Bnaya Eshet : MEF

MEF 2.0 - mini series: part 8 (Composition options and exception handling)

bnaya — Sat, 16 Feb 2013 14:05:57 GMT

MEF 2.0 - mini series: part 8 (Composition options and exception handling)

this is the last post in the MEF 2.0 mini series.
you can see other posts of this series in here.

this post will wrap-up the series with a quick survey to to the to some changes made for the underline composition process.

Exception

one of the most painful experience of MEF 1 was its misleading exception's description. some time it was really hard to figure out the exception roots. you can read more about MEF 1 issues in here.

MEF 2 was doing a much better job, but still some composition failure are misleading.
my recommendation is to use the IPartImportsSatisfiedNotification which can give you an interception point where you can figure out what went wrong, before MEF is throwing a composition exception.

the pattern that I'm using is to allow default on any imports and validate it on the OnImportsSatisfied method, see the next snippet:

Code Snippet
 public class Foo : IPartImportsSatisfiedNotification
{
    [Import(AllowDefault=true)]
    public IIdentity Identity { get; private set; }
 
    [ImportMany]
    public IReflect[] Reflects { get; private set; }
 
    public void OnImportsSatisfied()
    {
        if (Identity == null)
        {
            Trace.WriteLine("Composition failure (Identity)");
            throw new ArgumentNullException("Identity");
        }
 
        if (Reflects == null || Reflects.Length == 0)
        {
            Trace.WriteLine("Composition failure (Reflects)");
            throw new ArgumentNullException("Reflects");
        }
    }
}
 

Composition options

MEF 2 is exposing some composition tuning options that wasn't available on MEF 1.
there was some hidden composition assumptions which you couldn't modified.
I will focus on the DisableSilentRejection options.
silent rejection was added into MEF composition in order to support robust composition of ill functioning plug-ins. the requirement for this feature came from the Visual Studio team which is using MEF for their plug-in model.
anyway you might want to alter this behavior. MEF 2 doe's let you the option to disable it.
the original behavior will exclude the ill functioning plug-in from the composition (for import many) without throwing an exception.
this led to scenarios when you had a real hard time to figure out why some of the plug-in wasn't loading and still everything seem to be working.

the following snippet is demonstrating this scenario:

Code Snippet
 class Program
{
    static void Main(string[] args)
    {
        var p = new Program();
 
        var cat = new AssemblyCatalog(Assembly.GetExecutingAssembly());
        var container = new CompositionContainer(cat, CompositionOptions.DisableSilentRejection);
 
        try
        {
            container.ComposeParts(p);
            Console.WriteLine("{0} Plug-ins loaded", p.Plugins.Length);
        }
        catch (CompositionException)
        {
            Console.WriteLine("Composition error");
        }
    }
 
    [ImportMany]
    public Iplugin[] Plugins { get; set; }
}
 
[InheritedExport]
public interface Iplugin { }
 
public class BadPlugin : Iplugin
{
    [Import(typeof(IIdentity))]
    public IIdentity MissingDependency { get; set; }
}
 
public class FinePlugin : Iplugin { }
 

line 8: disabling the silent rejection behavior.
line 28 - 32: a class that is having a missing dependencies, therefore cannot be construct. on silent rejection mode (which is the default) it will be simply removed from the composition and the Plugins property at line 22 will get a single plug-in rather then 2.

Summary

MEF 2 is having improvements both in the engine and the API level.
you do have more control over the composition and better descriptive exception.

MEF 2.0 - mini series: Part 7 (Catalog filter and Deep hierarchic scoping)

bnaya — Thu, 24 Jan 2013 16:31:18 GMT

MEF 2.0 - mini series: Part 7 (Catalog filter and Deep hierarchic scoping)

this is the 7th post in the MEF 2.0 mini series.
you can see the following TOC for other posts in this series.

in the previous post I was talking about composition scoping and lifetime management.
on this one, I will extend the composition scoping topic toward hierarchic along with catalog filtering capability.

hierarchic scoping is not trivial, you must understand the hierarchic behavior and what it was design for.

MEF hierarchic was design to enable parts from higher scope to access lower scope's part without re-instantiation, lower part do not target higher scope's part directly.

each hierarchic scope construct by importing a ExportFactory<T> (which we saw in the previous post).

I will try to make it as simple as I can.

the following diagram demonstrate the idea of sub scope that interact with its parent scope's items.

let try to have a similar code structures.

