Attributes

Bielu.AspNetCore.AsyncApi uses attributes to discover and describe your AsyncAPI components.

Main Attributes

Attribute	Description
[AsyncApi]	Marks a class for scanning by the generator. Without this, other attributes on the class or its methods will be ignored.
[Channel]	Defines a channel. Can be applied to a class or a method.
[SendOperation]	Defines an operation through which the application sends messages (AsyncAPI 3 action: send; a subscribe operation in 2.x).
[ReceiveOperation]	Defines an operation through which the application receives messages (AsyncAPI 3 action: receive; a publish operation in 2.x).
[Message]	Provides metadata for a message, such as its name, summary, or headers.
[MessageExample]	Defines an example for a message.
[ChannelParameter]	Defines a parameter in a channel path (e.g., {id}).

[PublishOperation] and [SubscribeOperation] still work but are obsolete. They were named the AsyncAPI 2.x way, from the client's point of view, so they read backwards next to AsyncAPI 3 output: [PublishOperation] is replaced by [ReceiveOperation], and [SubscribeOperation] by [SendOperation].

Usage Examples

Channel and Operations

[AsyncApi]
public class MyMessageBus
{
    [Channel("users/{userId}/signed-up")]
    [ChannelParameter("userId", Description = "The unique user ID")]
    [SendOperation(typeof(UserSignedUp), "UserSignUp", Summary = "Emitted when a user signs up.")]
    public void HandleUserSignUp(UserSignedUp msg) { }
}

Message Metadata

You can use the [Message] attribute to customize how a message appears in the document.

[Message(Name = "UserEvent", Summary = "A generic user event")]
public class UserEvent { ... }

On a class the payload type is the class itself, so no type argument is needed. The values become the defaults for every channel that uses the class as a payload. A [Message] on a method still names the payload and overrides whatever it sets:

[Channel("users/created")]
[Message(typeof(UserEvent), Title = "User created")] // Name and Summary come from the class
public void UserCreated() { }

The source generator picks up class-level [Message] attributes too, so the same document is produced under Native AOT.

Message Examples

Examples can be defined as JSON literals or via a provider class.

[MessageExample("Basic", Json = "{\"id\": 123, \"name\": \"John\"}")]
public class UserSignedUp { ... }

Or using a provider:

[MessageExample(typeof(MyExampleProvider))]
public class UserSignedUp { ... }

public class MyExampleProvider : IAsyncApiMessageExampleProvider
{
    public object GetExample() => new UserSignedUp { Id = 123, Name = "John" };
}

XML Documentation Support

If enabled, the generator will also use your C# XML documentation comments (/// <summary>, /// <remarks>) as fallbacks for descriptions if the attributes don't provide them.

See Configuration to enable XML comments.