The svcutil tool fails when one of the contracts implements IXmlSerializable

[lipchev]

Describe the bug
The "typeReuseMode": "All" option no longer works when one of the [DataMember]s of a [DataContract] is itself not a [DataContract] but instead implements the IXmlSerializable interface. This isn't the case in net472 (using the "Update Service Reference").

To Reproduce
I've created a simple repository in order to demonstrate the issue (tested with all versions up to the latest build version: 8.0.0-dev.24427.1).
The solution contains the following projects:

1. CommonTypes - this contians the "shared" data contracts (including the problematic CustomSerializableType contract). It is currently multi-targeting net472 and net8.0 but that's not a problem here.
2. SerializationTests - a simple test that demonstrates that the DataContractSerializer is capable of dealing with all of my CommonTypes
3. CoreWCF-Service - a .NET8 service hosting a vanilla IService returning the contracts from CommonTypes.
4. NetFramework-Client - a simple net472 client referencing the CommonTypes which demonstrates that the scenario used to work before.
5. CoreWCF-Client - a simple net8.0 client referencing the CommonTypes which demonstrates that the scenario doesn't work any more (tested using the latest build svcutil tool: 8.0.0-dev.24427.1, as well as the current version 2.2.0-preview1.23462.5)

The following json file

{
  "ExtendedData": {
    "inputs": [
      "https://localhost:7246/Service.svc"
    ],
    "collectionTypes": [
      "System.Array",
      "System.Collections.Generic.Dictionary`2"
    ],
    "namespaceMappings": [
      "*, ServiceReference1"
    ],
    "targetFramework": "net8.0",
    "typeReuseMode": "All"
  }
}

is passed to the svcutil using:
dotnet svcutil -u "Connected Services/ServiceReference1" -v Debug.

Here are the relevant parts of the WSDL:

<xs:schema elementFormDefault="qualified" targetNamespace="http://schemas.datacontract.org/2004/07/CommonTypes">
<xs:import schemaLocation="https://localhost:7246/Service.svc?xsd=xsd1" namespace="http://schemas.microsoft.com/2003/10/Serialization/"/>
<xs:complexType name="SomeSharedType">
<xs:annotation>
<xs:appinfo>
<IsValueType>true</IsValueType>
</xs:appinfo>
</xs:annotation>
<xs:sequence>
<xs:element minOccurs="0" name="SomeProperty" type="xs:int"/>
</xs:sequence>
</xs:complexType>
<xs:element name="SomeSharedType" nillable="true" type="tns:SomeSharedType"/>
<xs:complexType name="AnotherSharedType">
<xs:annotation>
<xs:appinfo>
<IsValueType>true</IsValueType>
</xs:appinfo>
</xs:annotation>
<xs:sequence>
<xs:element minOccurs="0" name="SomeProperty" type="xs:int"/>
<xs:element minOccurs="0" name="SomeXmlSerializableType" type="tns:CustomSerializableType"/>
</xs:sequence>
</xs:complexType>
<xs:element name="AnotherSharedType" nillable="true" type="tns:AnotherSharedType"/>
<xs:complexType name="CustomSerializableType">
<xs:annotation>
<xs:appinfo>
<IsValueType>true</IsValueType>
</xs:appinfo>
</xs:annotation>
<xs:sequence>
<xs:any namespace=""/>
</xs:sequence>
</xs:complexType>
<xs:element name="CustomSerializableType" nillable="true" type="tns:CustomSerializableType"/>
</xs:schema>

And here is the "problematic" type schema:

<xs:schema id="CustomSerializableTypeSchema">
<xs:simpleType name="BigIntegerString">
<xs:restriction base="xs:string"/>
</xs:simpleType>
</xs:schema>

The "xml" returned by this operation contract

    [OperationContract]
    AnotherSharedType GetComplexValue();

should contain something like this:

<AnotherSharedType xmlns="http://schemas.datacontract.org/2004/07/CommonTypes" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><SomeProperty>42</SomeProperty><SomeXmlSerializableType><BigIntegerString>42</BigIntegerString></SomeXmlSerializableType></AnotherSharedType>

Expected behavior
It should be possible to have one [DataContract] contain a [DataMember] that is itself not a [DataContract] but an IXmlSerializable (possibly [Serializable] as well, I'm not sure anymore), as used to be the case in .net framework.

Alternatively setting the "serializer": "DataContractSerializer" option explicitly should work, as long as the types are supported by the DataContractSerializer.

Additional context

If we removed the GetComplexValue operation from the interface, the type-reuse works once again (at least for the one remaining operation, which returns the simple contract).

[lipchev]

FYI, I've move on to using data-contract-surrogates so I'm no longer directly affected by this issue...