WCF channels, proxies, and addresses

These days I am building a tiny distributed application that communicates via inherent one-way messages but optionally correlates replies with possible request messages. The transport being leveraged is MSMQ.
The actual reply endpoint address for the service that receives the request message is embedded in a message header. Yes, a default address may already be defined in the configuration of the endpoint, but in our case it changes at runtime. There is one queue for the incoming messages to be processed by the service - and another queue for the service to send back e.g. status or report messages from which the client can get updates on where the service-side processing is at.

 

Oops, did I mention that I am building this app with WCF? Ah, OK, now I did.

 

Now, in the WCF service that receives request messages for processing purchase orders, I want to create a proxy to communicate back to the client (or better: to the queue for sending messages which will then be processed by client(s)).
If I create a proxy like this:

 

 ChannelFactory<IOrderStatus> fac =
   new ChannelFactory<IOrderStatus>("MsmqClient");
 IOrderStatus proxy = fac.CreateChannel();

 

Everything looks fine. I can use the proxy object and proceed calling the operation on the interface - great. But wait...

The return endpoint address where to send the reply messages is hidden in a message header (materialized as a SOAP header on the wire). When reading this value from the header (more on this in another post) I need to be able to set the dynamically retrieved URI of the endpoint in my code. Hm, nothing really hard to do. Look here:

 

  ((IClientChannel)proxy).RemoteAddress = ...

 

Erm, it is read-only. OK. Next try:

 

  ((IClientChannel)proxy).RemoteAddress.Uri = ...
 
Ha, it is read-only as well... and now?

 

Man, there must be way, right?
When investigating a bit further we dig out that we are able to repoint the endpoint URL of a proxy if it extends ClientBase, but you unfortunately we can't do it with IClientChannel.

The problem here is that proxies are bound to the channels when created. We cannot repurpose a proxy without creating a new repurposed channel underneath. However, we can modify the factory and get back a modified proxy to use.

 

  ChannelFactory<IOrderStatus> fac =
    new ChannelFactory<IOrderStatus>("MsmqClient");
  IOrderStatus proxy = fac.CreateChannel();
  // ... do the work you need to do ...

  // Eventually we have a new endpoint adress!
  fac.Description.Endpoint.Address = new EndpointAddress("<my new address>");
  proxy = fac.CreateChannel();  

 

Or in a lot of scenarios even more desirable, is to modify the factory config info beforehand creating any proxy instance and then call CreateChannel.

 

  ChannelFactory<IOrderStatus> fac =
    new ChannelFactory<IOrderStatus>("MsmqClient");
  fac.Description.Endpoint.Address = new EndpointAddress("<my new address>");
  IOrderStatus proxy = fac.CreateChannel();

 

Lessons learnt:
A WCF proxy is a light-weight, fly-away beast - the channel is the heavy monster.

Comments

[John Cavnar-Johnson]

They really didn't intend to make messaging apps easy, did they?

[Javier G. Lozano]

Great info! I tried doing the same thing a while back using the ClientBase object. You can see this example here: http://blogs.thinktecture.com/cweyer/archive/2006/01/07/414354.aspx It's nice to know that there's another way of doing this without the need of a proxy.

[Javier G. Lozano]

I'm sorry, I posted the URL to your post, not mine. Here's the link in case you want to take a look at it: http://www.lozanotek.com/archive/2005/10/18/4305.aspx

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