Nullable datetime picker

I have for the first time used datetimepicker control in my project. Although it is a great control I found it useless because of two things that are related:

• Default value cannot be set to nothing (null in C#).
• Datetimepicker canot display null (nothing in VB) values.

When I checked the msdn there was a caution notice saying just that.

I checked the codeproject website to look for an alternative. I found this control that suited my needs I only changed it to recognize the DateTime.MinValue to be nothing and set default to nothing. What I liked about it that it was the extended control, so it kept all the nice things I liked about default control. Here it is:

Imports System
Imports System.ComponentModel
Imports System.Globalization
Imports System.Runtime.InteropServices
Imports System.Threading
Imports System.Windows.Forms

' Copyright (c) 2005 Claudio Grazioli, http://www.grazioli.ch
'
' This code is free software; you can redistribute it and/or modify it.
' However, this header must remain intact and unchanged.  Additional
' information may be appended after this header.  Publications based on
' this code must also include an appropriate reference.
' 
' This code is distributed in the hope that it will be useful, but 
' WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 
' or FITNESS FOR A PARTICULAR PURPOSE.
'

''' <summary>
''' Represents a Windows date time picker control. It enhances the .NET standard <b>DateTimePicker</b>
''' control with the possibility to show empty values (null values).
''' </summary>
<ComVisible(False)> _
Public Class DateTimePickerCtrl
  Inherits System.Windows.Forms.DateTimePicker
#Region "Member variables"
  ' true, when no date shall be displayed (empty DateTimePicker)
  Private _isNull As Boolean

  ' If _isNull = true, this value is shown in the DTP
  Private _nullValue As String

  ' The format of the DateTimePicker control
  Private _format As DateTimePickerFormat = DateTimePickerFormat.[Long]

  ' The custom format of the DateTimePicker control
  Private _customFormat As String

  ' The format of the DateTimePicker control as string
  Private _formatAsString As String
#End Region

#Region "Constructor"
  ''' <summary>
  ''' Default Constructor
  ''' </summary>
  Public Sub New()
    MyBase.New()
    MyBase.Format = DateTimePickerFormat.[Custom]
    NullValue = " "
    Format = DateTimePickerFormat.[Long]
  End Sub
#End Region

#Region "Public properties"

  ''' <summary>
  ''' Gets or sets the date/time value assigned to the control.
  ''' </summary>
  ''' <value>The DateTime value assigned to the control
  ''' </value>
  ''' <remarks>
  ''' <p>If the <b>Value</b> property has not been changed in code or by the user, it is set
  ''' to the current date and time (<see cref="DateTime.Now"/>).</p>
  ''' <p>If <b>Value</b> is <b>null</b>, the DateTimePicker shows 
  ''' <see cref="NullValue"/>.</p>
  ''' </remarks>
  <Bindable(True)> _
  <Browsable(False)> _
  Public Shadows Property Value() As [Object]
    Get
      If _isNull Then
        Return Nothing
      Else
        Return DateTime.MinValue
      End If
    End Get
    Set(ByVal value As [Object])
      If value Is Nothing OrElse value Is DBNull.Value OrElse (value.GetType() Is GetType(DateTime) And CDate(value) = DateTime.MinValue) OrElse (CDate(value) = New Date(CType(0, Long))) Then
        SetToNullValue()
      Else
        SetToDateTimeValue()
        MyBase.Value = DirectCast(value, DateTime)
      End If
    End Set
  End Property

  ''' <summary>
  ''' Gets or sets the format of the date and time displayed in the control.
  ''' </summary>
  ''' <value>One of the <see cref="DateTimePickerFormat"/> values. The default is 
  ''' <see cref="DateTimePickerFormat.Long"/>.</value>
  <Browsable(True)> _
  <DefaultValue(DateTimePickerFormat.[Long]), TypeConverter(GetType([Enum]))> _
  Public Shadows Property Format() As DateTimePickerFormat
    Get
      Return _format
    End Get
    Set(ByVal value As DateTimePickerFormat)
      _format = value
      If Not _isNull Then
        SetFormat()
      End If
      OnFormatChanged(EventArgs.Empty)
    End Set
  End Property