I will use a plug-in base class to trace the instantiations:

Code Snippet
 public abstract class PluginBase
{
    private static int _globalId = 0;
    protected int _id;
 
    public PluginBase()
    {
        _id = _globalId++; // not thread-safe
    }
}
 

derived class's ids will be incremented per instantiation.

the following snippet include the classes which will be used for our hierarchic demonstration:

Code Snippet
 public class Level1ScopeRoot : PluginBase
{
    [Import]
    public Level1ItemA ItemA { get; private set; }
    [Import]
    public Level1ItemB ItemB { get; private set; }
}
 
public class Level1ItemShared : PluginBase { }
 
public class Level1ItemA : PluginBase
{
    [Import]
    public Level1ItemShared ItemZ { get; private set; }
}
 
public class Level1ItemB : PluginBase
{
    [Import]
    public Level1ItemShared SharedItem { get; private set; }
 
    [Import]
    public ExportFactory<Level2SubScopeRoot> SubScope { get; private set; }
}
 
public class Level2SubScopeRoot : PluginBase
{
    [Import]
    public Level2SubItem1 SubItem1 { get; private set; }
 
    [Import]
    public Level2SubItem2 SubItem2 { get; private set; }
}
 
public class Level2SubItem1 : PluginBase
{
    [Import]
    public Level1ItemShared ItemShared { get; private set; }
    [Import]
    public Level2SubItemShared SubItemShared { get; private set; }
}
 
public class Level2SubItem2 : PluginBase
{
    [Import]
    public Level2SubItemShared SubItem { get; private set; }
}
 
public class Level2SubItemShared : PluginBase { }
 

all the classes prefixed with the scoping 'Level'.

referring a sub-scope required using of ExportFactory<T> (line 23).

the following diagram is showing their hierarchic and dependencies graph:

our final step is to define a catalog per scope's hierarchic and to set hierarchic structure.

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForTypesDerivedFrom<PluginBase>()
    // add ScopeLevel metadata 
    .AddMetadata("ScopeLevel", t => t.Name.StartsWith("Level1") ? 1 : 2)
    .Export<PluginBase>();
 
var catalog = new AssemblyCatalog(typeof(Program).Assembly, picker);
 
var catalogL0 = catalog.Filter(p => p.ContainsPartMetadata("ScopeLevel", 1));
var catalogL1 = catalog.Filter(p => p.ContainsPartMetadata("ScopeLevel", 2));
 
var scopeL1 = new CompositionScopeDefinition(catalogL1, null);
var scopeL0 = new CompositionScopeDefinition(catalogL0, new[] { scopeL1 });
 
var container = new CompositionContainer(scopeL0);
 

at line 2-5, I export everything inherits from 'PluginBase' and adding a 'ScopeLevel' metadata with the right scope leveling value.

line 7, define a general assembly catalog (which will be the base for the filtered catalogs).

lines 9,10, are defining a filtered catalog for each scoping level (filtering an existing assembly catalog).

lines 12,13, defines the hierarchic between the scoped catalogs.

line 15, create a container with the top level scope.

now we're ready for a composition.

here you can download a sample code.

Summary

I consider this feature as MEF 2's most complex one.

Hierarchic can become complex, so use it with caution.

This Week On Channel 9

bnaya — Wed, 16 Jan 2013 06:59:33 GMT

This Week On Channel 9

my blog was manage to get on Channel 9 week's top developer news.

MEF 2.0 - mini series: part 6 (Composition scoping and lifetime management)

bnaya — Wed, 16 Jan 2013 06:21:01 GMT

MEF 2.0 - mini series: part 6

(Composition scoping and lifetime management)

this is the 6th post in the MEF 2.0 mini series.
you can see the following TOC for other posts in this series.

in this post I will cover a new concept of scoping and part lifetime management, which is a great improvement over MEF 1.

MEF 1 was coming with a fairly naïve lifetime management.
part's lifetime could be either shared or non-shared (you could also apply 'any' but eventually 'any' will be created as shared or non-shared).
shared is a singleton instantiation, while non-shared will create a new instance each time.

MEF 1's instantiation model doesn't support a complex scenario where some dependency's lifetime should be dictate by the lifetime of other unites.
you can conceder a UI window that is having plug-ins that should be dispose while the UI window is closing.

for example consider you have the following components (parts) dependency flow:

the application can have multiple processes.
each process is having some plugins, but each view should have different instantiation of the plugins.
until now you can argue that a non-shared instantiation will do the job.
the next dependency level (DAL) should be shared under the boundary of a process but shouldn't be shared across processes.
this one can neither handle by the share nor by non-shared instantiation.
single instance of DAL should be created under each view scope.

