1. В чем разница между subscriptionClient.AbandonAsyn c против subscriptionClient.closeAsyn c.
Согласно моим исследованиям и тестам, метод subscriptionClient.AbandonAsync используется для определения службы шина, что сообщение оставлено, и оно должно обработать сообщение снова. Как только DeliveryCount сообщения достигает MaxDeliveryCount, сообщение перемещается в DLQ с указанием кода причины MaxDeliveryCountExceeded. Для получения более подробной информации, пожалуйста, обратитесь к документу 
Метод subscriptionClient.closeAsync используется для закрытия subscriptionClient, тогда ваш клиент не может получить сообщение от подписка.
Мой тест:
Код:
namespace CoreReceiverApp
{
using System;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Azure.ServiceBus;
class Program
{
const string ServiceBusConnectionString = "<your_connection_string>";
const string TopicName = "<your_topic_name>";
const string SubscriptionName = "<your_subscription_name>";
static ISubscriptionClient subscriptionClient;
public static async Task Main(string[] args)
{
subscriptionClient = new SubscriptionClient(ServiceBusConnectionString, TopicName, SubscriptionName);
Console.WriteLine("======================================================");
Console.WriteLine("Press ENTER key to exit after receiving all the messages.");
Console.WriteLine("======================================================");
// Register subscription message handler and receive messages in a loop
RegisterOnMessageHandlerAndReceiveMessages();
Console.ReadKey();
await subscriptionClient.CloseAsync();
}
static void RegisterOnMessageHandlerAndReceiveMessages()
{
// Configure the message handler options in terms of exception handling, number of concurrent messages to deliver, etc.
var messageHandlerOptions = new MessageHandlerOptions(ExceptionReceivedHandler)
{
// Maximum number of concurrent calls to the callback ProcessMessagesAsync(), set to 1 for simplicity.
// Set it according to how many messages the application wants to process in parallel.
MaxConcurrentCalls = 1,
// Indicates whether MessagePump should automatically complete the messages after returning from User Callback.
// False below indicates the Complete will be handled by the User Callback as in `ProcessMessagesAsync` below.
AutoComplete = false
};
// Register the function that processes messages.
subscriptionClient.RegisterMessageHandler(ProcessMessagesAsync, messageHandlerOptions);
}
static async Task ProcessMessagesAsync(Message message, CancellationToken token)
{
// Process the message.
Console.WriteLine($"Received message: SequenceNumber:{message.SystemProperties.SequenceNumber} Body:{Encoding.UTF8.GetString(message.Body)}");
// This can be done only if the subscriptionClient is created in ReceiveMode.PeekLock mode (which is the default).
await subscriptionClient.AbandonAsync(message.SystemProperties.LockToken);
Console.WriteLine(message.SystemProperties.DeliveryCount);
}
static Task ExceptionReceivedHandler(ExceptionReceivedEventArgs exceptionReceivedEventArgs)
{
Console.WriteLine($"Message handler encountered an exception {exceptionReceivedEventArgs.Exception}.");
var context = exceptionReceivedEventArgs.ExceptionReceivedContext;
Console.WriteLine("Exception context for troubleshooting:");
Console.WriteLine($"- Endpoint: {context.Endpoint}");
Console.WriteLine($"- Entity Path: {context.EntityPath}");
Console.WriteLine($"- Executing Action: {context.Action}");
return Task.CompletedTask;
}
}
}
