Generic interface desserialization failure

[Lexy2]

Consider the following scenario:
I have a method defined as
public Task<IList<string>> GetListAsync(), in an IRpcService. The client application is targeting net472, and the server application is targeting net10.0.
The server implementation is returning just new List().
The client is calling var list = await GetListAsync();

Expected result
Empty list is returned.

Actual result

System.Runtime.Serialization.SerializationException: Cannot deserialize inbound call arguments. ---> System.Collections.Generic.KeyNotFoundException: Type 'System.Collections.Generic.List`1[[System.String, System.Private.CoreLib]], System.Private.CoreLib' is not found.
   at Type ActualLab.Reflection.TypeRef.Resolve()
   at Type ActualLab.Rpc.Serialization.Internal.ByteTypeSerializer.FromBytes(ByteString bytes)+(ByteString b) => { }
   at TValue System.Collections.Concurrent.ConcurrentDictionary<TKey, TValue>.GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)
   at Type ActualLab.Rpc.Serialization.Internal.ByteTypeSerializer.FromBytes(ByteString bytes)
   at Type ActualLab.Rpc.Serialization.Internal.ByteTypeSerializer.ReadItemType(ref ReadOnlyMemory<byte> data)
   at Type ActualLab.Rpc.Serialization.Internal.ByteTypeSerializer.ReadDerivedItemType(ref ReadOnlyMemory<byte> data, Type expectedType)
   at void ActualLab.Rpc.Serialization.RpcByteArgumentSerializerV4.PolyDeserialize(ArgumentList arguments, ReadOnlyMemory<byte> data)
   at void ActualLab.Rpc.Serialization.RpcByteArgumentSerializerV4.Deserialize(ref ArgumentList arguments, bool needsPolymorphism, ReadOnlyMemory<byte> data)
   at ArgumentList ActualLab.Rpc.Infrastructure.RpcInboundCall.DeserializeArguments()
   at Task ActualLab.Rpc.Infrastructure.RpcInboundCall.Process(CancellationToken cancellationToken)

If I change the interface to request Task<List<string>>, the call succeeds.

Tried with 12.1.61.

[alexyakunin]

@Lexy2 hi, yep, it's pretty hard to workaround that:

• If your return type is an abstract one, ActualLab.Rpc assumes it has to use polymorphic deserialization, i.e. serialize the TypeRef
• And IList<T> assembly isn't the same as it used to be in .NET Framework 4.7.

So ultimately, you either have to fix the reference somehow (e.g., on the client), or avoid polymorphic serialization at all for types which changed location from .NET Fx.

[alexyakunin]

Currently there is no way in ActualLab.Rpc to "rewrite" such TypeRef-s. It's fairly easy to add, but I am a bit hesitant to add a feature you may not really need. Here is what I'd recommend you to do:

1. Avoid returning generic collection interfaces and interfaces in general in all RPC types. What MS folks recommend here is a huge mistake, especially returning types like IEnumerable instead of actual lists, arrays, etc. - if you send such types over the wire, you let the client to use any implementation they like, even though typically you want a very specific implementation, so that client-side code could handle it accordingly to the implementation. E.g., if you'd return an array, the client-side code could copy it using span API, etc. - so overall, there are no wins from returning a generic type instead of a specific one. All you get is more complex logic that handles such args.
2. Moreover, you return IList - do you actually mean at least IReadOnlyList? Coz most likely you don't want the client to modify it. And if it's a read-only list, probably it's better to return T[] or ImmutableList/Array<T>; Fusion also offers ApiArray, which is ~ an array exposing IReadOnlyList API with a set of .WithXxx mutators. There is also ApiMap, which guarantees stable order of their items on serialization (that's what you need if you also use Fusion's local cache for compute methods, coz you need a stable hash for the same map).
3. Avoid polymorphic serialization in general - e.g. I'd rather avoid it at all for the public APIs. You can use [RpcSerializable] tag for abstract types in the most recent Fusion to let it know the type doesn't need polymorphic serialization (e.g., a type tagged with [Union] w/ MessagePack, etc.).