Action On A Missing Request Type When Mapping from Cloud Events Event Type

^{Originally posted by **dssldavidmorgan** July 2, 2026}
I set up my `RmqSubscription` objects like below, so that a cloud events type string maps to a `Type` which is a request type. The `getRequestType` gets the associated type from the message's cloud events type header and returns null if it's not found.

What I want to know is how I should handle the case where there may be message types received that the service isn't interested in consuming.

Currently, the `getRequestType` method signature doesn't accept a null value returned, and it logs a noisy warning whenever a message type is received that doesn't have a handler.

```csharp
var messageTypes = subscriptions.ToDictionary(x => x.Type, x => x.RequestType);

var rmqSubscriptions = subscriptions.ConvertAll(x => new RmqSubscription(
...
getRequestType: message => messageTypes.TryGetValue(message.Header.Type.Value, out var messageType) ? messageType : null!,
...));
```

In both the Proactor and the Reactor the getRequestType function is used like this:

```csharp
var requestType = _mapRequestType(message);
if (requestType == null)
throw new MessageMappingException($"Failed to find request type for message {message.Id} ",
new ArgumentNullException(nameof(requestType), "The request type cannot be null."));
```

It expects that requestType might be null, although the signature doesn't suggest that it should return null, and it always throws an exception, which gets logged as a warning. Useful if you've made a configuration error, but noisy if you just want to ignore certain message types.