InvalidOperationException Reverse binding not allowed on FieldValue attributes

[WilbertOnGithub]

First of all, great library when having to deal with this stuff. However, I think I encountered a little bit of a snag.

I'm trying to use the FieldCrc16 attribute to calculate the CRC. I've noticed that when the CRC field/property is at the end of the message it works fine (see example below).

public class Packet
{
    [FieldOrder(0)]
    [FieldCrc16(nameof(Crc))]		
    public int PacketType { get; set; }	
	
    [FieldOrder(1)]
    public ushort Crc { get; set; }
}

However, if I swap the fieldorder (I have a requirement where the CRC value needs to be at the beginning of the message) like this:

public class Packet
{
    [FieldOrder(0)]
    public ushort Crc { get; set; }

    [FieldOrder(1)]
    [FieldCrc16(nameof(Crc))]		
    public int PacketType { get; set; }		
}

The serialization fails with the exception InvalidOperationException: Reverse binding not allowed on FieldValue attributes.

What am I missing here?

[jefffhaynes]

Unfortunately you're not missing anything. The nature of serializers is such that they go forward, not backwards. Supporting reverse bindings like this starts to open up all sorts of universe-ending time loops that I just wasn't prepared to tackle (e.g. what if that CRC was part of itself 🤯). This is similar to why things like lookup tables, databases, etc. are also not supported. It's all possible, for some definition of "possible", but at some point it just isn't a serializer anymore.

If that seems like a copout answer, the short answer is because it's really complicated and I probably don't have time to figure it out :). If I'm missing something obvious, please feel free to submit a PR!

[WilbertOnGithub]

Hi Jeff, thanks for the speedy and honest reply - I don't consider it to be a copout answer. I suspected that the backtracking could be problematic. I'm not going to say I will submit a quick PR but I will try to see how we can solve this with our specific issue for this project. Will update this comment with that (upcoming) result.

Might be a good idea to keep this issue alive so that other people who use the library can find this?

Keep up the good work - I've added your library to my list of packages to consider for new projects.