Unable to serialize session because SerializationFieldInfo is not marked as serializable

[cremor]

I can't update an application from NHibernate 5.1.4 to 5.2.2 (I've also tested 5.2.0). If I update and try to serialize an ISession the following exception is thrown in some cases:

System.Runtime.Serialization.SerializationException: Type 'System.Runtime.Serialization.SerializationFieldInfo' in Assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' is not marked as serializable.
   at System.Runtime.Serialization.FormatterServices.InternalGetSerializableMembers(RuntimeType type)
   at System.Runtime.Serialization.FormatterServices.<>c__DisplayClass9_0.<GetSerializableMembers>b__0(MemberHolder _)
   at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
   at System.Runtime.Serialization.FormatterServices.GetSerializableMembers(Type type, StreamingContext context)
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitMemberInfo()
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.InitSerialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder)
   at System.Runtime.Serialization.Formatters.Binary.WriteObjectInfo.Serialize(Object obj, ISurrogateSelector surrogateSelector, StreamingContext context, SerObjectInfoInit serObjectInfoInit, IFormatterConverter converter, ObjectWriter objectWriter, SerializationBinder binder)
   at System.Runtime.Serialization.Formatters.Binary.ObjectWriter.Serialize(Object graph, Header[] inHeaders, __BinaryWriter serWriter, Boolean fCheck)
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph, Header[] headers, Boolean fCheck)
   at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Serialize(Stream serializationStream, Object graph)
   at <my code calling binaryFormatter.Serialize(memoryStream, session)>

Since SerializationFieldInfo is internal in mscorlib neither my code nor NHibernate can reference it directly. But something must have changed between NHibernate 5.1 and 5.2 that causes an ISession to indirectly contain a SerializationFieldInfo somewhere.

I'm not sure why some sessions cause this issue for me and others don't. I haven't been able to find a specific entity that causes it yet.

My application targets Framework 4.7.1 and I'm currently on the 4.7.2 runtime (Windows 10 1709).

[fredericDelaporte]

NHibernate has many session serialization tests, and they do not fail. Can you give more information on how to reproduce this?

[cremor]

Not yet, but I'll continue to investigate how to reproduce it. Although it is 100% reproduceable on my failing tests, only very few tests are affected for my application too (less than 10% of those that use NHibernate).

I assume only very specific entity mappings trigger the bug, so I'll try to find out which ones tomorrow.

[cremor]

I think I found the problem. Although I don't have a unit test for NHibernate yet or even fully understand it, that's what I found by debugging the .NET Framework serialization code:

Objects of the type FieldInterceptorObjectReference have a field named "proxy$additionalMembers" of type MemberInfo[]. This array contains the SerializationFieldInfo objects that can't be serialized. The fields that are wrapped by the SerializationFieldInfo objects are those of base classes of entities (see background information below). The FieldInterceptorObjectReference objects itself are referenced by <entityType>ProxyForFieldInterceptor objects. Since both of those FieldInterceptor classes didn't exist yet in 5.1 it makes sense that that version didn't fail.

Some background information:

1. The only place that SerializationFieldInfo is instantiated is the same method that also throws the exception: FormatterServices.InternalGetSerializableMembers
2. That line is called for all fields that are in parent types of entities, e.g. for Entity.Id or Entity.Version properties (or to be exact - for their backing fields) in a Entity base class. For one of my unit tests which fails with 5.2.2, the line seems to be called the same amout of times in 5.1.4 and 5.2.2. At least the call count is the same for the successful run of 5.1.4 and the run of 5.2.2 until the exception is thrown. The fields that are wrapped in that object are also the same, just the order is very slightly different (6 differences in 316 objects). So it seems like the creation of the SerializationFieldInfo objects is not the problem, just that they are referenced somewhere.
3. I printed out obj and objectInfo.obj that are enqueued for serialization in ObjectWriter.Schedule. The difference between 5.1.4 and 5.2.2 is that 5.2.2 tries to serialize arrays of MemberInfo objects and those arrays contain the SerializationFieldInfo objects. I couldn't find any MemberInfo array serialization being done with 5.1.4.

[bahusoid]

So it's related to entities with lazy properties. And the issue can easily be reproduced on existing test

nhibernate-core/src/NHibernate.Test/StaticProxyTest/StaticProxyFixture.cs

Line 253 in 6b8c692

public void SessionSerializationWithLazyProperty()

If LazyTextEntity is modified in the following way (added base class with unmapped property) :

[Serializable]
public class LazyTextEntity : BaseTextEntity
{
	public virtual Guid Id { get; set; }
	public virtual string Name { get; set; }
	public virtual string Text { get; set; }
}

[Serializable]
public class BaseTextEntity
{
	public virtual int Test { get; set; }
}

[cremor]

So it's related to entities with lazy properties.

I'm not in front of my code right now, but I'm 99% sure that I don't have any lazy properties. So there has to be another case where these classes are used.

[hazzik]

The following kind of contradicts each other:

Objects of the type FieldInterceptorObjectReference

but I'm 99% sure that I don't have any lazy properties.

[hazzik]

I have seen this error in regarding to arrays (and particularly arrays of MemberInfo). This is the reason why we have SerializationHelper.AddValueArray and SerializationHelper.GetValueArray.

[cremor]

The following kind of contradicts each other:

I'll check that tomorrow.