Use source generators to remove the boiler plate of creating encodeable objects

[romanett]

Discussed in #3605

^{Originally posted by lkomanetz March 9, 2026}
Right now I use Claude to generate the boiler plate involved in inherited from EncodeableObject. It would be wonderful if there was a source generator that would handle, at least, overriding Decode, Encode, and IsEqual. I wouldn't mind if it was done by convention or through attributes (like the CommunityToolkit.Mvvm package does for view models). I could see it a couple of ways.

Convention

public partial class MyEncodeableObject : EncodeableObject
{
    private long p_BigNumber; // The p_ tells the source generator that it is an OPC-UA field

    public override ExpandedNodeId TypeId => ExpandedNodeId.Parse("node_id");
    public override ExpandedNodeId BinaryEncodingId => ExpandedNodeId.Parse("node_id");
    public override ExpandedNodeId XmlEncodingId => ExpandedNodeId.Parse("node_id");
}

// What it generates
public partial class MyEncodeableObject : EncodeableObject
{
    public long BigNumber
    {
        get => p_BigNumber;
        set -> p_BigNumber = value;
    }

    public override void Decode(IDecoder decoder)
    {
        BigNumber = decoder.ReadInt64("BigNumber");
    }

    public override void Encode(IEncoder encoder)
    {
        encoder.WriteInt64("BigNumber", BigNumber);
    }

    public override bool IsEqual(IEncodeable encodeable)
    {
        if (encodeable is not MyEncodeableObject other)
        {
            return false;
        }

        if (ReferenceEquals(this, encodeable))
        {
            return true;
        }

        return (
            BigNumber == other.BigNumber
        );
    }
}

And now the Attribute style of handling it (which might provide some more flexibility

Attributes

[Encodeable(TypeId = "node_id", BinaryEncodingId = "node_id", XmlEncodingId = "node_id")]
public partial class MyEncodeableObject : EncodeableObject
{
    [UaProperty(Order=2)] // Use the property name by convention
    private long _bigNumber;

    [UaProperty(Order=1, Name = "ErrorMessage")] // Use the provided UDT property name
    private string _someText;
}

This would basically generate the same thing (adding the ExpandedNodeId properties that the other one wouldn't) but also use information in the attribute to know in what order to read from the decoder and write to the encoder. I can continue to use AI tools to do it but a source generator would be the superior method to handling the boiler plate in this case. I'd love to hear others' thoughts on this.

[marcschier]

Yes, this was one big goal with the source generators, generate not just from model, but also from "partial" implementation. I have not gotten to it yet, but it is on my list. That said, help welcome :-)

[lkomanetz]

@marcschier
If I can find some additional time I'll try and provide a PR for a source generator like that. At the very least I figured it would be a good idea to bring the idea forward so it doesn't get lost. I'm glad I wasn't the only one thinking it!

[marcschier]

There are 2 source generators in this repo, one is used internally to generate the stack, the other compiles model/nodeset2.xml, and that is a good place to add this, as it could take advantage of the templates used for model compilation. At least that was the idea.