MEF 2 added a scoped lifetime management.
I will use the attribute based model for this sample, but it will work in the same way in the fluent model.

Code Snippet
 [Export]
public class Application
{
    [Import]
    private ExportFactory<ProcessA> ProcAFactory { get; set;}
    [Import]
    private ExportFactory<ProcessB> ProcBFactory { get; set;}
}
 
[Export]
public class ProcessA 
{
    [Import]
    public PluginA PlugA { get; private set; }
    [Import]
    public PluginB PlugB { get; private set; }
}
 
[Export]
public class ProcessB 
{
    [Import]
    public PluginA PlugA { get; private set; }
    [Import]
    public PluginB PlugB { get; private set; }
}
 
[Export]
public class PluginA 
{
    [Import]
    public DAL Dal { get; private set; }
}
 
[Export]
public class PluginB
{
    [Import]
    public DAL Dal { get; private set; }
}
 
[Export]
public class DAL
{
}
 

at line 4-7 you can see a new type of importing target called ExportFactory<T> this type will initialize the scope.
It is having a CreateExport method that return a ExportLifetimeContext<T> can control the part life time.

the full implementation of the Application class is:

Code Snippet
   [Export]
  public class Application
  {
      [Import]
      private ExportFactory<ProcessA> ProcAFactory { get; set;}
      [Import]
      private ExportFactory<ProcessB> ProcBFactory { get; set;}
 
      public void WriteLayoutA()
      {
          using (ExportLifetimeContext<ProcessA> lifeOfA = ProcAFactory.CreateExport())
          {
              ProcessA a = lifeOfA.Value;
              Console.WriteLine("Proc A");
              Console.WriteLine("\tPlug A: {0}", a.PlugA.GetHashCode());
              Console.WriteLine("\t\tDal: {0}", a.PlugA.Dal.GetHashCode());
              Console.WriteLine("\tPlug B: {0}", a.PlugB.GetHashCode());
              Console.WriteLine("\t\tDal: {0}", a.PlugB.Dal.GetHashCode());
          }
      }
 
      public void WriteLayoutB()
      {
          using (ExportLifetimeContext<ProcessB> lifeOfB = ProcBFactory.CreateExport())
          {
                ProcessBb = lifeOfB.Value;
              Console.WriteLine("Proc B");
              Console.WriteLine("\tPlug A: {0}", b.PlugA.GetHashCode());
              Console.WriteLine("\t\tDal: {0}", b.PlugA.Dal.GetHashCode());
              Console.WriteLine("\tPlug B: {0}", b.PlugB.GetHashCode());
              Console.WriteLine("\t\tDal: {0}", b.PlugB.Dal.GetHashCode());
          }
      }
  }
 

you can see the usage of ExportFactory<T> at lines 11 and 24.

having a lifetime handled part is the first step for the scoping.
now we can define which part will be managed by the scope.
for each scope we except to find a single instantiation of each scoped part.
as you can see in the diagram below, the Plugins and the DAL should be instantiate once for each scope.

in order to define what's goes within the scope we should use the CompositionScopeDefinition as shown in the following code snippet:

Code Snippet
 static void Main(string[] args)
{
    var scopeDependentCatalog = new TypeCatalog(
        typeof(ProcessA),
        typeof(ProcessB), 
        typeof(PluginA),
        typeof(PluginB),
        typeof(DAL));
    var scopeDefDependent = new CompositionScopeDefinition(scopeDependentCatalog, null);
 
    var appCatalog = new TypeCatalog(typeof(Application));
    var scopeDefRoot = new CompositionScopeDefinition(appCatalog, new[] { scopeDefDependent });
 
 
    var container = new CompositionContainer(scopeDefRoot);
 
    var app = container.GetExportedValue<Application>();
 
    app.WriteLayoutA();
    Console.WriteLine("------------------------------------");
    app.WriteLayoutB();
}
 

you can see the scope dependent definition at line 9
and the hierarchic between the scope and the application at line 12.

as you may notice it is possible to have deeper hierarchic, but this will be shown in future posts.

the output is:

remember that we was printing the part's hash code.
you can easily see that each process is having different plugin and DAL's instantiations, but even those the DAL was consumed by both plugins it has a single instantiation per scope.

