Microsoft.XmlSerializer.Generator generated serializers are not used in runtime

[skrecc]

Hello,

I have a very simple project testing that pre-generated serializers by Microsoft.XmlSerializer.Generator are used during runtime.

Issue:
Pre-generated serializers are not used during runtime

Current behavior:
Dynamically generated System.Xml.Serialization.XmlSerializer from System.Private.Xml assembly is used instead

Test performed for type:

[XmlRoot]
public class XmlRootAttributedClass
{
    public XmlRootAttributedClass()
    {
    }

    public string? XmlContent { get; set; }
}

Serializers assembly XMLSerializerGeneratorTests.XmlSerializers.dll is created correctly in output directory and contains XmlRootAttributedClassSerializer

Output log of testing app:

Loaded: System.Collections.NonGeneric, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Loaded: System.Xml.ReaderWriter, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Resolving: XMLSerializerGeneratorTests.XmlSerializers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Loaded: XMLSerializerGeneratorTests.XmlSerializers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Loaded: System.Text.RegularExpressions, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
SGEN Serializer implementation assembly: System.Private.Xml loaded for serialized type name: XMLSerializerGeneratorTests.XmlRootAttributedClass. Serializer type name: System.Xml.Serialization.XmlSerializer
SGEN Location: 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\9.0.10\System.Private.Xml.dll'  IsDynamic: False
SGEN Using runtime-generated (reflection) serializer implementation.

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<TargetFramework>net9.0-windows</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
	</PropertyGroup>
	<ItemGroup>
		<PackageReference Include="Microsoft.XmlSerializer.Generator" Version="9.0.10" />
	</ItemGroup>
	<PropertyGroup>
		<SGenVerbose>true</SGenVerbose>
		<OutputType>Exe</OutputType>
	</PropertyGroup>  

</Project>

public static class Program
{
    [STAThread]
    static void Main(string[] args)
    {
        AssemblyLoadContext.Default.Resolving += (ctx, name) =>
        {
            Console.WriteLine($"Resolving: {name}");
            return null;
        };

        AppDomain.CurrentDomain.AssemblyLoad += (_, e) =>
        {
            Console.WriteLine($"Loaded: {e.LoadedAssembly.FullName}");
        };

        var serializedTypeName = typeof(XmlRootAttributedClass);
        var serializer = new XmlSerializer(serializedTypeName);

        var serializerType = serializer.GetType();
        var implAsm = serializerType.Assembly;
        Console.WriteLine($"SGEN Serializer implementation assembly: {implAsm.GetName().Name} loaded for serialized type name: {serializedTypeName}. Serializer type name: {serializerType.FullName}");
        Console.WriteLine($"SGEN Location: '{implAsm.Location}'  IsDynamic: {implAsm.IsDynamic}");

        bool usedPreGenerated = implAsm.GetName().Name != null
                                && implAsm.GetName().Name.EndsWith("XmlSerializers")
                                && !string.IsNullOrEmpty(implAsm.Location);

        Console.WriteLine(usedPreGenerated
            ? "SGEN Using pre-generated serializer implementation."
            : "SGEN Using runtime-generated (reflection) serializer implementation.");

        Console.ReadLine();
    }
}

[petr-sebesta]

I can confirm same issue:
Pre-generated serializers are not used during runtime.

.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <!-- This property is useful for inspecting the generated files -->
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.XmlSerializer.Generator" Version="8.0.0" />
  </ItemGroup>

</Project>

global.json

{
  "sdk": {
    "version": "8.0.415"
  }
}

Program.cs

using System.Xml.Serialization;

public class Person
{
    public string Name { get; set; } = "X";
    public int Age { get; set; }
}

class Program
{
    static void Main()
    {
        var person = new Person { Name = "Alice", Age = 30 };
        var serializer = new XmlSerializer(typeof(Person));

        // Serialize to XML
        using (var sw = new StringWriter())
        {
            serializer.Serialize(sw, person);
            var xml = sw.ToString();
            Console.WriteLine("Serialized XML:\n" + xml);
        }

        var assembly = serializer.GetType().Assembly.GetName().FullName;
        Console.WriteLine($"XmlSerializer type: {serializer.GetType().Name}");
        Console.WriteLine($"XmlSerializer is using assembly: {assembly}");
    }
}

Output:

Serialized XML:
<?xml version="1.0" encoding="utf-16"?>
<Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Name>Alice</Name>
  <Age>30</Age>
</Person>
XmlSerializer type: XmlSerializer
XmlSerializer is using assembly: System.Private.Xml, Version=8.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51