MEF 'Export is not assigned an error of type

Question

I just started using MEF and got into an early problem.

I have an interface called DataService:

namespace DataAccess
{
  interface IDataService
  {
    string Name { get; }
    string Description { get;}

    List<String> GetPeople();
  }
}

There are 2 implementations of this interface: one for SQL Server and one for Oracle. The following is the implementation of Oracle, the implementation of SQL Server is exactly the same.

namespace DataAccess
{
[Export(typeof(IDataService))]
[ExportMetadata("Name","Oracle")]
[ExportMetadata("Description","Oracle Data Service")]
public class Oracle : IDataService
{

    #region IDataService Members

    public string Name
    {
        get { return "Oracle"; }
    }

    public string Description
    {
        get { return "Provides data access to Oracle database"; }
    }

    public List<string> GetPeople()
    {
        return new List<String>() { "Oracle boo", "Oracle boo1" };
    }

    #endregion
}
}

Name and description properties no longer function as I replaced them with metadata. As you can see, these are very simple objects, I wanted to make sure that I could get this to work before I started doing the hard work.

This is the code I use to detect assemblies:

    private static CompositionContainer _container;
    private const string ASSEMBLY_PATTERN = "*.dll";
    private AggregateCatalog _catalog; 

    [ImportMany]
    IEnumerable<DataAccess.IDataService> services { get; set; }

    private void button3_Click(object sender, EventArgs e)
    {


        _catalog = new AggregateCatalog(
            new DirectoryCatalog(txtLibPath.Text, ASSEMBLY_PATTERN),
            new AssemblyCatalog(Assembly.GetExecutingAssembly()));
        _container = new CompositionContainer(_catalog);
        _container.ComposeParts(this);
        MessageBox.Show(services.Count().ToString());
    }

This is mistake:

The composition produced a single compositional error. The main reason is given below. See the CompositionException.Errors property for more information.

1) 'DataAccess.Oracle(ContractName = "DataAccess.IDataService" )' 'DataAccess.IDataService'.

: "MEFTest.Form1.services(ContractName =" DataAccess.IDataService ")" "MEFTest.Form1". : MEFTest.Form1.services(ContractName = "DataAccess.IDataService" ) → MEFTest.Form1

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, , ...

Answer 1

, ( DataAccess.IDataService). , , , . MEF, MSDN Best .

Answer 2

.

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, . nuget .

, , nugets MEF.

Answer 3

I have to say that I had such a mistake in a completely idiotic context. Incidentally, I incorrectly set the export directive and put it not in a class, but in a function inside the class:

interface MyInterface
{
  void MyFunction();
}

public class MyClass : MyInterface
{
  [Export(typeof(MyInterface))]
  void MyFunction() { }
}

Surprisingly, the code compiled very well, without any warnings. But then I spent hours trying to figure out why MEF is failing in my silly typo!