PrivateClassMessagePack.cs

var mes = new TObs()
{
    messageid = Guid.NewGuid().ToString(),
    source = this.SourceName,
    status = "OK",
};
var options = MessagePack.MessagePackSerializerOptions.Standard.WithResolver(MessagePack.Resolvers.StandardResolverAllowPrivate.Instance);
var jsonString = MessagePack.MessagePackSerializer.ConvertToJson(MessagePack.MessagePackSerializer.Serialize<TObs>(mes, options));


[MessagePack.MessagePackObject(true)]
private class TObs
{
    public string error { get; set; }
    public string messageid { get; set; }
    public string status { get; set; }
}