How do you access the dead letter sub-queue on an Azure subscription?

There is a convention naming for Dead letter queue using Azure Service Bus:

• For Service Bus Queue: queuePath/$DeadLetterQueue
• For Service Bus Subscription: topicPath/subscriptionName/$DeadLetterQueue

So you can access your dead letter queues the same way you access your queues.

From @SamVanhoutte answer, you can see that the ServiceBus framework provides methods to format the dead letter queue name:

• For Service Bus Queue: QueueClient.FormatDeadLetterPath(queuePath)

• For Service Bus Subscription: SubscriptionClient.FormatDeadLetterPath(topicPath, subscriptionName)

I wrote two small methods to create a message receiver for queue and subscription where you can set if you want the deal letter queue or not:

/// <summary>
/// Create a new <see cref="MessageReceiver"/> object using the specified Service Bus Queue path.
/// </summary>
/// <param name="connectionString">The connection string to access the desired service namespace.</param>
/// <param name="queuePath">The Service Bus Queue path.</param>
/// <param name="isDeadLetter">True if the desired path is the deadletter queue.</param>
public static MessageReceiver CreateMessageReceiver(string connectionString, string queuePath,
    bool isDeadLetter = false)
{
    return MessagingFactory.CreateFromConnectionString(connectionString)
        .CreateMessageReceiver(isDeadLetter
            ? QueueClient.FormatDeadLetterPath(queuePath)
            : queuePath);
}

/// <summary>
/// Create a new <see cref="MessageReceiver"/> object using the specified Service Bus Topic Subscription path.
/// </summary>
/// <param name="connectionString">The connection string to access the desired service namespace.</param>
/// <param name="topicPath">The Service Bus Topic path.</param>
/// <param name="subscriptionName">The Service Bus Topic Subscription name.</param>
/// <param name="isDeadLetter">True if the desired path is the deadletter subqueue.</param>
public static MessageReceiver CreateMessageReceiver(string connectionString, string topicPath,
    string subscriptionName, bool isDeadLetter = false)
{
    return MessagingFactory.CreateFromConnectionString(connectionString)
        .CreateMessageReceiver(isDeadLetter
            ? SubscriptionClient.FormatDeadLetterPath(topicPath, subscriptionName)
            : SubscriptionClient.FormatSubscriptionPath(topicPath, subscriptionName));
}

If you are using Microsoft.Azure.ServiceBus instead of Microsoft.ServiceBus, it is slightly different.

var deadQueuePath = EntityNameHelper.FormatDeadLetterPath(your_queue_name);
var deadQueueReceiver = new MessageReceiver(connectionString, deadQueuePath);

As per EntityNameHelper class in Microsoft.Azure.ServiceBus namespace, for topics, use the subscription path instead of your_queue_name.

The name of the queue, or path of the subscription.

/// <summary>
/// Formats the dead letter path for either a queue, or a subscription.
/// </summary>
/// <param name="entityPath">The name of the queue, or path of the subscription.</param>
/// <returns>The path as a string of the dead letter entity.</returns>
public static string FormatDeadLetterPath(string entityPath)
{
    return EntityNameHelper.FormatSubQueuePath(entityPath, EntityNameHelper.DeadLetterQueueName);
}

Does this approach work for you ?

MessagingFactory factory = MessagingFactory.CreateFromConnectionString(cnxString);
var deadLetterPath = SubscriptionClient.FormatDeadLetterPath(topicPath,subName);
var dlqReceiver = factory.CreateMessageReceiver(deadLetterPath, ReceiveMode.ReceiveAndDelete);

I haven't test it out here (in a meeting), but give it a try

cheers