Why does XmlSerializer fail to serialize this class even though I added the XmlInclude attribute for-CodePudding

Problem

I am trying to XML serialize the class "ProfileSerialize" but I get this inner exception when calling xsSubmit.Serialize(writer, obj) in the Serialize function shown below.

Exception

Message = "The type ProfileFormatNumber was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically."

What I already tried

I tried to implement the solutions mentioned in these threads:

Reproducible example

Imports System
Imports System.Collections.Generic
Imports System.Collections
Imports System.IO
Imports System.Text
Imports System.Linq
Imports System.Reflection
Imports System.Xml.Serialization
Imports System.XML

' https://stackoverflow.com/questions/72856211/why-does-xmlserializer-fail-to-serialize-this-class-even-though-i-added-the-xmli

<XmlInclude(GetType(ProfileFormatNumber))>
Public Class ProfileFormat
    <XmlElement(ElementName:="e1")>
    Public Property Name As String = String.Empty      
End Class

Public Class ProfileFormatNumber
    Inherits ProfileFormat

    <XmlElement(ElementName:="e1")>
    Public Property Divider As Integer = 1
End Class

Public Class Serializer(Of T)
    Public Shared Function Serialize(ByVal obj As T) As String

            Dim xsSubmit As XmlSerializer = New XmlSerializer(GetType(T))

            Using sww = New StringWriter()

                Using writer As XmlTextWriter = New XmlTextWriter(sww) With {.Formatting = Formatting.Indented}
                    xsSubmit.Serialize(writer, obj)
                    Return sww.ToString() ' I would recommend moving this out of the inner Using statement to guarantee the XmlWriter is flushed and closed
                End Using

            End Using

    End Function
End Class
                
Public Module Module1
    Public Sub Main()
        Dim profileFormat = New ProfileFormatNumber With { .Name = "my name", .Divider = 111 }

        Dim xml2 = Serializer(Of ProfileFormat).Serialize(profileFormat)
        Console.WriteLine(xml2)

    End Sub
End Module

My question

How do I need to modify my code to correctly use the <XmlInclude(GetType())> attribute? I tried adding it in multiple places but always receive the same exception.

CodePudding user response：

Your problem is not with XmlInclude. Your problem is that you are trying to assign the two properties of ProfileFormatNumber, Divider and (from the base class) Name, to the same element name <e1> by setting <XmlElement(ElementName:="e1")> on both. I.e. if were able to serialize your ProfileFormatNumber as-is, the XML would look like:

<ProfileFormat xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xsi:type="ProfileFormatNumber">
  <e1>my name</e1>
  <e1>111</e1>
</ProfileFormat>

However XmlSerializer does not support this (perhaps because there would be ambiguity in deserialization), hence the (slightly inscrutable) error complaining that the element name e1 is already present:

The XML element 'e1' from namespace '' is already present in the current scope. Use XML attributes to specify another XML name or namespace for the element.]

Instead, use a different obfuscated element name for ProfileFormatNumber.Divider such as <f1>:

<XmlInclude(GetType(ProfileFormatNumber))>
Public Class ProfileFormat
    <XmlElement(ElementName:="e1")>
    Public Property Name As String = String.Empty      
End Class

Public Class ProfileFormatNumber
    Inherits ProfileFormat

    <XmlElement(ElementName:="f1")>
    Public Property Divider As Integer = 1
End Class

Demo fiddle here.