Box value types implementing IXmlSerializable in XmlSerializer generated IL

[daeghanelkin]

Value types implementing the IXmlSerializable interface were not being property boxed when they declared a parameterless constructor. This caused the XmlSerializer to fail to deserialize them with an InvalidProgramException exception when the generated IL tried to call the XmlSerializationReader.ReadSerializable method with the value.

Fixes #99613

---

I would like to note that I am not really that familiar with IL generation, so all corrections are welcome ☺️.

I took the C# output of the serializer produced by the Microsoft.XmlSerializer.Generator tool for a different types and simplified it down to determine what the IL should look like for the call to the XmlSerializationReader.ReadSerializable method.

My simplified snippet was the following:

public class Class {
    public void Method() {
        Read(new StructWithParameterlessConstructor());

        Read(new StructWithParameterlessConstructor(), true);
        
        Read(new StructWithoutParameterlessConstructor());

        Read(new StructWithoutParameterlessConstructor(), true);
    }
    
    private static void Read(IInterface @interface) { }
    
    private static void Read(IInterface @interface, bool b) { }
}

public interface IInterface;

public struct StructWithParameterlessConstructor : IInterface
{
    public StructWithParameterlessConstructor() { }
}

public struct StructWithoutParameterlessConstructor : IInterface;

The following is the resulting IL for each method:

C#	IL
Read(new StructWithParameterlessConstructor())	newobj instance void StructWithParameterlessConstructor::.ctor() box StructWithParameterlessConstructor call void Class::Read(class IInterface)
Read(new StructWithParameterlessConstructor(), true)	newobj instance void StructWithParameterlessConstructor::.ctor() box StructWithParameterlessConstructor ldc.i4.1 call void Class::Read(class IInterface)
Read(new StructWithoutParameterlessConstructor())	ldloca.s 0 initobj StructWithoutParameterlessConstructor ldloc.0 box StructWithoutParameterlessConstructor call void Class::Read(class IInterface)
Read(new StructWithoutParameterlessConstructor(), true)	ldloca.s 0 initobj StructWithoutParameterlessConstructor ldloc.0 box StructWithoutParameterlessConstructor ldc.i4.1 call void Class::Read(class IInterface)

No boxing was being written by the IL generator for the value types with parameterless constructors when the XML serializer was generating the runtime assembly, so a simple addition of that boxing seems to resolve the issue.
Value types without parameterless constructors do not run into this issue because they fall into the sm.TypeDesc.CannotNew condition above the new condition I've added, and avoid the style of instantiation that's demonstrated in my simplified example.

[daeghanelkin]

@dotnet-policy-service agree

[Copilot]

Pull Request Overview

This PR fixes a missing boxing step for value types implementing IXmlSerializable during IL generation in XmlSerializationReader, preventing runtime errors when deserializing such structs. It also adds test types and unit tests to verify correct behavior for structs with and without parameterless constructors.

• Added a new branch in the IL generator to convert (box) value types to IXmlSerializable.
• Introduced two struct types in tests to exercise both cases (with and without parameterless constructor).
• Added corresponding unit tests to validate serialization and deserialization of those structs.

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.cs	Added two IXmlSerializable structs (with and without parameterless ctor)
src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs	Added tests for serializing/deserializing the new structs
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReaderILGen.cs	Updated IL gen to box value types implementing IXmlSerializable

[daeghanelkin]

@filipnavara Thanks for the explicit label! I also notice that there's an entry called area-Serialization which also includes System.Xml.XmlSerializer, should that label also be added here or is that a more legacy label than the one you've already attached?

[filipnavara]

area-Serialization recently changed ownership, but it can be more appropriate. cc @StephenMolloy