Matching MEF imports to exports

Question

In the code below I am attempting to use MEF to match an import to a matching export. The TestMEFClass has an import and an export which share a matching contract name. The export should increment a counter every time it's called.

When I printed the export to the console, it did not increment the counter. Could someone point out my mistake?

Thank you very much,

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Reflection;

namespace MEFConsoleTest {

    public class TestMEFClass {

        /// <summary>
        /// This counter should increment everytime the getter in the ExportString property gets called.
        /// </summary>
        private int counter = 0;

        [Export("Contract_Name")]
        public string ExportString {

            get {
                return "ExportString has been called " + counter++.ToString();
            }
        }

        [Import("Contract_Name")]
        public string ImportString { get; set; }

        /// <summary>
        /// Default Constructor.
        /// Make a catalog from this assembly, add it to the container and compose the parts.
        /// </summary>
        public TestMEFClass() {

            AggregateCatalog catalog = new AggregateCatalog();
            catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
            var container = new CompositionContainer(catalog);
            container.ComposeParts(this);
        }


    }

    class Program {

        static void Main(string[] args) {

            TestMEFClass testClass = new TestMEFClass();
            Console.WriteLine(testClass.ImportString);
            Console.WriteLine(testClass.ImportString);
            Console.ReadLine();

        }
    }

Answer 1

For reasons I cannot explain at this moment I wasn't able to get MEF and properties imports/exports to work on a mutable property. However, using functions did the trick. I hope this code helps someone else.

Thanks,

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.Reflection;

namespace MEFConsoleTest {

    public class TestMEFClass {

        /// <summary>
        /// This counter should increment everytime the getter in the ExportString property gets called.
        /// </summary>
        private int counter = 0;

        [Export("Contract_Name")]
        string ExportMethod() {
            return ExportString;
        }


        public string ExportString {

            get {
                return "ExportString has been called " + counter++.ToString();
            }
        }

        [Import("Contract_Name")]
        Func<string> ImportMethod;

        public string ImportString { get { return ImportMethod(); } }

        /// <summary>
        /// Default Constructor.
        /// Make a catalog from this assembly, add it to the container and compose the parts.
        /// </summary>
        public TestMEFClass() {

            AggregateCatalog catalog = new AggregateCatalog();
            catalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
            var container = new CompositionContainer(catalog);
            container.ComposeParts(this);
        }


    }

    class Program {

        static void Main(string[] args) {

            TestMEFClass testClass = new TestMEFClass();

            for (int x = 0; x < 10; x++) {
                Console.WriteLine(testClass.ImportString);
            }

            Console.ReadLine();

        }
    }
}