MEF Import null

Question

First of all: you are all healthy, I very often look for this site for MEF answers.

My problem is this:

I have several assemblies with many [Import] in them and one main application where the assembly takes place. Now the problem is that these Imports are not filled, they always remain zero.

I tried to reproduce this behavior in a simple small project and came up with the following source code.

Did I miss some things about MEF?

Please, help! Thanks everyone!

Build Interfaces:

namespace Interfaces { public interface IClass1 { void Trigger(); } public interface IClass2 { void Trigger(); } public interface IClass3 { void Trigger(); } } 

Assembly Library1:

 namespace Library1 { [Export(typeof(IClass1))] public class Class1:IClass1 { #region IClass1 Members public void Trigger() { } #endregion } } 

Assembly Library2:

 namespace Library2 { [Export(typeof(IClass2))] public class Class2:IClass2 { [Import] public IClass1 Class1 { get; set; } public void Trigger() { } } } 

In the main program, I collect all the Mef stuff by doing the following:

 namespace MEFTest { public class mefStart { public CompositionContainer Container { get; private set; } public void Start() { AggregateCatalog catalog = new AggregateCatalog(); AssemblyCatalog assemblyCatalog = new AssemblyCatalog(typeof(Program).Assembly); DirectoryCatalog directoryCatalog = new DirectoryCatalog(".", "Library*.dll"); catalog.Catalogs.Add(directoryCatalog); catalog.Catalogs.Add(assemblyCatalog); this.Container = new CompositionContainer(catalog); CompositionBatch batch = new CompositionBatch(); batch.AddExportedValue(this.Container); this.Container.Compose(batch); this.Container.ComposeParts(this); } } } 

But after that, the following class has no populated imports:

 namespace MEFTest { public class Class3:IClass3 { [Import] public IClass1 Class1 { get; set; } [Import] public IClass2 Class2 { get; set; } public void Trigger() { Class1.Trigger(); Class2.Trigger(); } } } 

When I look at the container, I see that IClass1 and ICLass2 have been compiled.

Why is [Import] not performed in Class3? I guess I'm missing something ...

Thank you all for your help!

Michael

Answer 1

until class 3 is NOT instantiated by the MEF, you will not see the import.

btw, if you are not importing through [ImportingConstructor] - make sure the import is satisfied (IPartImportsSatisfiedNotification) before you use them.

this will work, but I don't know where you need your class

 public class mefStart { [Import] private IClass3 my3; public CompositionContainer Container { get; private set; } public void Start() { AggregateCatalog catalog = new AggregateCatalog(); AssemblyCatalog assemblyCatalog = new AssemblyCatalog(typeof(Program).Assembly); DirectoryCatalog directoryCatalog = new DirectoryCatalog(".", "Library*.dll"); catalog.Catalogs.Add(directoryCatalog); catalog.Catalogs.Add(assemblyCatalog); this.Container = new CompositionContainer(catalog); CompositionBatch batch = new CompositionBatch(); batch.AddExportedValue(this.Container); this.Container.Compose(batch); this.Container.ComposeParts(this); //from here you can use Class3 with all imports } } [Export(typeof(IClass3)] public class Class3:IClass3 { [Import] public IClass1 Class1 { get; set; } [Import] public IClass2 Class2 { get; set; } public void Trigger() { Class1.Trigger(); Class2.Trigger(); } }