Bnaya Eshet : extension

Immutable Collections

bnaya — Sun, 17 Feb 2013 19:45:12 GMT

Immutable Collections

Immutability is a pattern which is suit well parallel programming,
but you have to be aware of a potential memory pressure risk when it's not implemented right or used wisely.

this post will cover a new BCL library (still in its preview stage) which is targeting immutable collections.

.NET is already having Concurrent implementation for Queue, Stack, Bug and Dictionary, which is thread-safe, but other type of collection like List is missing.
another type of collection are read only collection, but those type of collection are not truly thread-safe because it is just a wrapper upon other collection which can be mutate by it's owner.
so iterating on a read only collection doesn't guarantee thread-safety because the collection can be mutate during the iteration by it's original owner.
currently we do have one option to create an immutable collection by using the ToArray() extension method, but as I mentioned before it can lead to a memory pressure while using a large collection.

the BCL team was targeting this issue and are working on a new kind of collections which are immutable.
those collection is having a cool concept which guarantee that an immutable sequence will not changed, but it can be part of another immutable sequence. this mean that you can take an immutable collection and add an item. this will keep the immutable part as is and you will get a new collection that include your addition and the original collection, but if someone is using the original collection he won't be affected by your mutation.
as you can see the BCL immutable collection can be mutate but the mutation will return a new collection and leave the original collection untouched.

Start using the immutable collection

you can start using it though NuGet.
on the current preview version it includes:

ImmutableStack<T>
ImmutableQueue<T>
ImmutableList<T>
ImmutableHashSet<T>
ImmutableSortedSet<T>
ImmutableDictionary<K, V>
ImmutableSortedDictionary<K, V>

Fluent API

I will speak about 3 APIs:

Add
AddRange
Builder

Add

Code Snippet
 [Test]
public void List_AddItem_Test()
{
    var col = ImmutableList<int>.Empty;
    var col1 = col.Add(1);
    var col2 = col1.Add(2);
 
    Assert.AreEqual(0, col.Count);
    Assert.AreEqual(1, col1.Count);
    Assert.AreEqual(2, col2.Count);
}
 

whenever you want an empty instance of immutable collection you should use the static Empty property (there is no public constructor available).

you should always remember that Adding an item will not affect the original collection therefore you must handle the return value (is is the same concept as string.Replace).

finally you can notice at lines 8-10 that the above code was actually construct 3 different immutable collections.

AddRange

both from performance and usability it is better using the AddRange API when you intend to add multiple items.

Code Snippet
 [Test]
public void List_AddRange_Test()
{
    var col = ImmutableList<int>.Empty;
    var col1 = col.AddRange(Enumerable.Range(0, 1000));
 
    Assert.AreEqual(0, col.Count);
    Assert.AreEqual(1000, col1.Count);
}
 

Builder

if you want to efficiently add multiple items and AddRange doesn't do the job for you, you can use a Builder (the concept is quit similar to StringBuilder).
you should remember that unlike the immutable collection a builder is not a thread-safe structure, therefore you shouldn't share its state between threads.
when you complete the mutation, you can extract an immutable collection from the builder.

Code Snippet
 [Test]
public void List_Builder_Test()
{
    var col = Enumerable.Range(0, 1000).ToImmutableList();
    var builder = col.ToBuilder();
    for (int i = 0; i < 100; i++)
    {
        builder.Remove(i * 10);
    }
    var col1 = builder.ToImmutable();
    Assert.AreEqual(1000, col.Count);
    Assert.AreEqual(900, col1.Count);
}
 

line 5: getting a builder from immutable collection.
line 10: getting an immutable back from the builder (after the mutation).

Migration

if you are using a read only collection though the IReadOnlyCollection, IReadOnlyList or IReadOnlyDictionary, migration will be easy because the immutable collections are implementing those interfaces.

Optimization

we already spoke about the memory pressure optimization, but what is the characterize of those collection in compare with the well known BCL mutable collection?
as the BCL team argue that immutable collections have been heavily tuned for maximum performance and minimum GC pressure.

the following table was published by the BCL team.
it compare the performance behavior of the BCL immutable and immutable collection (and remember that this is only a preview version which mean that further optimization can be applied).

you can read more on this at this post.

summary

Immutability is a great pattern for parallelism.
the BCL team are giving us a robust and well design immutable collection.
the potential memory pressure has reduced by the internal data-structure design.
so it is definitely something to wait for its release.

you can read more about it at here.
and if you want more background and comparison to the read only collection, you can check out this post.

