Why can't I have an custom mapper when mapping from an attribute?

[gareththackeray]

Hi,

Just wondering what the reasoning is for banning custom mappers when mapping from attributes?

Cheers,

Gareth

[edpollitt]

Hi Gareth

I don't remember ever considering it as a use case. Attribute mapping was added primarily as a shorthand way to map simple types - i.e. everything that TypeConverter handles out of the box.

From a technical perspective the problem is that the IMapper interface acts on XElements, and attributes are not XElements. So it would probably require adding a separate interface, e.g. IAttributeMapper, and then there would be the problem of how to handle someone trying to configure an XElement mapping to use an IAttributeMapper or vice versa.

Do you have an example of what you're trying to do? I have plans for a fairly radical rewrite of the whole custom mapping/strategy API that would allow constraining certain options according to context. It might be possible to do something with custom attribute mappers as part of that.

[gareththackeray]

Hi Ed,

Actually what I was trying to do was to hook into AutoConfig.Map to allow a developer to save some local overrides without committing them to source control - see https://gist.github.com/gareththackeray/aacc8c503006ebea631b51888d041e55.

I got around the attribute thing by just changing the config file in question to use elements instead, but as you can see from the gist there are some restrictions of what I'm doing anyway - maybe this will give you some thoughts for your rewrite!

Regardless, I think we might create a wrapper around AutoConfig to do this, rather than trying to use a custom mapper.

BTW I work with @kbrimble at Just Eat.

G

[edpollitt]

Many thanks for the Gist.

I don't think there is any elegant way to do this in AutoConfig at the moment. It looks to me like what you really need is something like an OnPropertyMapped event that you could hook into to do the override.

It's something I'll try to think about in the rewrite if I have time. I think for now you're right that the easiest way to do this is by wrapping AutoConfig with your own service.