Summary

scoping is a very powerful instantiation model which can solve some of the real-life scenario which wasn't fall into the shared or non-shared models.

in future post I will present a deeper hierarchic and the catalog filtering capability.

MEF 2.0 - mini series: part 5 (Fluent export properties)

bnaya — Tue, 15 Jan 2013 06:04:54 GMT

MEF 2.0 - mini series: part 5 (Fluent export properties)

this is the 5th post in the MEF 2.0 mini series.
you can see the following TOC for other posts in this series.

in this post I will cover the fluent property's export.

Exporting properties is a less known feature of MEF.
MEF 1 was supporting this feature by using the attribute model.
you could decorate a property with a [Export] attribute and then it become available for imports.

the following code demonstrate property exporting in MEF 1:

the Foo class is importing multiple SymmetricAlgorithm:

Code Snippet
 [Export]
public class Foo
{
    [ImportMany]
    public SymmetricAlgorithm[] CryptoProviders { get; private set; }
}
 

and the CryptoComposer class is exporting a few symmetric algorithms:

Code Snippet
 [Export]
public class CryptoComposer
{
    [Export]
    public SymmetricAlgorithm Aes { get { return new AesManaged(); } }
    [Export]
    public SymmetricAlgorithm TripleDES { get { return new TripleDESCryptoServiceProvider(); } }
    [Export]
    public SymmetricAlgorithm RC2 { get { return new RC2CryptoServiceProvider(); } }
}
 

with the following composition the Foo instance will have the CryptoComposer's symmetric algorithms:

Code Snippet
 var catalog = new AssemblyCatalog(typeof(Program).Assembly);
var container = new CompositionContainer(catalog);
 
var foo = container.GetExportedValue<Foo>();
 

so, how do we do it with the fluent export API?

to keep it simple I will leave the Foo class with the attribute model (we learned how to use fluent import in the previous post):

Code Snippet
 [Export]
public class Foo
{
    [ImportMany]
    public SymmetricAlgorithm[] CryptoProviders { get; private set; }
}
 
public class CryptoComposer
{
    public SymmetricAlgorithm Aes { get { return new AesManaged(); } }
    public SymmetricAlgorithm TripleDES { get { return new TripleDESCryptoServiceProvider(); } }
    public SymmetricAlgorithm RC2 { get { return new RC2CryptoServiceProvider(); } }
}
 

in order to export the CryptoComposer's properties you should use the following code:

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForType<CryptoComposer>()
    .ExportProperties(p => p.PropertyType == typeof(SymmetricAlgorithm));
 
var catalog = new AssemblyCatalog(typeof(Program).Assembly, picker);
var container = new CompositionContainer(catalog);
 
var foo = container.GetExportedValue<Foo>();
 

summary

exporting properties using the fluent API is fairly straightforward.

MEF 2.0 - mini series: part 4 (Fluent Import)

bnaya — Sat, 12 Jan 2013 05:18:51 GMT

MEF 2.0 - mini series: part 4 (Fluent Import)

this is the 4th post in the MEF 2.0 mini series.
you can see the following TOC for other posts in this series.

in this post I will discuss the Fluent import API.

we was talking about Fluent Export and Constructor Injection in the previous posts. this post will complete the main scenarios of Import's fluent API.

I will demonstrate the import technique using the following classes:

Code Snippet
 public class Foo
{
    public Logger Log { get; set; }
}
 
public class Logger
{
    public void Write(string message) { /* ... */ }
}
 

we can Export both classes by one of the fluent techniques which we discuss in previous posts, for example:

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForType<Logger>()
    .Export<Logger>();
picker.ForType<Foo>()
    .Export();
 
var catalog = new AssemblyCatalog(typeof(Foo).Assembly, picker);
var container = new CompositionContainer(catalog);
 

but this is not enough, because Foo's Log property has no [Import] attribute, therefore it will be ignored by the composition.

using the fluent ImportProperties, you can introduce it into the composition :

Code Snippet
 picker.ForType<Foo>()
    .Export()
    .ImportProperties<Logger>(pi => pi.Name == "Log");
 

the property's Type is referred by the generics parameter while the Lambda used for filtering the target properties.

actually you can take it further and configure the import behaviors.
for example you can set the import to Allow Default:

