在ClassLibrary中使用MEF

Question

I would like to use MEF in a ClassLibrary and not in an application project (neither ConsoleApplication nor WebApplication...).

When I try do this (like MSDN: http://msdn.microsoft.com/en-us/library/dd460648.aspx )

class Program
{
    private CompositionContainer _container;

    [Import(typeof(IPlugin))]
    public IPlugin Plugins;


    private Program()
    {
        //An aggregate catalog that combines multiple catalogs
        var catalog = new AggregateCatalog();
        //Adds all the parts found in the same assembly as the Program class
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(Program).Assembly));
        catalog.Catalogs.Add(new DirectoryCatalog(@".\Extensions"));


        //Create the CompositionContainer with the parts in the catalog
        _container = new CompositionContainer(catalog);

        //Fill the imports of this object
        try
        {
            this._container.ComposeParts(this);
        }
        catch (CompositionException compositionException)
        {
            Console.WriteLine(compositionException.ToString());
        }
    }


    static void Main(string[] args)
    {
        Program p = new Program(); //Composition is performed in the constructor
    }
}

The Property :

    [Import(typeof(IPlugin))]
    public IPlugin Plugins;

works fine.

EDIT:

However, I would like to move the import property & the aggregate catalog creation in a ClassLibrary and not in my ConsoleApplication.

Something like below.

In ClassLibrary [Manager.dll]:

public class Manager
{
    private CompositionContainer _container;

    [Import(typeof(IPlugin))]
    public IPlugin Plugins;


    public Manager()
    {
        //An aggregate catalog that combines multiple catalogs
        var catalog = new AggregateCatalog();
        //Adds all the parts found in the same assembly as the Program class
        catalog.Catalogs.Add(new AssemblyCatalog(typeof(Manager).Assembly));
        catalog.Catalogs.Add(new DirectoryCatalog(@".\Extensions"));


        //Create the CompositionContainer with the parts in the catalog
        _container = new CompositionContainer(catalog);

        //Fill the imports of this object
        try
        {
            this._container.ComposeParts(this);
        }
        catch (CompositionException compositionException)
        {
            Console.WriteLine(compositionException.ToString());
        }
    }
}

And in the ConsoleApplication:

class Manager
{
    static void Main(string[] args)
    {
        Manager m = new Manager(); //Composition is performed in the constructor
    }
}

But when I do this change, I'm not able to get "Plugins" property which is always "null" in my ClassLibrary.

Anyone have a solution to do this ?

Answer 1

This is probably just a problem with your catalog. If you are moving your property into a class library, that implies a separate assembly. Currently, only the Assembly hosting "Program" is in your catalog, along with whatever is in "Extensions". So, did you make sure that your new class library is in the extensions folder?

Also, you are composing this meaning you are composing an instance of "Program" If you moved your property to a different class in a different library, then you need to compose an instance of whatever class that is.

Something like this:

    class Program
   {
    private CompositionContainer _container;       

    private Program()
    {
        var helper = new MyOtherHelperClass();
        var catalog = helper.CreateCatalog();

        //Create the CompositionContainer with the parts in the catalog
        _container = new CompositionContainer(catalog);

        //Fill the imports of some object
        try
        {
            var thingIWantToCompose = new OtherClassWithAnImport();
            this._container.ComposeParts(thingIWantToCompose);
        }
        catch (CompositionException compositionException)
        {
            Console.WriteLine(compositionException.ToString());
        }
    }


    static void Main(string[] args)
    {
        Program p = new Program(); //Composition is performed in the constructor
    }
}

If that doesn't work then try looking at your parts in Visual MEFX. Here is a short guide to setting it up: Getting Started with Visual MEFX