DatagridView RTFColumn funktioniert noch nicht ganz

Question

Halo zusammen,

ich habe versucht aus dem Beispiel für eine CalenderColumn, siehe hier:

https://msdn.microsoft.com/en-us/library/7tas5c80.aspx?f=255&MSPPError=-2147217396

eine Klasse für eine RTFColumn für das DatagridView zu erstellen. Beim Test zeigt sich, dass in eine Zelle RTF-Text eingegeben werden kann über das EditingControl, da sieht man deutlich den formatierten Text. Verlässt man dann allerdings diese Zelle, wird nicht mehr der formatierte Text angezeigt, sondern der RTF-Code.
Ich poste mal den Klassen-Code und vielleicht kann mir jemand sozusagen auf die Sprünge helfen, dass der RTF-Text auch in der Zelle angezeigt wird, denn das ist ja das eigentliche Ziel.
Hier der Code:

Public Class RTFColumn
	Inherits DataGridViewColumn

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

	Public Overrides Property CellTemplate() As DataGridViewCell
		Get
			Return MyBase.CellTemplate
		End Get
		Set(ByVal value As DataGridViewCell)
			' Ensure that the cell used for the template is a RTFCell.
			If (value IsNot Nothing) AndAlso
				Not value.GetType().IsAssignableFrom(GetType(RTFCell)) Then
				Throw New InvalidCastException("Must be a RTFCell.")
			End If
			MyBase.CellTemplate = value
		End Set
	End Property

End Class

Public Class RTFCell
	Inherits DataGridViewTextBoxCell

	Public Sub New()
	End Sub

	Public Overrides Sub InitializeEditingControl(ByVal rowIndex As Integer,
		ByVal initialFormattedValue As Object,
		ByVal dataGridViewCellStyle As DataGridViewCellStyle)
		' Set the value of the editing control to the current cell value.
		MyBase.InitializeEditingControl(rowIndex, initialFormattedValue, dataGridViewCellStyle)
		Dim ctl As RTFEditingControl =
			CType(DataGridView.EditingControl, RTFEditingControl)
		' Use the default row value when Value property is null.
		If (Me.Value Is Nothing) Then
			ctl.Rtf = CType(Me.DefaultNewRowValue, String)
		Else
			ctl.Rtf = CType(Me.Value, String)
		End If
	End Sub

	Public Overrides ReadOnly Property EditType() As Type
		Get
			' Return the type of the editing control that RTFCell uses.
			Return GetType(RTFEditingControl)
		End Get
	End Property

	Public Overrides ReadOnly Property ValueType() As Type
		Get
			' Return the type of the value that RTFCell contains.
			Return GetType(String)
		End Get
	End Property

	Public Overrides ReadOnly Property DefaultNewRowValue() As Object
		Get
			Return ""
		End Get
	End Property

End Class

Class RTFEditingControl
	Inherits RichTextBox
	Implements IDataGridViewEditingControl

	Private dataGridViewControl As DataGridView
	Private valueIsChanged As Boolean = False
	Private rowIndexNum As Integer

	Public Sub New()
	End Sub

	Public Property EditingControlFormattedValue() As Object _
		Implements IDataGridViewEditingControl.EditingControlFormattedValue
		Get
			Return Me.Rtf
		End Get
		Set(ByVal value As Object)
			Try
				' This will throw an exception of the string is 
				' null, empty, or not in the format of a date.
				Me.Rtf = Rtf
			Catch
				' In the case of an exception, just use the default
				' value so we're not left with a null value.
				Me.Rtf = ""
			End Try
		End Set
	End Property

	Public Function GetEditingControlFormattedValue(ByVal context _
		As DataGridViewDataErrorContexts) As Object _
		Implements IDataGridViewEditingControl.GetEditingControlFormattedValue
		Return Me.Rtf
	End Function

	Public Sub ApplyCellStyleToEditingControl(ByVal dataGridViewCellStyle As _
		DataGridViewCellStyle) _
		Implements IDataGridViewEditingControl.ApplyCellStyleToEditingControl
		'Me.Font = dataGridViewCellStyle.Font
		'Me.ForeColor = dataGridViewCellStyle.ForeColor
		'Me.Backgroundcolor = dataGridViewCellStyle.BackColor
	End Sub

	Public Property EditingControlRowIndex() As Integer _
		Implements IDataGridViewEditingControl.EditingControlRowIndex
		Get
			Return rowIndexNum
		End Get
		Set(ByVal value As Integer)
			rowIndexNum = value
		End Set
	End Property

	Public Function EditingControlWantsInputKey(ByVal key As Keys,
		ByVal dataGridViewWantsInputKey As Boolean) As Boolean _
		Implements IDataGridViewEditingControl.EditingControlWantsInputKey
		' Let the cell handle the keys listed.
		Select Case key And Keys.KeyCode
			Case Keys.Left, Keys.Up, Keys.Down, Keys.Right,
				Keys.Home, Keys.End, Keys.PageDown, Keys.PageUp
				Return True
			Case Else
				Return Not dataGridViewWantsInputKey
		End Select
	End Function

	Public Sub PrepareEditingControlForEdit(ByVal selectAll As Boolean) _
		Implements IDataGridViewEditingControl.PrepareEditingControlForEdit
		' No preparation needs to be done.
	End Sub

	Public ReadOnly Property RepositionEditingControlOnValueChange() _
		As Boolean Implements _
		IDataGridViewEditingControl.RepositionEditingControlOnValueChange
		Get
			Return False
		End Get
	End Property

	Public Property EditingControlDataGridView() As DataGridView _
		Implements IDataGridViewEditingControl.EditingControlDataGridView
		Get
			Return dataGridViewControl
		End Get
		Set(ByVal value As DataGridView)
			dataGridViewControl = value
		End Set
	End Property

	Public Property EditingControlValueChanged() As Boolean _
		Implements IDataGridViewEditingControl.EditingControlValueChanged
		Get
			Return valueIsChanged
		End Get
		Set(ByVal value As Boolean)
			valueIsChanged = value
		End Set
	End Property

	Public ReadOnly Property EditingControlCursor() As Cursor _
		Implements IDataGridViewEditingControl.EditingPanelCursor
		Get
			Return MyBase.Cursor
		End Get
	End Property

	Protected Overrides Sub OntextChanged(ByVal eventargs As EventArgs)
		' Notify the DataGridView that the contents of the cell have changed.
		valueIsChanged = True
		Me.EditingControlDataGridView.NotifyCurrentCellDirty(True)
		MyBase.OnTextChanged(eventargs)
	End Sub

End Class

Danke im Voraus für Ideen...
Grüße-

---

Dietrich