Code Snippet
 picker.ForType<Foo>()
    .Export()
    .ImportProperties<Logger>(pi => pi.Name == "Log",
                (propInfo, builder) => builder.AllowDefault());
 

it's consider as best practice to Allow Default and implement IPartImportsSatisfiedNotification interface which is an interception point which occurs right after the composition.
the idea is to check the composition state within the OnImportsSatisfied()
method in order to verify the composition state.
It is true that MEF exceptions have made better, but it still not perfect and sometimes it is really hard to trace the root cause of the exception. implementation of IPartImportsSatisfiedNotification will make the tracing easier.

so what about Select Many?

let change our Foo class to the following code:

Code Snippet
 public class Foo: IPartImportsSatisfiedNotification
{
    public Logger[] Log { get; set; }
 
    public void OnImportsSatisfied()
    {
    }
}
 

the following code will define the Log property as [ImportMany]:

Code Snippet
 picker.ForType<Foo>()
    .Export()
    .ImportProperties<Logger>(pi => pi.Name == "Log",
                (propInfo, builder) => builder.AsMany());
 

as you can see we are using the same overload that can manipulate the Import's behaviors.

by the same technique you can define a re-composition and the creation policy:

Code Snippet
 picker.ForType<Foo>()
    .Export()
    .ImportProperties<Logger>(pi => pi.Name == "Log",
                (propInfo, builder) =>
                {
                    builder.AsMany();
                    builder.AllowRecomposition();
                    builder.RequiredCreationPolicy(CreationPolicy.NonShared);
                });
 

summary

fluent Import is complimentary to fluent Export.
it's fairly flexible API which enable to assign your own or 3rd party class's properties.

actually you can take a 3rd party class and inject it's public properties through those techniques.

MEF 2.0 - mini series: part 3 (Fluent import constructor)

bnaya — Wed, 09 Jan 2013 21:09:10 GMT

MEF 2.0 - mini series: part 3 (Fluent import constructor)

this is the 3rd post in the MEF 2.0 mini series.
you can see the following TOC for other posts in this series.

in this post I will discuss the fluent constructor injection API.

Constructor Injection

the following code is having a Logger and Worker class which is having 2 constructor (one has no parameters and the other is getting ILogger):

Code Snippet
  
public class Logger: ILogger
{
    public void Write(string message) { /* ... */}
}
 
public class Worker
{
    private ILogger _logger;
 
    public Worker()
    {
        /* ... */
    }
 
    public Worker(ILogger logger)
    {
        _logger = logger;
    } 
}
 

the question is, which of the construction will be invoked by the following code?

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForTypesDerivedFrom<ILogger>()
    .Export<ILogger>();
picker.ForType<Worker>()
    .Export();
 
var catalog = new AssemblyCatalog(typeof(Program).Assembly, picker);
var container = new CompositionContainer(catalog);
container.Compose(new CompositionBatch());
var w = container.GetExportedValue<Worker>();
 

the code Export the Logger and the Worker but doesn't define explicitly which of the Worker's construction should be invoke.

the answer is that it will invoke the constructor that is getting the ILogger parameter, but could we be more precise?

actually we do have 2 different options.

first option is to use SelectConstructor before the Export method.
see the following code:

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForTypesDerivedFrom<ILogger>()
    .Export<ILogger>();
picker.ForType<Worker>()
    .SelectConstructor(builder => new Worker(builder.Import<ILogger>()))
    .Export();
 
var catalog = new AssemblyCatalog(typeof(Program).Assembly, picker);
var container = new CompositionContainer(catalog);
container.Compose(new CompositionBatch());
var w = container.GetExportedValue<Worker>();
 

this way we can define an expression that define which constructor should be invoke.
the expression is getting a ParameterImportBuilder which I used to define the logger (builder.Import<ILogger>()).

the next option is to set the SelectConstructor after the Export.
now we are getting a ConstructorInfo[] and we can select a single constructor out of this array.
ConstructorInfo

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForTypesDerivedFrom<ILogger>().Export<ILogger>();
picker.ForType<Worker>()
    .Export()
    .SelectConstructor(ctors => 
        ctors.First(info => info.GetParameters().Length == 1));
 

the code select a constructor which is having a single parameter, but the real implementation is up to you. you can ask on whatever constructor's aspect which you like to.

Summary

as cool as it is to control the constructor injection using the fluent API,
you may consider to use imported properties rather than constructor injection because it can be more stable is during the composition state and can apply on more scenarios, including implementation of IPartImportsSatisfiedNotification.