  ''' <summary>
  ''' Gets or sets the custom date/time format string.
  ''' <value>A string that represents the custom date/time format. The default is a null
  ''' reference (<b>Nothing</b> in Visual Basic).</value>
  ''' </summary>
  Public Shadows Property CustomFormat() As [String]
    Get
      Return _customFormat
    End Get
    Set(ByVal value As [String])
      _customFormat = value
    End Set
  End Property

  ''' <summary>
  ''' Gets or sets the string value that is assigned to the control as null value. 
  ''' </summary>
  ''' <value>The string value assigned to the control as null value.</value>
  ''' <remarks>
  ''' If the <see cref="Value"/> is <b>null</b>, <b>NullValue</b> is
  ''' shown in the <b>DateTimePicker</b> control.
  ''' </remarks>
  <Browsable(True)> _
  <Category("Behavior")> _
  <Description("The string used to display null values in the control")> _
  <DefaultValue(" ")> _
  Public Property NullValue() As [String]
    Get
      Return _nullValue
    End Get
    Set(ByVal value As [String])
      _nullValue = value
    End Set
  End Property
#End Region

#Region "Private methods/properties"
  ''' <summary>
  ''' Stores the current format of the DateTimePicker as string. 
  ''' </summary>
  Private Property FormatAsString() As String
    Get
      Return _formatAsString
    End Get
    Set(ByVal value As String)
      _formatAsString = value
      MyBase.CustomFormat = value
    End Set
  End Property

  ''' <summary>
  ''' Sets the format according to the current DateTimePickerFormat.
  ''' </summary>
  Private Sub SetFormat()
    Dim ci As CultureInfo = Thread.CurrentThread.CurrentCulture
    Dim dtf As DateTimeFormatInfo = ci.DateTimeFormat
    Select Case _format
      Case DateTimePickerFormat.[Long]
        FormatAsString = dtf.LongDatePattern
        Exit Select
      Case DateTimePickerFormat.[Short]
        FormatAsString = dtf.ShortDatePattern
        Exit Select
      Case DateTimePickerFormat.Time
        FormatAsString = dtf.ShortTimePattern
        Exit Select
      Case DateTimePickerFormat.[Custom]
        FormatAsString = Me.CustomFormat
        Exit Select
    End Select
  End Sub

  ''' <summary>
  ''' Sets the <b>DateTimePicker</b> to the value of the <see cref="NullValue"/> property.
  ''' </summary>
  Private Sub SetToNullValue()
    _isNull = True
    MyBase.CustomFormat = If((_nullValue Is Nothing OrElse _nullValue = [String].Empty), " ", "'" & _nullValue & "'")
  End Sub

  ''' <summary>
  ''' Sets the <b>DateTimePicker</b> back to a non null value.
  ''' </summary>
  Private Sub SetToDateTimeValue()
    If _isNull Then
      SetFormat()
      _isNull = False
      MyBase.OnValueChanged(New EventArgs())
    End If
  End Sub
#End Region

#Region "Events"
  ''' <summary>
  ''' This member overrides <see cref="Control.WndProc"/>.
  ''' </summary>
  ''' <param name="m"></param>
  Protected Overloads Overrides Sub WndProc(ByRef m As Message)
    If _isNull Then
      If m.Msg = &H4E Then
        ' WM_NOTIFY
        Dim nm As NMHDR = DirectCast(m.GetLParam(GetType(NMHDR)), NMHDR)
        If nm.Code = -746 OrElse nm.Code = -722 Then
          ' DTN_CLOSEUP || DTN_?
          SetToDateTimeValue()
        End If
      End If
    End If
    MyBase.WndProc(m)
  End Sub

  <StructLayout(LayoutKind.Sequential)> _
  Private Structure NMHDR
    Public HwndFrom As IntPtr
    Public IdFrom As Integer
    Public Code As Integer
  End Structure

  ''' <summary>
  ''' This member overrides <see cref="Control.OnKeyDown"/>.
  ''' </summary>
  ''' <param name="e"></param>
  Protected Overloads Overrides Sub OnKeyUp(ByVal e As KeyEventArgs)
    If e.KeyCode = Keys.Delete Then
      Me.Value = Nothing
      OnValueChanged(EventArgs.Empty)
    End If
    MyBase.OnKeyUp(e)
  End Sub

  Protected Overloads Overrides Sub OnValueChanged(ByVal eventargs As EventArgs)
    MyBase.OnValueChanged(eventargs)
  End Sub

#End Region
End Class