Polymorphic XML Serialized Array

[Guest]

Hello Folks

I am trying to create an xml file from a class containing a
polymorphic array. The example: AutoCompany contains an array of
Vehicles(base class) which can either be Cars or Trucks(derived classes).
Everything works accept the XMLSerialization. The error is very vague. The
simplified code follows and the entire code can be downloaded from a
compressed file from http://www.Cartner.Net/PolymorphismExample.zip

Please let me know what is causing this. I have not found any
examples on the internet that address this specific problem. Thank you for
your help in advance.

Best Regards
Todd

-------------------------------------------------------------------
Option Explicit On
Option Strict On

Imports System.IO
Imports System.Xml.Serialization

Public Class frmMain
Inherits System.Windows.Forms.Form

Private GeneralMotors As clsAutoCompany

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
GeneralMotors.ToString()
End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button2.Click
Dim XSer As XmlSerializer = New XmlSerializer(GetType(clsAutoCompany))
Dim FIO As TextWriter = New StreamWriter("PolyTest.xml")

' ********** Error occurs on the next line **********
XSer.Serialize(FIO, Me.GeneralMotors)
FIO.Close()
End Sub
End Class
-------------------------------------------------------------------
Option Explicit On
Option Strict On

<Serializable()> Public Class clsAutoCompany
Public Vehicles() As clsVehicle

Public Sub New()
Dim VT As clsTruck
Dim VC As clsCar
ReDim Vehicles(3)
VT = New clsTruck
VT.Index = 0
VT.TruckType = "SUV"
Vehicles(0) = VT
VC = New clsCar
VC.Index = 1
VC.CarType = "Sports"
Vehicles(1) = VC
VC = New clsCar
VC.Index = 2
VC.CarType = "Luxury"
Vehicles(2) = VC
VT = New clsTruck
VT.Index = 3
VT.TruckType = "4X4"
Vehicles(3) = VT
End Sub

Public Overrides Function ToString() As String
Dim Counter As Int32
For Counter = 0 To Vehicles.Length - 1
Debug.WriteLine(Vehicles(Counter).ToString & " - " &
Vehicles(Counter).Index.ToString)
Next
End Function
End Class
-------------------------------------------------------------------
Option Explicit On
Option Strict On

<Serializable()> Public MustInherit Class clsVehicle
Public Index As Int32

Public Sub New()
Index = -1
End Sub

Public MustOverride Shadows Function ToString() As String
End Class

<Serializable()> Public Class clsCar
Inherits clsVehicle

Public CarType As String

Public Overrides Function ToString() As String
Dim Output As String
Output = "Car Type = " & CarType
Return Output
End Function
End Class

<Serializable()> Public Class clsTruck
Inherits clsVehicle

Public TruckType As String

Public Overrides Function ToString() As String
Dim Output As String
Output = "Truck Type = " & TruckType
Return Output
End Function
End Class

 

[Samuel R. Neff]

Would have helped if you posted the error message, even if it is
vague. With XmlSerialization the errors usually have InnerExceptions
(often nested several levels deep) and the inner-most exception is
what provides the real information about what's wrong.

From looking at your code, the problem is most likely related to:

1. clsVehicle is mustinherit. Not sure you can xml serialize a class
with this attribute set. It goes against the public constructor
requirement, but perhaps if there are no actual vehicle instances it
won't cause any problems (and there can't be since the class is
abstract).

2. Need XmlInclude attributes to indicate the sub-classes that should
also be included in the serialization type mapping.

3. Not that it will cause any errors, but note that Serializable
attribute has nothing to do with XmlSerialization--it's related to
binary serialization.

4. Not that it will cause any errors, but those prefixes are very old
school (clsVehicle should just be Vehicle). Of course you can do
whatever you want and name things however you want, but the standard
practice with VB.NET is to not use prefixes.

HTH,

Sam



B-Line is now hiring one Washington D.C. area VB.NET
developer for WinForms + WebServices position.
Seaking mid to senior level developer. For
information or to apply e-mail resume to
sam_blinex_com.

 

[Samuel R. Neff]

As I said, you need the last InnerException. Catch the error, walk
through the InnerExceptions, and you'll get more info.

Did you implement the items I mentioned in my previous post and did
they not address the problem?

Sam

Here is the incredibly helpful and descriptive error message...

B-Line is now hiring one Washington D.C. area VB.NET
developer for WinForms + WebServices position.
Seaking mid to senior level developer. For
information or to apply e-mail resume to
sam_blinex_com.