MEF 2.0 - mini series: part 2 (Fluent and Conventional Export)

bnaya — Sun, 06 Jan 2013 14:51:05 GMT

MEF 2.0 - mini series: part 2 (Fluent and Conventional Export)

this is the second post in the MEF 2.0 mini series.
you can see the following TOC for more.

in this post I will start to cover the fluent discovery model and the idea of discovery by convention.

traditionally, out of the box, MEF come with a single discovery model, which was the Attribute model.

it is true that you could extend MEF's discovery model by using either a custom catalog or a custom export provider, but this task wasn't as simple as the MEF 2 fluent discovery model and therefore wasn't a common practice.

Registration Builder

MEF 2.0 is having the new RegistrationBuilder class which can be used to apply a custom picker (filter) for specific types within the scope of a catalog.

the idea is that the catalog will apply a sandbox which is a searching area scope (like Assembly, Directory, ext...), while the registration builder will pick up the relevant exported part within the scope.

actually the catalog will compose both the old attribute model and the picked up types.

this technique can be used either as a replacement for the Attribute model or as a complementary for 3rd party types which we cannot recompile with the [Export] attribute.

in order to work with the registration builder, you should add reference to

System.Reflection.Context
System.ComponentModel.Composition
System.ComponentModel.Composition.Registration

at its simplest form it can be used to append a specific type into the composition.

Code Snippet
 static void Main(string[] args)
{
    Program p = new Program();
 
    var picker = new RegistrationBuilder();
    picker.ForType<AesManaged>()
        .Export<SymmetricAlgorithm>();
 
    var catalog = new TypeCatalog(
        new []{typeof(AesManaged)}, picker);
 
    var container = new CompositionContainer(catalog);
    container.ComposeParts(p);
}
 
[Import]
private SymmetricAlgorithm Crypto { get; set; }
 

the above code exporting the AesManaged cryptography algorithm as a symmetric crypto algorithm.

currently the Import is still using the old Attribute model. I will show you how to define a fluent Imports in future post.

another aspect that you can control is the instantiation strategy by using the SetCreationPolicy:

Code Snippet
 var picker = new RegistrationBuilder();
picker.ForType<AesManaged>()
    .Export<SymmetricAlgorithm>()
    .SetCreationPolicy(CreationPolicy.NonShared);
 

in future post I will talk about creation scope and lifetime, which is a fine tuning option for the part lifetime control.

Export inheritance

you can choose to export anything that derived from a Type or implement an Interface:

Code Snippet
 picker.ForTypesDerivedFrom<SymmetricAlgorithm>()
    .Export<SymmetricAlgorithm>()
    .SetCreationPolicy(CreationPolicy.NonShared);
 

Export by conventions

a common IoC strategy is to discover extensions by convention.
for example loading all the type with a 'Plugin' suffix.
this strategy reduce the maintenance of either xml files or bootstrapper code.

this type of discovery made easy in MEF 2.0.

the following code is loading any type which is ending with a 'Plugin' extension into the composition.

Code Snippet
 picker.ForTypesMatching(pl => pl.Name.EndsWith("Plugin"))
    .ExportInterfaces();
 

as you can see that the code is exporting all of the interfaces that was implemented by the plugins.

foe example, if you are having the following interfaces and implementation:

Code Snippet
 public interface ILogPlugin 
{
    void Write(string message);
}
public class LoggerPlugin : ILogPlugin
{
    public void Write(string message) { /* ... */}
}
 
public interface ISavePlugin 
{
    void Save(string message);
}
public class FilePlugin : ISavePlugin
{
    public void Save(string message) { /* ... */}
}
 

LoggerPlugin will be export as ILogPlugin and FilePlugin will be export as ISavePlugin.

ExportInterfaces is having a few overloads which you can use to refine the interface exporting, instead of exporting all of the implemented interfaces, you can define a convention, for example:

Code Snippet
 picker.ForTypesMatching(pl => pl.Name.EndsWith("Plugin"))
    .ExportInterfaces(t => t.Name.EndsWith("Plugin"));
 

Summary

MEF 2.0 fluent and conventional model is a great improvement over the Attribute model. as you can see you can use both model together, but the convention model can ease your maintenance.

in the next post I will discuss it further.

