Support generating using Azure.Core CloudEvent

[JoshLove-msft]

The CloudEvent type is used in the generated surface for Event Grid, and will likely be used in more services in the future. It would be good if there was some way to annotate a model to indicate that it is a CloudEvent (or maybe more generally an Azure.Core model) in typespec, and then use that information when generating as an indication that the Azure.Core type should be used. I'm not sure if the annotation piece would belong in a different repo, but I imagine the code gen piece would need to happen in this repo for C#.

[JoshLove-msft]

/cc @m-nash @lmazuel @jsquire

[m-nash]

We do this today by annotating the type with a set of attributes like this ReferenceTypeAttribute. Here is an example of how we annotated ResponseError to be able to be picked up automatically in the generator and used instead of creating duplicates in all of our libraries.

There was discussion at one point about changing this mechanism since at minimum it might be an improvement if all languages could utilize the same mechanism, but that was never fully investigated.

If this issue is about enabling CloudEvent we should simply use the existing mechanism. If this issue is about a proposal on how we should be doing this going forward can you please provide details on what options you think we should consider?

[JoshLove-msft]

This issue is about generating CloudEvent. If there is already support for this using an existing pattern, that seems like it would be the best path forward.

@lmazuel mentioned that there was some investigation being done into a more generic way to enable using Core types that would not require attributing each type, but I don't have more information about that.

[m-nash]

This issue is about generating CloudEvent. If there is already support for this using an existing pattern, that seems like it would be the best path forward.

@lmazuel mentioned that there was some investigation being done into a more generic way to enable using Core types that would not require attributing each type, but I don't have more information about that.

In this case we should move this to Azure.Core issue since that is where the change would happen.

[JoshLove-msft]

In this case we should move this to Azure.Core issue since that is where the change would happen.

No changes are required in Autorest? How would the code gen know to use the Core CloudEvent type based on the typespec?

[m-nash]

It scans for those attributes today and uses them.

[JoshLove-msft]

This can be accomplished by adding the TypeReferenceTypeAttribute to CloudEvent.
We would need to pass ignoreExtraProperties as true since the swagger contains properties that are not exposed in the type.

[lmazuel]

We should wait for Azure/typespec-azure#1960