C# 4 and .NET 4

Transactional Queues ❘ 1377
to that message holds the ID of the originating message as its correlation ID. The messages from the
acknowledgment queue can be read using MessageQueue.ReceiveByCorrelationId() to receive
the associated acknowledgment.
Instead of using acknowledgments, the dead-letter queue can be used for messages that didn’t arrive at
their destination. With the UseDeadLetterQueue property of the Message class set to true, the message is
copied to the dead-letter queue if it didn’t arrive at the target queue before the timeout was reached.
Timeouts can be set with the Message properties TimeToReachQueue and TimeToBeReceived.
response queues
If more information than an acknowledgment is needed from the receiving application, a response queue can
be used. A response queue is like a normal queue, but the original sender uses the queue as a receiver and the
original receiver uses the response queue as a sender.
The sender must assign the response queue with the ResponseQueue property of the Message class. The
sample code here shows how the receiver uses the response queue to return a response message. With
the response message responseMessage, the property CorrelationId is set to the ID of the original
message. This way the client application knows to which message the answer belongs. This is similar to
acknowledgment queues. The response message is sent with the Send() method of the MessageQueue object
that is returned from the ResponseQueue property:
public void ReceiveMessage(Message message)
{
Message responseMessage = new Message("response");
responseMessage.CorrelationId = message.Id;
}
message.ResponseQueue.Send(responseMessage);
TransaCTional queues
With recoverable messages, it is not guaranteed that the messages will arrive in order and just once. Failures
on the network can cause messages to arrive multiple times; this happens also if both the sender and receiver
have multiple network protocols installed that are used by Message Queuing.
Transactional queues can be used when these guarantees are required:
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Messages arrive in the same order they have been sent.
Messages arrive only once.
With transactional queues, a single transaction doesn’t span the sending and receiving of messages. The
nature of Message Queuing is that the time between send and receive can be quite long. In contrast,
transactions should be short. With Message Queuing, the first transaction is used to send the message into
the queue, the second transaction forwards the message on the network, and the third transaction is used to
receive the messages.
The next example shows how to create a transactional message queue and how to send messages using a
transaction.
A transactional message queue is created by passing true with the second parameter of the MessageQueue.
Create() method.
If you want to write multiple messages to a queue within a single transaction, you have to instantiate a
MessageQueueTransaction object and invoke the Begin() method. When you are finished with sending
all messages that belong to the transaction, the Commit() method of the MessageQueueTransaction object
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