MEF 2.0 TOC

bnaya — Sun, 06 Jan 2013 14:30:49 GMT

MEF 2.0 TOC

MEF 2.0 is a reflection of a community requests.
features like Open Generics, fluent and conventional discovery, lifetime handling of the part, better exception handling and more, was all requested by the community.

you can read more about MEF 1 series:

MEF 2.0 - mini series: part 1 (Open Generics)

bnaya — Fri, 04 Jan 2013 08:33:21 GMT

MEF 2.0 - mini series: part 1 (Open Generics)

this is the first post of a new mini series about MEF 2.0.

each post of this mini series will target a single enhancement.

Open Generics

one of the most annoying missing feature in MEF 1 was the lack of support for Open generics.

the following code (which is fully functioning in MEF 2.0) didn't worked in MEF 1.

Code Snippet
 public class Foo
{
    [Import]
    public EventAggregator<int> IntAggregator { get; set; }           
}
 
[Export]
public class EventAggregator<T>
{
    public event Action<T> Notify = (item) => { };
    public void Send(T item)
    {
        Notify(item);
    }
}
 

as obvious as it may seem, this piece of functionality is missing in MEF 1.
if you try to do it in MEF 1 it will look like the following code:

Code Snippet
 public class Foo
{
    [Import]
    public EventAggregatorInt IntAggregator { get; set; }           
}
 
[Export]
public class EventAggregatorInt : EventAggregator<int> { }
 
public class EventAggregator<T>
{
    public event Action<T> Notify = (item) => { };
    public void Send(T item)
    {
        Notify(item);
    }
}
 

it is quite obvious that extra EventAggregatorInt class shouldn't have be been there in the first place.

Summary

Open Generics if fairly straight forward and it is keeping our code smaller and better.

MEF - Best Practice

bnaya — Thu, 28 Jul 2011 02:32:22 GMT

MEF - Best Practice

In this post I will offer MEF practice that may ease your MEF life.

one of the MEF headache is to understand what's goes wrong,

when MEF reject the composition, you can read more about

MEF diagnostic in this post.

Most of the obscure loading failure happens when your Export dependencies doesn't

satisfied. in those case you solely relay on the MEF error message which is not enough

in cases of deep dependencies graph.

It will normally occurs when you are using the Import attribute (not ImportMany).

let take the following test case:

Code Snippet
 public interface IFoo
{
    void Write();
}
 
[Export(typeof(IFoo))]
public class Foo : IFoo
{ 
}
 
public class Bar
{
    [Import]
    public IFoo Foo { get; private set; }
}
 

class Bar importing IFoo, which mean that the composition will fail whenever

MEF find no valid IFoo export or find more than single IFoo export.

we can use 2 patterns:

[ImportMany] which will handle both the case of no export and too many export.
[Import(AllowDefault=true)] which cover the case of no export.

using ImportMany and the IPartImportsSatisfiedNotification pattern.

we can validate the composition right after the composition stage.

Code Snippet
 public class Bar : IPartImportsSatisfiedNotification
{
    public IFoo Foo { get; private set; }
 
    [ImportMany]
    private IFoo[] FoosInternal { get; set; }
 
    public void OnImportsSatisfied()
    {
        if (FoosInternal == null || FoosInternal.Length == 0)
            throw new Exception("MEF composition find no IFoo");
        if (FoosInternal.Length > 1)
            throw new Exception("MEF composition find to many IFoo");
    }
}
 

at the OnImportsSatisfied method you can check whether

you got the composition right (instead of letting the MEF infrastructure

doing this job).

in most cases it is enough to ignore the to many IFoo composition case

and to apply the [Import(AllowDefault=true)] pattern.

Code Snippet
 public class Bar : IPartImportsSatisfiedNotification
{
    [Import(AllowDefault=true)]
    public IFoo Foo { get; private set; }
 
    public void OnImportsSatisfied()
    {
        if (Foo == null)
            throw new Exception("MEF composition find no IFoo");
    }
}
 

after the composition we can validate whether IFoo composition goes right.

Summary

MEF failure at the composition phase can be frustrating.

in this post I have tried to offer pattern to ease that pain.

SDP - MEF (Real-Life patterns)

bnaya — Fri, 18 Mar 2011 13:36:51 GMT

SDP - MEF (Real-Life patterns)

On the last day of the SDP at Crown Plaza Hotel, Tel Aviv.

I was lecturing about Real-Life scenario of MEF failures.

the lecture was focusing on diagnostic ,solutions and testing.

I want to thanks all attendant's, It was the after lunch session and

you were bravely kept your eyes open.

the presentation pdf can be found here.

you can use the following check-list as recommended diagnostic process.

