Private m_Font As Font = New Font("arial", 8.0!, FontStyle.Regular, _
           GraphicsUnit.Point)

Public Property [Font]() As Font
   Get
       Return m_Font
   End Get
   Set(ByVal Value As Font)
       m_Font = Value
   End Set
End Property

Private Function ShouldSerializeFont() As Boolean
   Return Not (m_Font.SizeInPoints = 8.0! AndAlso _
           m_Font.Style = FontStyle.Regular AndAlso _
           m_Font.Name.ToLower = "arial")
End Function

Private Sub ResetFont()
   m_Font = New Font("arial", 8.0!, FontStyle.Regular, _
           GraphicsUnit.Point)
End Sub

In C#, I would do something like this.

private static readonly Font DefaultFontValue = new Font("Arial", 8.0F,
FontStyle.Regular);
private Font font = null;

[AmbientValueAttribute(null)]
public override Font Font
{
 get
 {
   if (font == null)
   {
     return DefaultFontValue;
   }
   return font;
 }
 set
 {
   if (font != value)
   {
     font = value;
     this.Invalidate();
   }
 }
}

protected virtual bool ShouldSerializeFont()
{
 return (font != null);
}