Sample Azure Function in .NET 5 using IAsyncCollector<T>to write multiple messages to a service bus

IAsyncCollector is currently not supported with out-of-process / worker process SDK according to the documentation:

Because .NET isolated projects run in a separate worker process, bindings can't take advantage of rich binding classes, such as ICollector, IAsyncCollector, and CloudBlockBlob. There's also no direct support for types inherited from underlying service SDKs, such as DocumentClient and BrokeredMessage. Instead, bindings rely on strings, arrays, and serializable types, such as plain old class objects (POCOs).

The documentation is not clear about how to achieve it. This issue highlights that you can by having a dedicated class with the ServiceBusOutput attribute on a collection property to dispatch serialized multiple values as separate messages. For example:

[Function("OneToMany")]
public static DispatchedMessages Run([ServiceBusTrigger("myqueue",
    Connection = "AzureServiceBus")] string myQueueItem,
    FunctionContext context)
{
    // Generate 5 messages
    var messages = new List<MyMessage>();
    for (var i = 0; i < 5; i++)
    {
        var message = new MyMessage { Value = $"Message #{i}" };
        messages.Add(message);
    }

    return new DispatchedMessages
    { 
        Messages = messages.Select(x => JsonSerializer.Serialize(x)) 
    };
}

Where each message would be of the type MyMessage:

class MyMessage
{
    public string Value { get; set; }
}

And the function would return it as a DispatchedMessages type which has a property Messages that would represent all the messages that need to be sent out as serialized strings:

public class DispatchedMessages
{
    [ServiceBusOutput(queueOrTopicName: "dest", Connection = "AzureServiceBus")]
    public IEnumerable<string> Messages { get; set; }
}

Note that the connection string name is necessary as by default Functions SDK will try to use AzureWebJobsServiceBus connection string name.

Once the function is completed, there will be 5 messages in the dest queue, with JSON content of the serialized MyMessage messages.