How to get plugins by setting a path - mef

Question

I created console c# project. and in the code I have made a module. My code looks like this.
[Import]
public IMessageSender MessageSender { get; set; }
public static void Main(string[] args)
{
Program p = new Program();
p.Run();
}
public void Run()
{
Compose();
Console.ReadLine(MessageSender.Send("Message Sent"));
}
private void Compose()
{
AssemblyCatalog catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
var container = new CompositionContainer(catalog);
container.ComposeParts(this);
}
public interface IMessageSender
{
string Send(string message);
}
[Export(typeof(IMessageSender))]
public class EmailSender : IMessageSender
{
public void Send(string message)
{
return message;
}
}
It works perfectly fine. But now I added a new project in my solution and added module into that
AnotherProject->EmailSender.cs
[Export(typeof(IMessageSender))]
public class EmailSender : IMessageSender
{
public void Send(string message)
{
return message;
}
}
Now in the main console program I changed some of my code.
private void Compose()
{
var catalog = new DirectoryCatalog(path);
//AssemblyCatalog catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
var container = new CompositionContainer(catalog);
container.ComposeParts(this);
}
But now when I run this program. It doesnt load the module. MessageSender in main program is null. What wrong I have done.

Answer 1

There are a few things you need to check:
Have you correctly referenced the assemblies?
The DirectoryCatalog by default uses the search pattern *.dll. Because you have a console application, which uses the .exe extension, no exports in that assembly will get picked up by the DirectoryCatalog - with the default search pattern. You'll likely want to use an AggregateCatalog, passing in the DirectoryCatalog (*.dll), and either another DirectoryCatalog (*.exe), or an AssemblyCatalog, of the entry assembly.
You currently have one [Import] where you may end up with multiple [Export(typeof(IMessageSender))], you didn't state that you have moved the EmailSender to the class library, merely that you have created a new one, which means you'll likely end up with a cardinality mismatch where it is expecting a sinple import, you have many exports. This will explicitly throw an exception, which is what will happen even it couldn't find a single instance of IMessageSender, because your [Import] attribute is not set to allow a default value where no part can be provided. If you need to be fault tollerant, you can use [Import(AllowDefault = true)]
Incidentally... the above code won't compile, I assume it was just an example and not a copy-paste from your current code?
public void SendMessage(string message)
{
return message;
}
You're retuning a message to a void method - that can't be done, and it also means that EmailSender doesn't correctly implement IMessageSender. Not too bothered, as I think it is an example more than actual code.