class Program
    {
        static void Main(string[] args)
        {
            var objRaw = new SomeClass
            {
                Obj = new string[] {"asd", "asd"}
            };

var objSer = MessagePackSerializer.Serialize(objRaw, TypelessContractlessStandardResolver.Instance);

            var objDes = MessagePackSerializer.Deserialize<SomeClass>(objSer, TypelessContractlessStandardResolver.Instance);

            var expectedTrue = objDes.Obj is string[];
        }
    }

    public class SomeClass
    {
        public object Obj { get; set; }
    }