MEF prevent class from being manually instatiated

Question

I wonder if I can somehow prevent class from being manually created? I want to make sure it is only imported.

[Export]
[PartCreationPolicy(CreationPolicy.Shared)]
public class TwoWayMessageHubService
{
    [ImportingConstructor]
    public TwoWayMessageHubService(ILoggerService loggerService)
    {
    }
}

So, I want to make sure this works:

[Import]
public TwoWayMessageHubService MHS {get; set;)

And make sure this doesn't:

var MHS = new TwoWayMessageHubService(logger);

Answer 1

In fact this is possible. Just have to apply [Import] attribute to your constructor's parameters and make your constructor private. I have made the following sample based on your code, it works and you can test it.

First, the TwoMessageHubService with the changes I mentioned:

[Export]
    [PartCreationPolicy(CreationPolicy.Shared)]
    public class TwoWayMessageHubService
    {
        [ImportingConstructor]
        private TwoWayMessageHubService([Import]ILogger logger) { }
    }

Notice that the constructor is private

Then a class that must be composed with an instance of TwoWayMessageHubService:

public class Implementer
    {
        [Import]
        public TwoWayMessageHubService MHS { get; set; }
    }

The Logger decorated with Export

   public interface ILogger { }

    [Export(typeof(ILogger))]
    public class Logger : ILogger { }

And Main:

var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
            var container = new CompositionContainer(catalog);

            var implementer = new Implementer();
            container.ComposeParts(implementer);
            //var IdoNotCompile = new TwoWayMessageHubService(new Logger());

            Console.ReadLine();

If you uncomment the comment (lol) then you will notice that it does not compile.

Hope this helps