DynamicProxies, C# 4.0 Keywords, and Cecil: A Vision

I'm going on vacation for nearly 2 weeks, starting this Wednesday, so my online time will probably be dimished for a while. Before I go, I wanted to outline some thoughts I had on a project I may pursue starting in 2009. This idea literally came to me as I was driving home from work late last week. I have no idea if this is a good idea or not - consider this post the "envisioning" portion of the project. It's all pie-in-the-sky with Visio-like drawings and whatnot, plus it may be based on incorrect evidence so don't take anything I say as "truth" (e.g. my descriptions of what's going in in 4.0)...but you have to start somewhere!

Consider the following C# 4.0 code:

dynamic x = new Something();
x.Configure(); 

If you've been following the new changes for C#, you know that dynamic is a new keyword. Essentially, what this does is it allows a C# developer to use members on an object dynamically. We have no idea if there is a method called Configure() available on x at compile-time; we're hoping that this will work at run-time. In fact, VS 2010 doesn't give you any Intellisense when you hit the period, because it can't. It has no idea what you could invoke on x. Note that the work that C# does underneath the scenes to get this to work is rather messy. Basically it creates a new CallSite class, provides it with a bunch of information, and then invokes the target. Feel free to write a small program in C# 4.0 and view it in ildasm if you're that curious to see this in action yourself.

Here's the thing. If your object implements IDynamicObject, then you can get notifications whenever the client wants to invoke [1]. This includes everything, even non-virtual calls.

This is of interest to me because I wrote my DynamicProxies framework years ago. And I've never been able to catch non-virtual calls. Now I can!

Well, there's a catch. Let's say I wrote my code like this:

// assume the "this" implement IInvocationHandler
dynamic x = new Something().CreateProxy(this);
x.Configure();

Assuming that I added the feature in DynamicProxies to implement IDynamicObject on the proxy (and of course giving the client the ability to enable or disable this feature whenever they wanted to), this would work. That is, if Configure() was non-virtual, I could now get the call. But this requires the user to switch from "dynamic" to "var" at the right time.

Enter MSBuild and Cecil.

Here's the idea. I create a post-build framework based on Cecil called Cecil.MSBuild (or something like that). It's a generic framework that allows developers to plug in various Cecil-based modifiers on the compilation result of C# (or VB or F# or any managed compiler). In this case, I'd create a plug-in that would look for any "CreateProxy" call, and replace subsequent invocations to that call as a "late-bound" dynamic call if the method is non-virtual, and keep it the same if it is virtual.

My guess is that this wouldn't be easy to pull off, but again, I'm just envisioning at this point :).

So my idea is this:

  • Create a new MSBuild-based framework using Cecil that would allow anyone the ability to modify an assembly post-build. Not too hard, just makes it a bit easier for .NET devs to add this modification business into a pre-existing MSBuild fle (which the .csproj and .vbproj files are).
  • Update DynamicProxies to implement IDynamicObject (based on a configuration value). All non-virtual calls would be handled by this interface; virtual calls would be deferred to the proxy implementation.
  • Create a post-build process that would change non-virtual calls to proxy-based objects to be "semi-dynamic". Virtual calls would still be done directly to the proxy. Non-virtual calls would be done against the MetaObject provided by the proxy's implementation of IDynamicObject. This would eliminate the need for developers to declare their variables as dynamic.

Let me know what you think.

[1] See http://blogs.msdn.com/cburrows/archive/2008/10/28/c-dynamic-part-ii.aspx for details.

* Posted at 12.21.2008 02:42:51 PM CST | Link *

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