Private fields of base-class doesn't get (de-)serialized.

[GunSharp]

Hi All,
Although the case is simple, I cannot get it to work.
I’m trying to serialize/deserialize (i.e. clone) an instance of a SubClass that derives a BaseClass. The baseclass has a private field which doesn’t seem to be (de-)serialized. I’m trying to avoid the annotation in the classes itself.

Bug description

Private members of base-class aren't (de-)serialized

Repro steps

    public class BaseClass
    {
        private int _baseField;

        public BaseClass() : this(-1)
        {
            
        }
        public BaseClass(int value)
        {
            this.SetBaseField(value);
        }
        public int GetBaseField()
        {
            return _baseField;
        }
        public void SetBaseField(int value)
        {
            _baseField = value;
        }
    }

    public class SubClass : BaseClass
    {
        private int _extField;

        public SubClass():
            this(-2, -2)
        {
            
        }

        public SubClass(int baseField, int extField)
        : base(baseField)
        {
            this.SetExtField(extField);
        }
        public int GetExtField()
        {
            return _extField;
        }
        public void SetExtField(int value)
        {
            _extField = value;
        }
    }


    static void Main(string[] args)
    {
        var orgBase = new BaseClass(42);
        var cloneBase = Clone(orgBase);
        Console.WriteLine($"BaseClass org  : [{orgBase.GetBaseField()}]");
        Console.WriteLine($"BaseClass clone: [{cloneBase.GetBaseField()}]");
        
        var orgSub = new SubClass(42, 73);
        var cloneSub = Clone(orgSub);
        Console.WriteLine($"SubClass org   : [{orgSub.GetBaseField()}, {orgSub.GetExtField()}]");
        Console.WriteLine($"SubClass clone : [{cloneSub.GetBaseField()}, {cloneSub.GetExtField()}]");

        Console.ReadLine();

    }

    static T Clone<T>(T sut)
    {
        var options = TypelessContractlessStandardResolver.Options.WithResolver(
            CompositeResolver.Create(
                DynamicObjectResolverAllowPrivate.Instance,
                ContractlessStandardResolverAllowPrivate.Instance,
                TypelessContractlessStandardResolver.Instance));

        byte[] seri = MessagePackSerializer.Serialize(sut, options);

        var clone = MessagePackSerializer.Deserialize<T>(seri, options);

        return (T)clone;
    }

Expected behavior

The cloned object should have the value member int _baseField set to
BaseClass org : [42]
BaseClass clone: [42]
SubClass org : [42, 73]
SubClass clone : [42, 73]

Actual behavior

What happened instead of what you expected.
BaseClass org : [42]
BaseClass clone: [42]
SubClass org : [42, 73]
SubClass clone : [-2, 73]

• Version used: 2.5.140
• Runtime: .NET framework 4.8

Additional context

Is it me or a bug (or both).?

Thank you for your replies in advance,

[AArnott]

Duplicate of #1756.