As Integer)
            _rowIndex = value
        End Set
    End Property

    Public Property EditingControlValueChanged() As Boolean Implements System.Windows.Forms.IDataGridViewEditingControl.EditingControlValueChanged
        Get
            Return _valueChanged
        End Get
        Set(ByVal value As Boolean)
            _valueChanged = value
        End Set
    End Property

    Public Function EditingControlWantsInputKey(ByVal keyData As System.Windows.Forms.Keys, ByVal dataGridViewWantsInputKey As Boolean) As Boolean Implements System.Windows.Forms.IDataGridViewEditingControl.EditingControlWantsInputKey
        Return True
    End Function

    Public ReadOnly Property EditingPanelCursor() As System.Windows.Forms.Cursor Implements System.Windows.Forms.IDataGridViewEditingControl.EditingPanelCursor
        Get
            Return MyBase.Cursor
        End Get
    End Property

    Public Function GetEditingControlFormattedValue(ByVal context As System.Windows.Forms.DataGridViewDataErrorContexts) As Object Implements System.Windows.Forms.IDataGridViewEditingControl.GetEditingControlFormattedValue
        Return Me.EditingControlFormattedValue
    End Function

    Public Sub PrepareEditingControlForEdit(ByVal selectAll As Boolean) Implements System.Windows.Forms.IDataGridViewEditingControl.PrepareEditingControlForEdit

    End Sub

    Public ReadOnly Property RepositionEditingControlOnValueChange() As Boolean Implements System.Windows.Forms.IDataGridViewEditingControl.RepositionEditingControlOnValueChange
        Get
            Return False
        End Get
    End Property

End Class

If paintParts And DataGridViewPaintParts.Background = DataGridViewPaintParts.Background Then
If paintParts And DataGridViewPaintParts.Border = DataGridViewPaintParts.Border Then