When using protobuf-net, how do I know what fields will be updated (or have been updated) when using merge on an existing object

Question

Using Protobuf-net, I want to know what properties of an object have been updated at the end of a merge operation so that I can notify interested code to update other components that may relate to those updated properties.

I noticed that there are a few different types of properties/methods I can add which will help me serialize selectively (Specified and ShouldSerialize). I noticed in MemberSpecifiedDecorator that the ‘read’ method will set the specified property to true when it reads. However, even if I add specified properties for each field, I’d have to check each one (and update code when new properties were added)

My current plan is to create a custom SerializationContext.context object, and then detect that during the desearalization process – and update a list of members. However… there are quite a few places in the code I need to touch to do that, and I’d rather do it using an existing system if possible.

It is much more desirable to get a list of updated member information. I realize that due to walking down an object graph that may result in many members, but in my use case I’m not merging complex objects, just simple POCO’s with value type properties.

Answer 1

Getting a delta log isn't an inbuilt feature, partly because of the complexity when it comes to complex models, as you note. The Specified trick would work, although this isn't the purpose it was designed for - but to avoid adding complexity to your own code,that would be something best handled via reflection, perhaps using the Expression API for performance. Another approach might be to use a ProtoReader to know in advance which fields will be touched, but that demands an understanding of the field-number/member map (which can be queried via RuntimeTypeModel).

Are you using habd-crafted models? Or are you using protogen? Yet another option would be to have code in the setters that logs changes somewhere. I don't think protogen currently emits partial method hooks, but it possibly could.

But let me turn this around: it isn't a feature that is built in right now, and it is somewhat limited due to complexity anyway, but: what would a "good" API for this look like to you?

As a side note: this isn't really a common features in serializers - you'd have very similar challenges in any mainstream serializer that I can think of.