Tuesday 15 June 2010

Extending ASP.NET MVC with custom binders and results

Recently, I was putting together a brief sample for someone who wanted to use protobuf-net with ASP.NET as part of a basic HTTP transport; fairly simple stuff – just using protobuf as the binary body of the HTTP request/response.

It was a simple enough demo, but it got me wondering: what would be required to do this (more cleanly) in ASP.NET MVC? In many ways we just want something pretty similar to how many people currently use JSON: forget the formal API – just tell me the routes and let me know what data I need.

It turns out that this is astonishingly easy…

The controller

A good way of understanding the big picture is to look at the controller. So let’s take a peek:


One very basic controller, with one action. Things to note:

  • We want to read the request parameter (“req”) from the request body
  • To do that, we’re using a custom binder, via [ProtoPost] which we’ll see shortly
  • We could have exposed different input arguments separately – it would have added complexity, and isn’t very compatible with the way that requests are written in the .proto language, so I’ve restricted myself to binding a single (composite) parameter
  • We run some logic, same as normal
  • We return an action-result that will write the response back as protobuf

The nice thing about this is that (no matter which option you use to control your routes) it is very simple to add actions (as different urls).

That might be all you need to know!

I’ll include more explanation for completeness, but if you want a simple way of throwing objects over http efficiently and in an interoperable way, that might be enough.

The binder

The binder’s job is it understand the incoming request, and map that into things the controller can understand, such as method variables. There are two parts to a binder: writing the code to handle the input, and telling the system to use it. Writing a binder, it turns out, can be pretty simple (well – if you already have the serialization engine available); here’s all there is:


This simply checks that the caller is doing a POST, and if they are it passes the input stream to protobuf-net, based on the parameter type (which is available from the binding-context).

The other part is to tell ASP.NET MVC to use it; in my case I’m using a new [ProtoPost] attribute which simply creates the binder:


The result

Just as a controller doesn’t need to know how to parse input, it also doesn’t know how to format output – that is the job of an action-result; we’ll write our own, that we can re-use to write objects to the response stream. Which is actually less work than writing the last sentence!


Again – some very basic code that simply takes an object and writes it to the output stream via protobuf-net.

Anything else?

OK, I’ve taken some shortcuts there. To do it properly you might want to check for an expected content-type, etc – and things like security is left open for your own implementation (just like it would be for any other similar object-passing API). I’m just showing the core object <===> stream code here. But it works, and is painless.

The example code (include the ASP.NET / IHttpHandler equivalent, and an example client) is all available in the protobuf-net trunk, here.