Tuesday, 10 November 2009

Controlling WCF / protobuf-net at the Endpoint

protobuf-net has had WCF hooks for a long time, but (to be frank) they’ve been a tad…. flakey.

Firstly, if you use “mex” generated proxy classes, it has a habit of discarding key metadata (the “Order” of the [DataMember]). This can be resolved either by using a shared DTO assembly, or by using fixups in the partial class – but…

Secondly, even if you have a shared DTO assembly, if you use the standard “add service reference” (or svcutil.exe), it duplicates the service-contract interface, and it drops the [ProtoBehavior] marker. Again, you can get around this by using the WCF channel directly or subclassing ClientBase<T> – but it would be nice if this wasn’t necessary.

Additionally, it would be very nice if the protobuf-net aspect was configurable (app.config/web.config); allowing greater reuse, and allowing parallel endpoints (with/without protobuf-net, to support vanilla and protobuf-net aware clients).

The community to the rescue

Step up Scott Prugh, who an embarrassingly long time ago very kindly supplied me with all the cogs to make this happen. Apologies for the delay, but things have been hectic and when I tried to test it my WCF brain-cells were switched off (in short: I borked my tests, couldn’t get it working, and had to shelve it for a while).

So what’s the shiny? Simply: rather than decorating your operations with [ProtoBehavior], we can apply it in the configuration instead, by adding an endpoint-behavior that references a custom WCF extension (which we must also add). This sounds worse than it is'; inside “system.ServiceModel” (in your app.config/web.config, this means adding a few lines of boilerplate:


And then to both client and server, we simply specify the endpoint configuration:


(in more complex scenarios you might want to add the custom extension to your existing endpoint configuration).

What’s the catch?

In the most part, that’s it. WCF will use protobuf-net for any suitable objects (data-contracts etc). Note that this is a coarser brush than the per-operation control, though (you could always split the interface into different endpoints, of course).

Also, protobuf-net does have some subtle differences (especially regarding empty objects), so run your unit tests etc.

Note that it only works on the full-fat WCF; it won’t help Silverlight etc, since it lacks the extension features – but that isn’t new here.

Finally, the resolver in WCF is a pain, and AFAIK wants the full assembly details including version number; so one more thing to maintain when you get new versions. If anyone knows how to get around this?


A minimal (but working) example is in the repo (client, server). This uses a shared DTO assembly (to avoid the need for fixups) and is based on the WCF project template. I’ll try to go back and retrofit the Northwind example shortly, and perhaps include the WCF samples in a simple download.