---
title: "Decorators"
toc_min_heading_level: 2
toc_max_heading_level: 3
---

# Decorators

## TypeSpec.Protobuf

### `@field` {#@TypeSpec.Protobuf.field}

Defines the field index of a model property for conversion to a Protobuf
message.

The field index of a Protobuf message must:

- fall between 1 and 2<sup>29</sup> - 1, inclusive.
- not fall within the implementation reserved range of 19000 to 19999, inclusive.
- not fall within any range that was [marked reserved](#

```typespec
@TypeSpec.Protobuf.field(index: valueof uint32)
```

#### Target

`ModelProperty`

#### Parameters

| Name  | Type                    | Description                          |
| ----- | ----------------------- | ------------------------------------ |
| index | `valueof scalar uint32` | The whole-number index of the field. |

#### Examples

```typespec
model ExampleMessage {
  @field(1)
  test: string;
}
```

### `@message` {#@TypeSpec.Protobuf.message}

Declares that a model is a Protobuf message.

Messages can be detected automatically if either of the following two conditions are met:

- The model has a `@field` annotation on all of its properties.
- The model is referenced by any service operation.

This decorator will force the emitter to check and emit a model.

```typespec
@TypeSpec.Protobuf.message
```

#### Target

`model {}`

#### Parameters

None

### `@package` {#@TypeSpec.Protobuf.package}

Declares that a TypeSpec namespace constitutes a Protobuf package. The contents of the namespace will be emitted to a
single Protobuf file.

```typespec
@TypeSpec.Protobuf.package(details?: TypeSpec.Protobuf.PackageDetails)
```

#### Target

`Namespace`

#### Parameters

| Name    | Type                                     | Description                         |
| ------- | ---------------------------------------- | ----------------------------------- |
| details | `model TypeSpec.Protobuf.PackageDetails` | the optional details of the package |

### `@reserve` {#@TypeSpec.Protobuf.reserve}

Reserve a field index, range, or name. If a field definition collides with a reservation, the emitter will produce
an error.

This decorator accepts multiple reservations. Each reservation is one of the following:

- a `string`, in which case the reservation refers to a field name.
- a `uint32`, in which case the reservation refers to a field index.
- a tuple `[uint32, uint32]`, in which case the reservation refers to a field range that is _inclusive_ of both ends.

Unlike in Protobuf, where field name and index reservations must be separated, you can mix string and numeric field
reservations in a single `@reserve` call in TypeSpec.

#### API Compatibility Note

Field reservations prevent users of your Protobuf specification from using the given field names or indices. This can
be useful if a field is removed, as it will further prevent adding a new, incompatible field and will prevent users
from utilizing the field index at runtime in a way that may break compatibility with users of older specifications.

See _[Protobuf Language Guide - Reserved Fields](https://protobuf.dev/programming-guides/proto3/#reserved)_ for more
information.

```typespec
@TypeSpec.Protobuf.reserve(...reservations: valueof string | [uint32, uint32] | uint32[])
```

#### Target

`model {}`

#### Parameters

| Name         | Type                                                   | Description                  |
| ------------ | ------------------------------------------------------ | ---------------------------- |
| reservations | `valueof model string \| [uint32, uint32] \| uint32[]` | a list of field reservations |

#### Examples

```typespec
// Reserve the fields 8-15 inclusive, 100, and the field name "test" within a model.
@reserve([8, 15], 100, "test")
model Example {
  // ...
}
```

### `@service` {#@TypeSpec.Protobuf.service}

Declares that a TypeSpec interface constitutes a Protobuf service. The contents of the interface will be converted to
a `service` declaration in the resulting Protobuf file.

```typespec
@TypeSpec.Protobuf.service
```

#### Target

`Interface`

#### Parameters

None

### `@stream` {#@TypeSpec.Protobuf.stream}

Set the streaming mode of an operation. See [StreamMode](./data-types#TypeSpec.Protobuf.StreamMode) for more information.

```typespec
@TypeSpec.Protobuf.stream(mode: TypeSpec.Protobuf.StreamMode)
```

#### Target

`Operation`

#### Parameters

| Name | Type                                | Description                                    |
| ---- | ----------------------------------- | ---------------------------------------------- |
| mode | `enum TypeSpec.Protobuf.StreamMode` | The streaming mode to apply to this operation. |

#### Examples

```typespec
@stream(StreamMode.Out)
op logs(...LogsRequest): LogEvent;
```

```typespec
@stream(StreamMode.Duplex)
op connectToMessageService(...Message): Message;
```
