One of the coolest thing in the newest version of MEF is scoping support. Not because I’ve participated in designing and eventually patented some aspects of it – not at all – but because it’s actually quite helpful.

Scopes, as you can imagine, allow you to define isolated “bubbles” of composition.

On the components side, we use an ExportFactory (should have been ScopeFactory, but dont get me started on this one) with CreationPolicy.NewScope (ughh) to achieve this behavior. On the hosting side, you need to define an hierarchy of CompositionScopeDefinition and give it to the container.

One thing that is particularly handy in IoC containers is supporting default services, but allow for overrides. Windsor even use this taxonomy. The following is a trick that uses scopes to achieve the same behavior.

Before saying that it’s easier, simpler, faster to achieve that using Container X, Y or Z, remember that MEF is all about federated systems, and that comes with challenges and constraints.

Here’s the trick: I define three scopes: Default, Overrides and App. All default services are bound to the Default scope. The app level components live in the App scope. In the middle, the Override scopes will be used to replace default services.

    using System.ComponentModel.Composition;
    using System.ComponentModel.Composition.Hosting;
    class Program
    {
        static void Main(string[] args)
        {
            var ubber = new AssemblyCatalog(typeof (Program).Assembly);
            var defLevelCat = ubber.Filter(cpd => cpd.ContainsPartMetadata("Scope", "Default"));
            var overLevelCat = defLevelCat.Complement.Filter(cpd => cpd.ContainsPartMetadata("Scope", "Override"));
            var appLevelCat = overLevelCat.Complement.Filter(cpd => cpd.ContainsPartMetadata("Scope", "App"));
            var appDef = new CompositionScopeDefinition(appLevelCat, new CompositionScopeDefinition[0],
                                                        appLevelCat.Parts.SelectMany(p => p.ExportDefinitions));
            var overDef = new CompositionScopeDefinition(overLevelCat, new [] { appDef },
                                                         overLevelCat.Parts.SelectMany(p => p.ExportDefinitions));
            var defDef = new CompositionScopeDefinition(defLevelCat, new[] { overDef },
                                                        defLevelCat.Parts.SelectMany(p => p.ExportDefinitions));
            var cont = new CompositionContainer(defDef);
            var entryPoint = cont.GetExportedValue<EntryPoint>();
            var sndLevel = entryPoint.Creator.CreateExport().Value;
            var lastLevel = sndLevel.Creator.CreateExport().Value;
            var component = lastLevel.com;
        }
    }
    [Export]
    [PartMetadata("Scope", "Default")]
    public class EntryPoint
    {
        [Import(RequiredCreationPolicy = CreationPolicy.NewScope, AllowDefault = true)]
        public ExportFactory<OverrideLevelScopeCreator> Creator { get; set; }
    }
    [Export]
    [PartMetadata("Scope", "Override")]
    public class OverrideLevelScopeCreator
    {
        [Import(RequiredCreationPolicy = CreationPolicy.NewScope, AllowDefault = true)]
        public ExportFactory<AppLevelScopeCreator> Creator { get; set; }        
    }
    [Export]
    [PartMetadata("Scope", "App")]
    public class AppLevelScopeCreator
    {
        [Import]
        public Component com { get; set; }
    }
    public interface IService
    {
    }
    [Export(typeof(IService))]
    [PartMetadata("Scope", "Default")]
    public class DefaultService : IService
    {
    }
    [Export(typeof(IService))]
    [PartMetadata("Scope", "Override")]
    public class OverrideService : IService
    {
        [Import(Source = ImportSource.NonLocal)]
        public IService ServParent { get; set; }
    }
    
    [Export, PartMetadata("Scope", "App")]
    public class Component
    {
        [Import]
        public IService Serv { get; set; }
    }

Have fun!

Weird post title, I know. But I refer to FAKE – F# Make.

It’s a nice and simple library to perform usual build related tasks. The fact that it’s pure F# makes it quite powerful – have you tried doing complex stuff with NAnt or MSBuild? If so, you know what I mean. Xml is *not* a programming language.

