Azure Function无法与QueueTrigger和非无效返回值一起使用？

Question

I published my 3 different versions of a function and the last one is where things start to go wrong. I think it's the combination of async, queue trigger and a return value, but I don't know for sure (and I can't find any documentation that helps me out).

V1: Out-of-box Visual Studio template

    [FunctionName("Function1")]
    public static void Run([QueueTrigger("myqueue-items", Connection = "")]string myQueueItem, TraceWriter log)
    {
        log.Info($"C# Queue trigger function processed: {myQueueItem}");
    }

Works as expected when I deploy & test.

V2: Change to async Task

    [FunctionName("Function1")]
    public static async Task([QueueTrigger("myqueue-items", Connection = "")]string myQueueItem, TraceWriter log)
    {
        await Task.Delay(1000);
        log.Info($"C# Queue trigger function processed: {myQueueItem}");
    }

Still good.

V3: Use Task<string>

    [FunctionName("Function1")]
    public static async Task<string> Run([QueueTrigger("myqueue-items", Connection = "")]string myQueueItem, TraceWriter log)
    {
        await Task.Delay(1000);
        log.Info($"C# Queue trigger function processed: {myQueueItem}");
        return new string(myQueueItem.Reverse().ToArray());
    }
}

Now when I test in the portal all I see is Status: 202 Accepted , no output, nothing logged, no error in log streaming, etc. Practically impossible to figure out what's going wrong other than this incremental guess-and-check that I did.

Any ideas what the issue is?

Answer 1

Queue trigger can't return any data to the caller: it's a queue after all.

You could use Output Binding feature, for instance to send the output to another queue. But then you need to annotate this binding with an extra attribute:

[FunctionName("Function1")]
[return: Queue("myqueue-output", Connection = "")]
public static async Task<string> Run([QueueTrigger("myqueue-items", Connection = "")]string myQueueItem, TraceWriter log)
{
    await Task.Delay(1000);
    log.Info($"C# Queue trigger function processed: {myQueueItem}");
    return new string(myQueueItem.Reverse().ToArray());
}