MEF import doesn't work with external assembly

Question

I have the following MEF test code:

[Import(AllowDefault = true)]
string importedString; 

[Import(typeof(IString), AllowDefault = true)]
public IString importedClass;

private void Import(bool fromDll)
{
    CompositionContainer MyContainer;
     if (fromDll)
    {
        DirectoryCatalog MyCatalog = new DirectoryCatalog("D:\\Source\\ClassLibrary\\bin\\Debug\\", "ClassLibrary.dll");
        MyContainer = new CompositionContainer(MyCatalog);
    }
    else
    {
        AssemblyCatalog MyCatalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
        MyContainer = new CompositionContainer(MyCatalog);
    }
    MyContainer.SatisfyImportsOnce(this);

    MessageBox.Show(importedString == null ? "String not found" : importedString, "fromDLL=" + fromDll.ToString());
    MessageBox.Show(importedClass == null ? "Class not found" : importedClass.getClassMessage(), "fromDLL=" + fromDll.ToString());
}

The export section is defined in the same file as follows:

public class MyString
{
    [Export()]
    public string message = "This string is imported";
}

public interface IString
{
    string getClassMessage();
}

[Export(typeof(IString))]
public class MyClass : IString
{
    public string getClassMessage()
    {
        return ("This class is imported");
    }
}

Now every thing works fine if I call Import(false), I do get two message boxes with the text "This string is imported" and "This class is imported"

However, if I create the ClassLibrary.dll (Which just has the exported section in its namespace) and call Import(true), I do get "This string is imported" message box but I get the "Class not found" message. Any reason for the difference in behavior? Am I doing something wrong?

You need to make sure that you use the *same* interface for the export. `string` ist from mscorlib, but if you have IString in app.exe and IString in ClassLibrary.dll, they are different interfaces to MEF. — Manuel Schweigert, Oct 07 '15 at 12:42
So how can I tell MEF that IString declared in the app is the same as IString declared in the dll? — user2852294, Oct 07 '15 at 13:17
Add the app as reference and use the referenced IString interface. (Or use a separate Interfaces.dll which is referenced from both app and library) — Manuel Schweigert, Oct 07 '15 at 13:19

score 3 · Accepted Answer · edited May 23 '17 at 12:29

For completion's sake I'll post the answer.

When using MEF, you need to watch out that you use the exact same types, which means, the same type from the same assembly. This is why MEF is not really useful as a plugin-system, because every time you rebuild the assembly containing the interfaces, you need to rebuild every plugin against that.

There are possibilities to do this of course, for example using the Managed AddIn Framework. See this post for more info on both: Choosing between MEF and MAF (System.AddIn)

MEF import doesn't work with external assembly

1 Answers1