FAKE is simply organized as a collection of modules, each exposing related functions and|or operator overloads. So it’s not exposed in an OOP fashion, rather in a very flat public surface. That *may* make up for the lack of documentation. It’s somewhat easy to understand how things work by browsing the source – for those who are comfortable with that.

That said, in my first experience I couldn’t get the FileSet to generate any result. I also couldn’t find a way to turn on a more verbose logging. I went on a trial-and-error and realized the fileset was missing a BaseDir setting.

Since I’m a big fan of F# Project Extender, which adds metadata to define the order of the build items that are not recognized by MSBuild. So I managed to create a FscTask that takes that in consideration. In the process I noticed how FAKE lacks a few common NAnt features, such as resolving reference assemblies. I guess the author didn’t need them, which is fair in the OSS world. That said, a simple Fsc.exe invocation turned into a sizable task.

My fsc task is dirty and ugly, so beware. You can find it here. For now I’m sticking to Fake for MR builds and see how it goes.

Fun times! Since I’ve left MS I’ve been working on a real web site/backend/frontend – you know, doing actual work instead of long meaningless meetings, doc writing and one-on-ones with managers setting you up for failure (I wonder if I should make one of my ‘reviews’ public, so you’ll see the amount of BS I had to put up with).

WebSockets

Anyway, the first work was to create a websockets server. That was fun especially since F# offers its async workflow support and MailboxProcessor. It’s fun to think about it. While at MS, one of my friends there wouldn’t stop bitching about how F# is incredibly superior to C#. Gosh, he was right..

The company I’m doing the work for and I are discussing whether we will make the websocket server available commercially. It’s a PITA to implement the protocols, to test them and keep up, since the spec is in progress. So, sorry, won’t be doing all this work for free.

Ideas playground

On the MonoRail end, we’ve been using it extensively and coding up functionality as needed – by we I mean Henry and I. It’s been quite a journey to reassess all the design decisions in MR1/2, and also evaluate what’s out there. I’m sure we learn more from our mistakes than anything else. Mauricio is pushing some interesting ideas on web frameworks that are making me reevaluate our proposed API over and over again. Now that I’m quite familiar with FParsec, the idea of using combinators to compose forms (formlets) is enticing. It’s just strike me as something hard to expose in C#, we would need a different API and I’m sure it would be awfully verbose.

Castle.Blade = ++Razor

I love Razor’s simplicity and tooling support. Achieving simplicity is a major accomplishment, and the guys at the ASP.NET team did it. I remember when Scott Hunter gave us a preview of what at the time was code named Plan9. Awesome work!

I’ve then decided to make Razor the main view engine for MR3. However, when I started to experiment with a better API for our helpers library I bumped into some not-so-nice limitations from Razor. For example, I wanted to be able to express something like:

@Form.For(..., {
	// this is a block
        @builder.EditorFor(m => m.Name)
})

Well, Razor does support translating a mix of content and code block into a delegate – which is neat. The limitation is that they cannot be nested, and the parameter name is restricted to “item”

@Form.For(..., @{
        @item.EditorFor(m => m.Name)
})

The issue is that “item” is not very expression. I’ve spent many hours digging into Razor’s code trying for find a way to work-around this limitation (by using its extension points, not changing their code). At some point it was clear that coding up my own parser and translator would be easier. Castle Blade then came to fruition.

Blade intends to be 100% backward compatible with Razor, and introduces a few (one?) different transition marks to overcome Razor’s limitations. For the example above, we would use:

@Form.For(..., @=> builder {
        @builder.EditorFor(m => m.Name)
})

The @=> transition signals that a delegate will be created for the block, and the parameter name is the one that follows. In theory more than a single parameter is supported.

