Question

i have some simple Export/Import Scenario that i can’t figur out why this not work. in my scenario i have one WPF app and 2 ClassLibrary, in Classlib1 i have one interface named ITestEx1 as below :

public interface ITestEx1
{
  string Name {get; set;}
}

and 1 derived class Named (TestEx1) as below :

using System.ComponentModel.Composition;

[Export(typeof(ITestEx1))]
public class TestEx1 : ITestEx
{
  public Name {get; set;}
}

as you can see this class exported as type of ITestEx1, now in Classlib2 i refrenced Classlib1 and have one class as below :

using System.ComponentModel.Composition;
using Classlib1;

public class TestMEF
{
 [Import(typeof(ITestEx1))]
 public ITestEx1 TestE {get; set;}
}

and in main WPF application i refrenced both Classlib1 and ClassLib2 and in constructor of MainWindow.xaml i wrote this code for initializing MEF :

private CompositionContainer _container;

...

public MainWindow()
{ 
  InitializeComponent();
  var catalog = new AggregateCatalog();
  catalog.Catalogs.Add(new AssemblyCatalog(typeof(MainWindow).Assembly));
  catalog.Catalogs.Add(new AssemblyCatalog(typeof(TestEx1).Assemble));
  _container = new CompositionContainer(catalog)
  _container.ComposeParts(this);
}

… and in button click i have this :

{
  ...
  var aa = new TestMEF();
  aa.TestE.Name = "abc";    // Error, object null refrence
}

Please Help Me to solve this problem

Answer 1

You have to grab the instance out of the container, not create it yourself. In this case, you would need a factory (in the container) to create the objects on-the-fly.