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.

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 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 - 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

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.

Post recommendation

bnaya — Tue, 01 Jun 2010 01:17:28 GMT

if you want to learn more about MEF backstage you can find

more information under the following post:

MEF: What and why

http://blogs.msdn.com/b/hammett/archive/2010/05/29/mef-what-and-why.aspx

C# IntelliSense extension for VS 2010 - UPDATE

bnaya — Tue, 06 Apr 2010 00:33:13 GMT

C# IntelliSense extension for VS 2010 – UPDATE

the extension has update and it is include the description.

enjoy :)

C# IntelliSense extension for VS 2010

bnaya — Fri, 02 Apr 2010 21:07:41 GMT

C# IntelliSense extension for VS 2010

the C# IntelliSense extension is now available at the Visual Studio Gallery.

the extension is adding filtering capability to the VS IntelliSense,

so for example when you are looking for methods you can filter out the namespace, fields, events and properties.

Credits

this work is heavily based on the Xaml IntelliSense extension that was written by Karl Shifflett.

Download

you can either download the extension from Visual Studio 2010 (tool->Add-in manage…

and search for CSharp IntelliSense).

or directly from Visual Studio Gallery site here.

Point of interest

you may also want to check the Spell Checker extension.

MEF for Beginner (Deployment Catalog) - part 12

bnaya — Fri, 02 Apr 2010 01:20:55 GMT

MEF for Beginner (Deployment Catalog) - part 12

this is the 12th post of the MEF for Beginner series, the series TOC is available here.

this post will focus on Deployment Catalog.

the code sample for this post can be found here.

What is MEF Deployment Catalogs?

the deployment catalog is actually a redesign of the older package catalog.

it enable to load parts from xap packages a-synchronically.

Code sample

the following code sample depend on 2 assembly that should be added to the project:

System.ComponentModel.Composition
System.ComponentModel.Composition.Initialization

at the startup (app.xaml)

Code Snippet
 private void Application_Startup(object sender,StartupEventArgs e)
{
    InitializeContainer();
    this.RootVisual = new MainPage();
}
 
private void InitializeContainer()
{
    var catalogs = new AggregateCatalog();
 
    var catalog = new TypeCatalog(typeof(PlugIns));
    catalogs.Catalogs.Add(catalog);
 
    // Create deployment catalog
    var deployCatalog = new DeploymentCatalog("PlugIns.xap");
    deployCatalog.DownloadCompleted += OnDownloadCompletedHandler;
    deployCatalog.DownloadProgressChanged += OnDownloadProgressChangedHandler;
    deployCatalog.DownloadAsync();
 
    catalogs.Catalogs.Add(deployCatalog);
 
    // initialize the main application composition host (container)
    CompositionHost.Initialize(catalogs);
}
 

line 3, initialize MEF before the creation of any windows that depend on MEF.

line 9, define aggregate catalog (this is basically a catalog collection).

lines 11-12, create type catalog for plug-ins within the main xap (plug-in within the main xap

can be assess directly) and add the catalog to the aggregate catalog.

line 15, create deployment catalog (it get the xap relative path as parameter).

lines 16-17, register for xap download process events (optional).

line 18, start downloading the xap.

line 20, add the deployment catalog to the aggregate catalog.

line 23, initialize the composition host with the aggregate catalog (so

latter using CompositionInitializer.SatisfyImports will operate against the aggregate catalog).

at the MainPage.xaml the code will look as follow:

Code Snippet
 public partial class MainPage:UserControl,INotifyPropertyChanged
{
    public MainPage()
    {
        InitializeComponent();
        this.DataContext = this;
        CompositionInitializer.SatisfyImports(this);
    }
 
    private IEnumerable<string> _plugIns;
    [ImportMany(AllowRecomposition = true)]
    public IEnumerable<string> PlugIns
    {
        get
        {
            return _plugIns;
        }
        set
        {
            _plugIns = value;
            if(PropertyChanged != null)
                PropertyChanged(this,new PropertyChangedEventArgs("PlugIns"));
        }
    }
 
    public event PropertyChangedEventHandler PropertyChanged;
}
 

line 7, asking MEF to satisfy any import of the current instance (in our case the PlugIns property).

lines 11, decorate the PlugIns property for being assigned by the MEF composition.

notice that the decoration define AllowRecomposition = true, this is very important

because the deployment catalog is a-synchronic, therefore we do not control the exact time of the assignment.