Diagnostic Check-List

Is the instance included in the composition
Are the plug-in assemblies included in the deployment path
Are the plug-in dependencies included in the probing path
Is the Silverlight XAP included in the application
Use Strong Name and versioning policy
Do not refer to imported properties during construction time, use IPartImportsSatisfiedNotification
Check that IEnumerable<T>’s are decorated with [ImportMany] as long as they come from multiple exports.
Remember that errors in nested dependencies are sometimes obscured
Use AllowDefault=true when nulls are permitted
Use ComposeExportedValue for including mocks of your dependencies

SDP - My sessions

bnaya — Sun, 27 Feb 2011 15:33:18 GMT

SELA Group was having a live broadcast on the SDP conference content,

if you happens to be at Israel at between the 13-16.3 and considering to attend the conference,

the on demand recording can help you to choose which session best suit you.

the SDP agenda site: http://www.sela.co.il/s/sdp/_new/practice2011.html

my talk about the Parallel tutorial day and MEF session:

http://scc.sela.co.il/scc/pages/showlecture/showlecture.aspx?lectureid=487.

Dynamic MEF isn't thread safe by nature

bnaya — Sat, 04 Dec 2010 17:05:54 GMT

Dynamic MEF isn't thread safe by nature

this post focus a pitfall which you should be aware in order to avoid sporadic crashes.

In general most of MEF operation are operate once at the application initialization stage,

and it normally happens on a single thread.

this may not be true when you need to dynamically load MEF parts (using GetExportedValue or SatisfyImportsOnce).

unfortunately MEF underline is using non concurrent dictionary which is not thread safe for the above operation implementation.

therefore the following code will crash sporadically.

Code Snippet
 [Export] // exported type which will instantiate using MEF
public class ExportedType {}
 
public class Program
{
    static void Main(string[] args)
    {
        var catalog = new AssemblyCatalog (typeof(Program).Assembly);
        var container = new CompositionContainer(catalog);
 
        for (int i = 0; i < 100; i++)
        {
            new Thread(() =>
                {   // get the exported part dynamically (instantiate by MEF)
                    container.GetExportedValue<ExportedType>();
                }).Start();
        }
    }
}
 

line 15: GetExportedValue call is a non protected critical section.

Summary

whenever using dynamic MEF techniques you should put a critical section synchronization over it.

the code sample can be found here.

Credits

Eran Stiller was the first one who's point my intention to this issue.

Exporting non Exportable types

bnaya — Sat, 30 Oct 2010 19:23:56 GMT

Exporting non Exportable types

this post extend Glenn Block's post about

"Poco, Mef, and custom type systems. Are you ready to take the red pill?"

the post is adding a compile time attribute export model, Directory catalog

and migrate Glenn code to VS 2010.

It is very recommended to read Glenn Block's post before reading

this one.

the code sample for this post can be download from here.

Summarizing Glenn's post

in general Glenn show how to add attributes (like Export) to 3rd party type

which doesn't support it (it is cool technique which doesn't related directly to MEF).

Glenn show how to write catalog which can be aware of types extension.

What was added?

I took Glenn's source, fix it to run against the CLR 4.0,

then I added a directory catalog which aware of the

type extension and I also add a nice clean way of exposing the type

by using assembly level attribute.

the directory catalog is very straight forward so I wont cover it in the post

(you can download the source and see the implementation).

the following code show how to use the assembly level attribute:

Code Snippet
 [assembly: ComposablePartRegistry(typeof(MyNoneExportedType), typeof(IMyNoneExportedType))]
 

as you can see, all you have to do is to declare the exported type followed by the exported contract.

the attribute snippet:

Code Snippet
 [AttributeUsage (AttributeTargets.Assembly)]
public class ComposablePartRegistryAttribute : Attribute
{
    public ComposablePartRegistryAttribute(
        Type implementation, Type contract, 
        CreationPolicy policy = CreationPolicy.Any)
    { 
        Type t = typeof (ComposablePartType<,>);
        t = t.MakeGenericType(new[] {implementation, contract});
        Type = Activator.CreateInstance(t, policy) as TypeDelegator;
    }
 
    public TypeDelegator Type { get; private set; }
}
 

line 8-10: creating ComposablePartType type which later be used by the catalog.

Summary

this was small extension to Glenn's concept which can be very useful

in cases where you want to embrace 3rs party component into your MEF model.