Class can't be used in WCF service, while derived class can - vb.net

Question

I have an abstract class FileFolderBase with 2 classes deriving from it File and Folder, the class ProjectFolder is again deriving from the Folder class.
I have a WCF service, called BrowseService with 2 functions: GetRoot, which returns a list of ProjectFolder instances, and OpenFolder, which returns one Folder instance.
Here's the service code
iBrowseService.vb:
Imports System.ServiceModel
<ServiceContract()>
Public Interface IBrowseService
<OperationContract()>
Function OpenFolder(ByVal path As String) As Domain.Folder
<OperationContract()>
Function GetRoot(ByVal projectCodes As String) As List(Of Domain.ProjectFolder)
End Interface
BrowseService.svc.vb:
Imports System.ServiceModel.Activation
<AspNetCompatibilityRequirements(RequirementsMode:=AspNetCompatibilityRequirementsMode.Allowed)>
Public Class BrowseService
Implements IBrowseService
Public Function OpenFolder(ByVal path As String) As Domain.Folder Implements IBrowseService.OpenFolder
Dim service As MyService = New MyService()
Dim folder As Domain.Folder = service.OpenFolder(path)
Return folder
End Function
Public Function GetRoot(ByVal projectCodes As String) As List(Of Domain.ProjectFolder) Implements IBrowseService.GetRoot
Dim service As MyService = New MyService()
Dim folders As List(Of Domain.ProjectFolder)
If Not String.IsNullOrEmpty(projectCodes) Then
Dim codes As List(Of String) = projectCodes.Split(",").ToList
folders = service.GetRoot(codes)
Else
folders = service.GetRoot(Nothing)
End If
Return folders
End Function
End Class
Nothing overly complicated there, now the odd thing is, the GetRoot function works perfectly and returns a set of ProjectFolder instances when I call it. The OpenFolder function however, won't work at all. The WCF test client gives a 'This operation is not supported in the WCF Test Client cause it uses the type Folder' message and when I use the service in an application, I get a CommunicationsException, most inner exception message 'An existing connection was forcibly closed by the remote host', when invoking it.
I can't find any cause why ProjectFolder can be used, while Folder can't, especially cause ProjectFolder derives from Folder.
Here are the sources for the 4 type classes:
FileFolderBase
Imports System.Runtime.Serialization
<Serializable()>
<DataContract(IsReference:=True)>
Public MustInherit Class FileFolderBase
Implements IComparable(Of FileFolderBase)
#Region "Attributes"
Protected varPkey As Long
Protected varName As String
Protected varFullName As String
Protected varProjectFolder As ProjectFolder
Protected varLastModified As Date
#End Region
#Region "Properties"
<DataMember>
Public Property Pkey() As Long
Get
Return varPkey
End Get
Set(value As Long)
varPkey = value
End Set
End Property
<DataMember>
Public Property Name() As String
Get
Return varName
End Get
Set(ByVal value As String)
varName = value
End Set
End Property
<DataMember>
Public Property FullName() As String
Get
Return varFullName
End Get
Set(ByVal value As String)
varFullName = value
End Set
End Property
<DataMember>
Public Property ProjectFolder() As ProjectFolder
Get
Return varProjectFolder
End Get
Set(value As ProjectFolder)
varProjectFolder = value
End Set
End Property
<DataMember>
Public Property LastModified() As Date
Get
Return Me.varLastModified
End Get
Set(ByVal value As Date)
Me.varLastModified = value
End Set
End Property
#End Region
#Region "Methods"
'Equals function and = and <> operators
#End Region
#Region "CompareMethods"
'Comparison functions, used for sorting and stuff
#End Region
End Class
File
Imports System.Runtime.Serialization
<Serializable()>
<DataContract(IsReference:=True)>
Public Class File
Inherits FileFolderBase
#Region "Attributes"
Private varSize As Long
Private varComment As String
#End Region
#Region "Constructors"
Public Sub New()
Me.varPkey = 0
End Sub
Public Sub New(ByVal pkey As Long, ByVal name As String, ByVal fullName As String, ByVal lastModified As Date, ByVal size As Long, ByVal comment As String, ByVal project As ProjectFolder)
Me.varPkey = pkey
Me.varName = name
Me.varFullName = fullName
Me.varLastModified = lastModified
Me.varSize = size
Me.varComment = comment
Me.varProjectFolder = project
End Sub
#End Region
#Region "Properties"
<DataMember>
Public Property Size() As Long
Get
Return Me.varSize
End Get
Set(ByVal value As Long)
Me.varSize = value
End Set
End Property
<DataMember>
Public Property Comment() As String
Get
Return Me.varComment
End Get
Set(ByVal value As String)
Me.varComment = value
End Set
End Property
#End Region
#Region "Methods"
'Equals function and = and <> operators
#End Region
#Region "CompareMethods"
'Comparison functions, used for sorting and stuff
#End Region
End Class
Folder
Imports System.Runtime.Serialization
<Serializable()>
<DataContract(IsReference:=True)>
Public Class Folder
Inherits FileFolderBase
#Region "Attributes"
Protected varItems As List(Of FileFolderBase)
Protected varParent As Folder
#End Region
#Region "Constructors"
Public Sub New()
Me.varPkey = 0
End Sub
Public Sub New(ByVal pkey As Long, ByVal name As String, ByVal fullName As String, ByVal lastModified As Date, ByVal parent As Folder, ByVal project As ProjectFolder)
Me.varPkey = pkey
Me.varName = name
Me.varFullName = fullName
Me.varLastModified = lastModified
Me.varParent = parent
Me.varProjectFolder = project
varItems = New List(Of FileFolderBase)()
End Sub
#End Region
#Region "Properties"
<DataMember>
Public Property Items() As List(Of FileFolderBase)
Get
Return Me.varItems
End Get
Set(ByVal value As List(Of FileFolderBase))
Me.varItems = value
End Set
End Property
<DataMember>
Public Property Parent() As Folder
Get
Return varParent
End Get
Set(ByVal value As Folder)
Me.varParent = value
End Set
End Property
#End Region
#Region "Methods"
'Equals function and = and <> operators
#End Region
End Class
ProjectFolder
Imports System.Runtime.Serialization
<Serializable()>
<DataContract(IsReference:=True)>
Public Class ProjectFolder
Inherits Folder
#Region "Attributes"
Private varProject As Project
#End Region
#Region "Constructors"
Public Sub New()
MyBase.New()
End Sub
Public Sub New(ByVal project As Project, ByVal parent As Folder)
MyBase.New(0, project.Name, project.Name, Date.Now, parent, Nothing)
Me.varProject = project
End Sub
#End Region
#Region "Properties"
<DataMember>
Public Property Project() As Project
Get
Return Me.varProject
End Get
Set(ByVal value As Project)
Me.varProject = value
End Set
End Property
<DataMember>
Public Overloads Property Items() As List(Of FileFolderBase)
Get
Return Me.varItems
End Get
Set(ByVal value As List(Of FileFolderBase))
Me.varItems = value
End Set
End Property
<DataMember>
Public Overloads Property ProjectFolder() As ProjectFolder
Get
Return Me
End Get
Set(value As ProjectFolder)
End Set
End Property
#End Region
#Region "Methods"
'Equals function and = and <> operators
#End Region
End Class
Please help, cause I have no clue how this can be possible.