Microsoft.XmlSerializer.Generator fails to generate serializer for assemblies not targeting netstandard2.0

[mconnew]

The runtime config for the Microsoft.XmlSerializer.Generator specifies to run on netcoreapp2.0. When reflecting over application assembly to generator serializer, if the assembly is referencing for example NetCoreApp3.0, the version of System.Runtime referenced by the library can't be loaded.
The fix is to change the assembly loading code to use System.Reflection.MetadataLoadContext to reflect over the application assembly. We do use Type.GetCustomAttributes at least in sgen.cs so we would also need to make changes to use Type.GetCustomAttributesData. XmlSerializer.GenerateSerializer does the bulk of the work so there might need to be changes there as well.

Original issue: dotnet/corefx#41286

• Port to 3.1.x

[vzalamea]

I have a netstandard2.0 project with a reference.cs file generated by the Visual Studio WCF web service reference generator. We point the tool to a WSDL to generate the reference.cs file. The project is referenced by a netcoreapp3.1 web application.

I gave up on the Microsoft.XmlSerializer.Generator due to the error:
Method 'System.Xml.Serialization.XmlSerializer.GenerateSerializer' was not found. This is likely because you are using an older version of the framework. Please update to .NET Core v2.1 or later.

So I added the package "dotnet-svcutil.xmlserializer" instead and it successfully created a myproject.XmlSerializers.dll file with all the read/write logic for serialization/deserialization. And I did notice a major reduction in start up time when this file is present in my bin folder.

So what is the difference between Microsoft.XmlSerializer.Generator and dotnet-svcutil.xmlserializer?

[ericstj]

I happened to be looking at this and prototyped a bit ran into more places that use GetCustomAttributes.

runtime/src/libraries/Microsoft.XmlSerializer.Generator/src/Sgen.cs

Line 263 in 6991ccd

object[] obsoleteAttributes = type.GetCustomAttributes(typeof(ObsoleteAttribute), false);

runtime/src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlAttributes.cs

Line 101 in 6991ccd

object[] attrs = provider.GetCustomAttributes(false);

runtime/src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlAttributes.cs

Line 200 in 6991ccd

object[] attrs = memberInfo.GetCustomAttributes(attrType, false);

runtime/src/libraries/System.Private.Xml/src/System/Xml/Serialization/Types.cs

Line 1227 in 6991ccd

object[] attrs = ctor.GetCustomAttributes(typeof(ObsoleteAttribute), false);

All of these were hit by XmlSerializer.Generator. There are other uses that may also be significant, but I stopped after looking at these:
https://github.com/search?q=GetCustomAttributes+repo%3Adotnet%2Fruntime+path%3A%2Fsrc%2Flibraries%2FSystem.Private.Xml%2Fsrc%2FSystem%2FXml&type=Code&ref=advsearch&l=&l=

Those would all be challenges to porting this code to use MetadataLoadContext. You'd need to fix all of them that are hit by XmlSerializer.Generator, and even once fixed it would limit XmlSerializer.Generatorto only work on the framework version with those fixes.