Runtime crash when deserialising under NativeAOT

[Nolram12345]

When attempting to deserialise a class containing a List of structs deriving from an interface (all tagged according to the MemoryPack documentation, i.e. with [MemoryPackUnion(tag, type)]), when compiling under NativeAOT, the following crash occurs:

Process terminated. Failed to create generic virtual method implementation

Declaring type: MemoryPack.Formatters.MemoryPackableFormatter`1<GameCore.Scenes.Scene>
Method name: Serialize
Instantiation:
  Argument 00000000: MemoryPack.Internal.ReusableLinkedArrayBufferWriter

   at System.RuntimeExceptionHelpers.FailFast(String, Exception, String, RhFailFastReason, IntPtr, IntPtr) + 0x249
   at Internal.Runtime.TypeLoader.TypeLoaderEnvironment.ResolveGenericVirtualMethodTarget(RuntimeTypeHandle, RuntimeMethodHandle) + 0x256
   at System.Runtime.TypeLoaderExports.<>c.<GVMLookupForSlotSlow>b__8_0(IntPtr, IntPtr, Object, IntPtr&) + 0x3e
   at System.Runtime.TypeLoaderExports.CacheMiss(IntPtr, IntPtr, RuntimeObjectFactory, Object) + 0x3c
   at System.Runtime.TypeLoaderExports.GVMLookupForSlotSlow(Object, RuntimeMethodHandle) + 0x68
   at System.Runtime.TypeLoaderExports.GVMLookupForSlot(Object, RuntimeMethodHandle) + 0x91
   at MemoryPack.MemoryPackWriter`1.WriteValue[T](T&) + 0x3d
   at MemoryPack.MemoryPackSerializer.Serialize[T](T&, MemoryPackSerializerOptions) + 0x342
   at Game.Main(String[] args) + 0xaa

This is after ensuring both a Serialize and Deserialize method are available (per #189 , in this case just with a dirty workaround by making a useless call to Serialize).

[MichalStrehovsky]

This is a bug in native AOT, tracked at dotnet/runtime#113664.

Once fixed, MemoryPack will run into the following exception:

MemoryPack/src/MemoryPack.Core/MemoryPackFormatterProvider.cs

Line 190 in fed6d9f

throw new InvalidOperationException("Type implements IMemoryPackFormatterRegister but can not found RegisterFormatter. Type: " + type.FullName);

That code is not trimming safe - native AOT optimizes away reflection metadata even if the method exists.

I would suggest replacing the inexact¹ interface method invocation with something trim and AOT safe, like:

static bool TryInvokeRegisterFormatter<T>()
{
    if (typeof(IMemoryPackFormatterRegister).IsAssignableFrom(typeof(T)))
    {
        // Will not trigger trimming or AOT warnings as of .NET 9 since the entire chain can be statically analyzed
        // You will see an AOT warning if you're still on .NET 8, but that's purely a point in time issue
        // More about intrinsically recognized AOT unsafe APIs: https://learn.microsoft.com/dotnet/core/deploying/native-aot/intrinsic-requiresdynamiccode-apis
        typeof(MemoryPackFormatterProvider).GetMethod("InvokeRegisterFormatter").MakeGenericMethod(typeof(T)).Invoke(null, null);
    }
}

static void InvokeRegisterFormatter<T>() where T : IMemoryPackFormatterRegister
{
    T.RegisterFormatter();
}

I would also recommend marking the project with <IsAotCompatible>true</IsAotCompatible> property so that trimming and AOT compatibility analyzers run on it and flag any other problematic code - I've not looked if there's more.

Footnotes

1. The fact that the name matches doesn't mean it actually implements the method ↩

[Nolram12345]

@neuecc Would it be possible to get these fixes rolled into MemoryPack itself and perhaps have a warning about this in the readme?