Check out example codes for "azure service bus topic example c# .net core". It will help you in understanding the concepts better.

Code Example 1

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 the message pump should automatically complete the messages after returning from user callback.
        // False below indicates the complete operation is handled by the user callback as in ProcessMessagesAsync().
        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)}");

    // Complete the message so that it is not received again.
    // This can be done only if the subscriptionClient is created in ReceiveMode.PeekLock mode (which is the default).
    await subscriptionClient.CompleteAsync(message.SystemProperties.LockToken);

    // Note: Use the cancellationToken passed as necessary to determine if the subscriptionClient has already been closed.
    // If subscriptionClient has already been closed, you can choose to not call CompleteAsync() or AbandonAsync() etc.
    // to avoid unnecessary exceptions.
}

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