Summary

deployment catalog is the current way for delay composition from xap packages.

it is a-synchronic and it will notify the catalog upon the download completion.

the code sample for this post can be found here.

Learn more

recommending VS 2010 XAML IntelliSense Extension

bnaya — Fri, 26 Mar 2010 18:12:38 GMT

recommending VS 2010 XAML IntelliSense Extension

a pal named Karl had published a very useful plug-in to VS 2010.

this plug-in is making Xaml writing so much better experience with enhanced IntelliSense.

if you doing some Xaml work using VS 2010 you should consider to download this plug-in.

the download is available from the following link:

http://karlshifflett.wordpress.com/2010/03/21/visual-studio-2010-xaml-editor-intellisense-presenter-extension/#comment-2228

Features

Pascal case lookup
Optional narrowing list filter
Filtering based on item type
xmlns IntelliSense options
- Show/hide only solution assemblies
- Show/hide schemas
Filter settings persist for the current Visual Studio session

MEF for Beginner (Import from Xaml) - part 11

bnaya — Sat, 20 Mar 2010 21:28:23 GMT

MEF for Beginner (Import from Xaml) - part 11

this is the 11th post of the MEF for Beginner series, the series TOC is available here.

this post will focus on Importing mef parts directly from the Xaml.

the code sample for this post is available here.

assuming that we have the following exports:

Code Snippet
 class DemoStrings
{
    [Export("MyTag")]
    public string Text1 { get { return "Hello world"; } }
    [Export("MyTag")]
    public string Text2 { get { return "export using custom attribute"; } }
    [Export("NotMyTag")]
    public string Text3 { get { return "not included (Not My Tag contract)"; } }
    [Export("MyTag")]
    public string Text4 { get { return "Wpf markup extension"; } }
    [Export]
    public string Text5 { get { return "not included (have no contract)"; } }
}
 

we may want to have it import into the Xaml as follow:

Code Snippet
 <Window x:Class="Bnaya.Samples.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mef="clr-namespace:Bnaya.Samples"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <ListBox
                 DataContext="{mef:ImportManyStrings MyTag}" 
                 ItemsSource="{Binding}"/>
    </Grid>
</Window>
 

as you can see at line 8 the data context is using special definition for importing all exported strings that
define the their contract as MyTag (looking at the exports it will import the exports at lines 4,6,10).

what should we have in order to achieve that functionality?

actually very little, all we have to do is to define a markup extension that return the exported parts.

the following code is how the markup extension should be define:

Code Snippet
 [MarkupExtensionReturnType(typeof(IEnumerable<string>))]
public class ImportManyStringsExtension : MarkupExtension
{
    private readonly IEnumerable<string> _dataSource;
    public ImportManyStringsExtension(string contract)
    {
        _dataSource = CompositionHost.Provider.GetExportedValues<string>(contract);
    }
 
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return _dataSource;
    }   
}
 

line 1, decorate that our markup extension for returning IEnumerable<string>.

line 2, derive from MarkupExtension.

line 7, at the construction time, dynamically ask mef for any discoverable parts that follow the contract.

the CompositionHost is a very small helper class that was taken from the MEF Silverlight implementation

with some very minor enhancements.

lines 10-13, returning the discoverable parts to the Xaml.

One last thing

we should remove the StartupUri from the App.Xaml and using the OnStartup override instead:

Code Snippet
 protected override void OnStartup(StartupEventArgs e)
{
    base.OnStartup(e);
 
    if (!Utils.IsInDesignTool)
    {
        var asmCatalog = new AssemblyCatalog(typeof(App).Assembly);
        var catalogs = new AggregateCatalog(asmCatalog);
        var container = new CompositionContainer(catalogs);
        CompositionHost.Initialize(container);
        container.Compose(new CompositionBatch());
        
        var root = new MainWindow(); 
        root.ShowDialog();
    }
}
 

line 5, preventing MEF operation during design time.

lines 7-9, setting the MEF container.

line 10, setting the MEF container as the main container of the application

(this way it can be reachable anywhere).

line 11, do the composition,

IMPOERANT: the composition should happens before the window instantiation,

because that when the markup extension will be invoke.

the exported part should be available at this time.

lines 13-14, starting the application.

Summary

we can extend the Xaml very easily in order to get better Xaml->Mef experience.

the code sample for this post is available here.

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