Use ReflectionOnly as serialization mode in case dynamic code runtime feature is not supported

[MaximLipnin]

Fixes #56533

[jkotas]

Suggested change

	internal static SerializationMode Mode { get; set; } = RuntimeFeature.IsDynamicCodeSupported ? SerializationMode.ReflectionAsBackup : SerializationMode.ReflectionOnly;
	internal static SerializationMode Mode => RuntimeFeature.IsDynamicCodeSupported ? SerializationMode.ReflectionAsBackup : SerializationMode.ReflectionOnly;

Is it possible to return the constant directly instead of caching it in a static variable, to enable linker to trim the unreachable code?

[MaximLipnin]

There are some tests checking for the Mode setter, e.g. https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs#L22-L27

[stephentoub]

Are those even used? Where are these compilation constants set?

runtime/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs

Line 21 in 8190713

#if ReflectionOnly|| XMLSERIALIZERGENERATORTESTS

cc: @StephenMolloy

[stephentoub]

Ah, nevermind, there's a separate project that sets at least one of them:

runtime/src/libraries/System.Private.Xml/tests/XmlSerializer/ReflectionOnly/System.Xml.XmlSerializer.ReflectionOnly.Tests.csproj

Line 3 in 8190713

<DefineConstants>$(DefineConstants);ReflectionOnly</DefineConstants>

It'd be good at some point to find a way to test this while not negatively impacting the size of what we ship.

[StephenMolloy]

Yeah, at this stage of the game, I'd stick with the original proposed change that caches the value at initialization so we maintain the ability to override when needed. (Which I believe is pretty much just testing right now.) We have a desire to potentially make the reflection-based serializer the primary option in the future, but we've got work to do to bring it up to par with the ILGen serializer before we can do that. Having a switch that is not stuck in one position is helpful.

[jkotas]

Revert Mode to usual property

You may be able to keep this optimization by using the same pattern as DataContractSerializer:

        private static SerializationMode s_mode = SerializationMode.ReflectionAsBackup;

        internal static SerializationMode Mode
        {
            get => RuntimeFeature.IsDynamicCodeSupported ? s_mode : SerializationMode.ReflectionOnly; }
            set => s_mode = value;
        }

[MaximLipnin]

We can merge it if there are no any other comments.