We also support nested blocks, which allows for the something like the following:

@Form.For(..., @=> builder {
        builder.FormTemplate(@=> t {
                <div>
                    @t.Label(): @t.Field()
                </div>                                                      
            });
 
        <fieldset id="contactForm">    
 
        @builder.TemplateFor( m => m.Name )
        @builder.TemplateFor( m => m.Email )
 
        </fieldset>
})

Feel free to give it a try.

MonoRail+++

MonoRail’s 3 goal is based on our my experience and perception of the “state of the union” and trends. If I could put them in three simple statements:

I’m mentally tired of crafting web sites despite huge functionality overlaps (combine/compose). I’m tired of REST being an afterthought to existing websites (rest support from the beginning). I’m tired of frameworks created by people without *actual* website building experience (frictionless).

The goals/roadmap/value-proposition were discussed in the past in our development list.

• Since our underlying runtime (CLR) is keen on static typing then fully embrace it
• Move forward: embrace HTML 5
• Simplify special render for different form factors
• Strive for simplicity, but no simpler

I’ll dive into what’s been done in practice to address each of the above bullets in upcoming blog posts.

Thanks for Quora + Reflector + MSDN, I’ve figured out how to expose extension methods from F#

1. Create a public module within a namespace
2. Add the extension method as a function
3. Decorate both the module and the function(s) with the ExtensionMethodAttribute

namespace Castle.MonoRail
 
[<System.Runtime.CompilerServices.ExtensionAttribute>]
module public ExtensionMethods = 
 
    open Castle.MonoRail.Routing
    open Castle.MonoRail.Hosting.Mvc
 
    [<System.Runtime.CompilerServices.ExtensionAttribute>]
    let Match(router:Router, path:string) = 
        router.Match(path, MonoRailHandlerMediator())

Long long time ago I promised a few people in my twitter that someday I’d post somewhere a sample on how to deal with “dependency hell”.

By that I mean something you’ve probably experienced yourself. Suppose you’re happily using log4net, and then you start using NHibernate which happens to use a different version of log4net. Hmmm. Easy. You can switch your own copy to the NHibernate uses. Then you add another dependency to your project, say SupperCoolWidget, and it happens to use yet another version of log4net.

You can rely on binding redirects, as long as you’re damn sure the API surfaced touched by these projects haven’t changed on the dependency (in this case log4net), otherwise you’ll get exceptions in runtime (cannot find member).

IMHO it’s especially problematic to rely on binding redirects because some code, somewhere, isn’t used very often, and in some special circumstance it may try to use an API that isn’t there.

log4net is an interesting example, but the problem applies to any scenario where you have common dependencies in different versions.

Looking at another camp – Java in particular – OSGi brought some interesting ideas to this very situation. There it’s even worse since jars are way more loose than assemblies. OSGi’s solution is to either have independent versioned bundles which your bundle may explicit say it depends upon, _or_ your bundle carries everything it needs to work. Multiple bundles can be loaded and executed in a single VM, and they are guarantee to not step on each others foot.

Java’s enabler to this magic is the ClassLoader.

In .NET there isn’t a concrete equivalent of the Class loader, but we have a loader. And it has different contexts. In fact, as many as you want. The Load and LoadFrom contexts are the typical ones you’re exposed to. More resources: Choosing a Binding Context and LoadFile vs LoadFrom

By using a combination of the right loading context and the AssemblyResolve event, you can achieve the behavior of isolated silos loading the same assemblies (with different versions) in the same AppDomain.

I created a sample to demonstrate the idea and you take it from there. I’ve tried to minimize the concepts, so no MEF, no Windsor, and it’s not a web app. The file structure is like the following

The build folder contains the app, which doesn’t do much:

private static readonly CustomBinder _binder = new CustomBinder();

static void Main()
{
    var curDir = AppDomain.CurrentDomain.BaseDirectory;

    // Each module is loaded in its own isolated context
    // so they can have conflict dependencies and work
    var modules = LoadModules(_binder, curDir);

    foreach (var module in modules)
    {
        Console.WriteLine(module);
    }
}

So it loads “modules” in a kind of late bound way, using a well-known contract: IModule.

