Deserialization of generic types fails due to inability to find property setters that use `init`

[AArnott]

Bug description

A generic type containing properties with an init accessor fails to serialize.

Repro steps

This type fails to serialize with the DynamicObjectResolver:

        [MessagePackObject]
        public class GenericPerson<T>
        {
            [Key(0)]
            public string Name { get; init; }
        }

This test fails:

        [Fact]
        public void RoundtripGenericClass()
        {
            var person = new GenericPerson<int> { Name = "bob" };
            byte[] msgpack = MessagePackSerializer.Serialize(person, MessagePackSerializerOptions.Standard);
            var deserialized = MessagePackSerializer.Deserialize<GenericPerson<int>>(msgpack, MessagePackSerializerOptions.Standard);
            Assert.Equal(person.Name, deserialized.Name);
        }

Expected behavior

Successful pass.

Actual behavior

  Message: 
    MessagePack.MessagePackSerializationException : Failed to deserialize MessagePack.Tests.DynamicObjectResolverTests+GenericPerson`1[[System.Int32, System.Private.CoreLib, Version=5.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]] value.
    ---- System.MissingMethodException : Method not found: 'Void GenericPerson`1.set_Data(!0)'.
  Stack Trace: 
    MessagePackSerializer.Deserialize[T](MessagePackReader& reader, MessagePackSerializerOptions options) line 251
    MessagePackSerializer.Deserialize[T](ReadOnlyMemory`1 buffer, MessagePackSerializerOptions options, CancellationToken cancellationToken) line 270
    DynamicObjectResolverTests.RoundtripGenericClass() line 321
    ----- Inner Stack Trace -----
       at MessagePack.Formatters.MessagePack_Tests_DynamicObjectResolverTests\.GenericPerson`1\[\[System_Int32\, System_Private_CoreLib\]\]Formatter1.Deserialize(MessagePackReader& reader, MessagePackSerializerOptions options)
    MessagePackSerializer.Deserialize[T](MessagePackReader& reader, MessagePackSerializerOptions options) line 246

[AArnott]

Based on this doc it seems likely that our IL code gen has to be updated to include modreq([System.Runtime]System.Runtime.CompilerServices.IsExternalInit) in the called method signature somehow. It seems all C# compiler generated calls to init methods require this, and it's evident in decompilers but not in our code-generated calls to it.

As to why it only fails in generic classes may be a bug/detail of the .NET 5 runtime.

[AArnott]

See linked dotnet/runtime bug for more info. I'm pretty certain this is a bug in .NET.

[AArnott]

This is a bug in .NET. .NET 6.0 has the fix, which may be ported to 5.x.
For now, a workaround is to use DynamicMethod to make the call instead. That translates in MessagePack to folks using the StandardResolverAllowPrivate resolver instead of StandardResolver.

[simonziegler]

@AArnott thanks for moving #1147 to here. I probably cannot use StandardResolverAllowPrivate because of side effects through my code base. For now I am selectively using set; rather than init;. To help me chart the best path forwards pending .NET 6, can you let me know if you expect some sort of resolution in MessagePack, and if so how long you think it may take before that resolution is in place? I'll then orientate my medium term serialization strategy around that advice. Thanks!