Reflection for oneOf fields

X Dev

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Jul 2, 2025, 12:28:45 PM Jul 2

to Protocol Buffers

Hi guys

Please, in `golang` having a oneOf field how at runtime to enumerate the types that fits the target interface (e.g. reflection)?

And a sibling question why actually the interface generated for the oneOf purpose is a package private. It is really logically not to expose the interface content it self to restrict the types as described in a proto.
Still the interface it self would be useful for e.g. to define a function that composes related result.

Thank you

scott beeker

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Jul 3, 2025, 4:20:51 AM Jul 3

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Use reflect.TypeOf() to get the concrete type of a oneOf interface value.

Package-Private Interface:

Exposing it could be useful for function composition but risks violating the protobuf contract.

Workarounds like type switches or custom interfaces can achieve similar goals without breaking encapsulation.

X Dev

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Jul 3, 2025, 7:20:36 AM Jul 3

to Protocol Buffers

Hi @Scott. Thanks for the reply

1.

I was interested not in the type it self. Instead how to enumerate all the underlying types that are implementing related `oneOf` interface. And how to make it in general in runtime without static knowledge of that types. E.g.

```

func ResolveUnderlyingInterfaceImplementations(interfaceType reflect.Type) []reflect.Type {

}

```

2.

* I understand this would not in help for the case solution still for the general understanding: What exactly does it violate. Do you mean leak of abstraction and backward capabilities?

* Could you get an example of workarounds you mentioned. Type switch is a static definition approach. How might it help for enumerating types in runtime.

* What is custom interfaces. Is it about just to redeclare the very same interfaces in the client code base

Any way importing from proto compiled package some type as

```

type SomeMessage struct {

SomeField isPrivateOneOfInterface

}

```

how may one defined a function returning related value or a variable for intermediate storage as

```

var someFieldValue: protos.isPrivateOneOfInterface

func ComposeSomeField() isPrivateOneOfInterface {

}

```

Could you please clarify the idea behind your workarounds

Thank you

четвер, 3 липня 2025 р. о 11:20:51 UTC+3 scott beeker пише:

Cassondra Foesch

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Jul 4, 2025, 5:57:42 AM Jul 4

to X Dev, Protocol Buffers

I keep responding privately to people by accident. (I’ve been out of mailing lists too long.)

1. There is not a way to dynamically enumerate the fields that form a `oneof`. The utility of doing such a thing would have _very_ narrow utility in general, and explaining why might be possible, but I think it would be hard.

2. In the private message I sent, I directed you to https://github.com/golang/protobuf/issues/1326 this issue has been fully and thoroughly hashed out there. The overly short answer is that we were against exposing the interface for a long time for Semantics reasons, and now that the Opaque API is going to do away with the interface type entirely, we will definitely not be exposing the interface type in the future.

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