Question

Consider the following code sample that uses MEF to create an object of type Importer that imports an object of type ImporterExporter which in turn imports an object of type Exporter, i.e. Importer -> ImporterExporter -> Exporter. The catalog is managed by a CompositionUtility (obviously simplified for this example).

I know that MEF will resolve imports recursively on imported parts. However, because I want to have the option to instantiate each of these classes independetly, every class with imports also composes itself in its constructor to resolve those imports.

using System;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Reflection;

namespace MefRecursionSample
{
    class Program
    {
        static void Main(string[] args)
        {
            // var importerExporter = new ImporterExporter(); // include this and composition will work
            var importer = new Importer();
            Console.Write(importer.ImporterExporter.Exporter.Value); // should print 7
            Console.ReadKey();
        }
    }

    class CompositionUtility
    {
        static CompositionUtility()
        {
            var executingAssembly = Assembly.GetExecutingAssembly();
            var assemblyCatalog = new AssemblyCatalog(executingAssembly);
            _compositionContainer = new CompositionContainer(assemblyCatalog);
        }

        private static CompositionContainer _compositionContainer;
        private static bool _isComposing;

        public static void Compose(object part)
        {
            _compositionContainer.ComposeParts(part);
        }
    }

    class Importer
    {
        public Importer()
        {
            CompositionUtility.Compose(this);
        }

        [Import]
        public ImporterExporter ImporterExporter { get; set; }
    }

    [Export]
    class ImporterExporter
    {
        public ImporterExporter()
        {
            CompositionUtility.Compose(this);
        }

        [Import]
        public Exporter Exporter { get; set; }
    }

    [Export]
    class Exporter
    {
        public int Value { get { return 7; } }
    }

}

Running the code as is leads to a composition error “The ComposablePart of type MefRecursionSample.Importer’ cannot be recomposed….”, obviously because I am trying to explictly compose something that MEF also wants to compose.

What surprised me, was the fact that when I included the first line of the Main method, i.e. create an object of type ImporterExporter without MEF, this “double-composition” no longer caused an exception. Why is that?

Also, how could I make it work such that I could instantiate each of these indepennetly, yet also make them compose themselves when chained as in the sample. I figured I would introduce a boolean flag _compositionInProgress on the CompositionUtility and immediately return from Compose() when the flag is set to avoid the recursive composition. Is there a better way?

Answer 1

The flag I considered setting in the CompositionUtility‘s Compose method does not work, because there can be cases where the string of automatic imports is interrupted. For instance, in the question’s example if Exporter instantiated a class in its constructor using new and this class would want to compose itself. Under the original solution, that class’ call to Ccompose would return immediately, leaving the class uncomposed.

Because I want classes to compose themselves (thus making it unneccessary for their users to even know about MEF), the only solution was to establish the rule that classes with an [Export] attribute must not call Compose(this). Because they will be composed automatically by MEF when being imported, this would result in “double composition” and thus throw an exception.

If it is a requirement that classes marked with [Export] have to instantiated independently via new instead of nly imported via MEF, they have to have an addional constructor with a boolean flag which when set well trigger composition of that class. The default behavior, however, has to be no composition in order to avoid the aforementioned “double composition”.