Each module implementation depends on – guess what – log4net. But different versions of it. Each implementation of IModule looks pretty much the same, but the dependency version is quite different:

namespace FakeMod1
{
    using WellKnownContracts;

    public class Mod1Impl : IModule
    {
        private static log4net.ILog logger = log4net.LogManager.GetLogger(typeof(Mod1Impl));

        public Mod1Impl()
        {
            logger.Info("constructed");
        }
    }
}

namespace FakeMod2
{
    using WellKnownContracts;

    public class Mod2Impl : IModule
    {
        private static log4net.ILog logger = log4net.LogManager.GetLogger(typeof(Mod2Impl));

        public Mod2Impl()
        {
            logger.Info("Mod2Impl constructed");
        }
    }
}

When we run the app we expect the following to happen

• module 1 is found
• A logical context is created for it
• Each dependency within the module 1 is satisfied within the boundary
• module 2 is found
• ditto ditto..

Running the app and watching the debugger confirms the expected behavior:

‘ParallelContexts.vshost.exe’: Loaded ‘ParallelContexts.exe’, Symbols loaded.

‘ParallelContexts.vshost.exe’: Loaded ‘WellKnownContracts.dll’, Symbols loaded.

‘ParallelContexts.vshost.exe’: Loaded ‘modules\mod1\FakeMod1.dll’, Symbols loaded.

‘ParallelContexts.vshost.exe’: Loaded ‘modules\mod1\log4net.dll’

log4net:ERROR No appenders could be found for logger (FakeMod1.Mod1Impl).

log4net:ERROR Please initialize the log4net system properly.

‘ParallelContexts.vshost.exe’: Loaded ‘modules\mod2\FakeMod2.dll’, Symbols loaded.

‘ParallelContexts.vshost.exe’: Loaded ‘modules\mod2\log4net.dll’

Notice that WellKnownContracts.dll isn’t loaded more than once, since it first loaded in the Load context, it’s always found – I’m a bit unsure if this one is even probed after it’s loaded for the first time.

How it works?

The code is simple. The class CustomBinder takes a “module folder”, and starts a new logical context for it.

public partial class CustomBinder : IDisposable
{
    ...


    public BindingContext Add(string modFolder)
    {
        var ctx = new BindingContext(this);
        var files = Directory.GetFiles(modFolder);

        string entryPointFromManifest = null;

        foreach (var file in files)
        {
            // manifest has an entry point which is the first type/assembly we load
            // this is just an optimization, so we dont have to load all assemblies found within a package/module
            if (Path.GetFileName(file).Equals("manifest.xml", StringComparison.InvariantCultureIgnoreCase))
            {
                entryPointFromManifest = GetEntryPointFromManifest(file);
                continue;
            }

            if (!file.EndsWith(".dll")) continue;

            var name = AssemblyName.GetAssemblyName(file);
            ctx.AddAssemblyName(name.Name, file);
        }

        if (entryPointFromManifest != null)
        {
            string[] split = entryPointFromManifest.Split(',');
            Debug.Assert(split.Length == 2);
            ctx.EntryPointTypeName = split[0];
            ctx.PreLoad(split[1].TrimStart());
        }

        return ctx;
    }

Then, whenever the loader probes for an assembly, we use the “requesting assembly” to bring the existing context back, and use it to load the right assembly.

private Assembly CurrentDomain_AssemblyResolve(object sender, ResolveEventArgs args)
{
    if (args.RequestingAssembly == null)
        return null;

    BindingContext ctx = GetBindingContext(args.RequestingAssembly);
    if (ctx == null) return null;

    Assembly assembly;
    if (ctx.TryGetAssembly(new AssemblyName(args.Name), out assembly))
    {
        return assembly;
    }

    return null;
}

As I mentioned before, this is just a proof of concept that shows what is possible. The sky is the limit for modular/composable frameworks out there. Enjoy!

Download sample