WCF and "Open" Operation Request

I wanted to write a contract with an operation that had the following signature:

public interface ITakeAnythingService
    [OperationContract(IsOneWay = true)]
    void TakeThis(object message);

The idea was to allow a client to post anything over to the server. Of course, this ended being a very bad idea, as I started getting all sorts of exception of the System.ServiceModel.CommunicationException kind. The error message was pretty verbose and actually contained information that started me down an alternative, successful path:

Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.

Hmmmm...KnownTypeAttribute. What's that? Well, to make a long story short, this post was very helpful in describing the nuances of KnownTypeAttribute, along with how you can configure known types. What I ended up doing is changing the contract like so:

public interface ITakeAnythingService
    [OperationContract(IsOneWay = true)]
    void TakeThis(Request message);

Where Request was defined like this:

public abstract class Request { }

Then I added the following configuration information:

<?xml version="1.0" encoding="utf-8" ?>
                <add type="ServiceDomain.Messages.Request, ServiceDomain">
                    <knownType type="ServiceDomain.Messages.AKnownRequest, ServiceDomain"/>
                    <knownType type="ServiceDomain.Messages.AnotherKnownRequest, ServiceDomain"/>

The key is the declaredTypes element. You can list all of the known types that derive from the base type that your service can handle. What I like about this is that it forces the client to make a request that derives from a known base type on the server side, and the server must know about this new type before it'll handle it. Usually I don't advocate an "open" design like this across the board. Messages should be carefully crafted and designed with a known structure upfront. That said, in this case I needed to make an exception to this rule-of-thumb as my service needed to be fairly open and configurable.

By the way, don't use System.Object as the "root" known type (i.e. the type defined in the type attribute of the add element). I don't remember what the error message was, but basically I wasn't allowed to do that. In retrospect, that was a good thing anyway as I was violating on of my core tenets of service-based design: one parameter in of a known base type and (if the operation is not one-way), one return value of a known base type out.

* Posted at 01.22.2007 07:06:54 AM CST | Link *

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