PrefetchCount not work as expected

Describe the bug

For some overloaded methods of IBus.Advanced.Consume , the prefetchcount parameter is not work as expected

To Reproduce

string MQConnectionStrFormat =
    "host={0};virtualHost={1};username={2};password={3};prefetchcount=10;publisherConfirms=true";

string conn = string.Format(MQConnectionStrFormat, "********", "****", "****","********");

var _bus = RabbitHutch.CreateBus(conn);

var queue=await _bus.Advanced.QueueDeclareAsync("testprefetch"
);
var exchange = _bus.Advanced.ExchangeDeclare("testprefetch", ExchangeType.Direct);
_=_bus.Advanced.Bind(exchange, queue, "testprefetch");


// publish test messages
Enumerable.Range(1, 20).ToList().ForEach( async i =>
{
    var msg = new Message<string>("prefetch msg " + i);
    await _bus.Advanced.PublishAsync(exchange, "testprefetch",false,msg);
});

then subscribe handlers in several ways

//work as expected , it consume 10 message per 5 seconds
_bus.Advanced.Consume(queue,  async (data, mp, mri) =>
{
     var message = Encoding.UTF8.GetString(data.ToArray());
     Console.WriteLine(message);
     await Task.Delay(5000);
     return AckStrategies.Ack;
 }, config =>
 {
     config.WithPrefetchCount(10);
 });

// not work as expected, it consume 1 message per 5 second
_bus.Advanced.Consume(queue,  async (data, mp, mri) =>
{
    var message = Encoding.UTF8.GetString(data.ToArray());
    Console.WriteLine(message);
    Task.Delay(5000).Wait();
    return AckStrategies.Ack;
}, config =>
{
    config.WithPrefetchCount(10);
});

// not work as expected, it consume 1 message per 5 second
_bus.Advanced.Consume(queue, (data, mp, mri) =>
{
    var message = Encoding.UTF8.GetString(data.ToArray());
    Console.WriteLine(message);
    Task.Delay(5000).Wait();
    return Task.FromResult(AckStrategies.Ack);
}, config =>
{
    config.WithPrefetchCount(10);
});

Only the first of the above methods can work as expected, which makes me very confused, Also, I dont know the difference between the prefetchcount parameter within the connection string and the WithPrefetchCount configured in the subscribe method, so i set both to 10…

I don’t know what caused the failure of the following two methods, which makes me very worried about unexpected problems in the program I wrote

So here again to summarize my question：

1. What could cause the prefetchcount parameter to not work?
2. wht is the difference between the prefetchcount parameter within the connection string and the WithPrefetchCount configured in the subscribe method?

Please complete the following information):

• EasyNetQ version: [e.g. 7.3.7]
• RabbitMQ version [e.g. 3.8.18]

[Pliner]

for my second question , after reading this issue https://github.com/EasyNetQ/EasyNetQ/issues/1467 , I guess the prefetchcount in the connection string is used to control the concurrent limit of messages sent by RabbitMQ to EasyNetQ for processing, and WithPrefetchCount method is used by EasyNetQ to set the concurrency limit to call Handler after receiving messages from rabbitmq , is my understanding correct?

I prefer to think about PrefetchCount as a count of messages which are prefetched from a queue without any knowledge of how many “threads” are used for processing. ConcurrencyLimit should be used to set up count of threads for processing.

Having said that, at the moment ConcurrencyLimit could be set only for all models of a connection, i.e. globally in case of EasyNetQ. RabbitMQ.Client needs to be patched to allow it on a per model basis.

[Pliner]

If you change code slightly, it will work as expected.

// 2nd example
_bus.Advanced.Consume(queue, async (data, mp, mri) =>
{
    await Task.Yield(); // Line changed
    var message = Encoding.UTF8.GetString(data.ToArray());
    Console.WriteLine(DateTime.UtcNow + " " + message);
    Task.Delay(5000).Wait();
    return AckStrategies.Ack;
}, config => { config.WithPrefetchCount(10); });

So the issue here is the following: if a message callback is executed synchronously(even if it is marked as async), then it could block dispatcher.

https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/6.x/projects/RabbitMQ.Client/client/impl/AsyncConsumerWorkService.cs#L153