Question

I want to create an delegate based export for an interface which is called upon every compose call to return every consumer a new instance. But apparently I do get always the same instance back. Is this a limitation of MEF (I am using .NET 4.0)?

Here is the sample code how I tried to create fresh instances for every compose call:

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using System.ComponentModel.Composition.Primitives;

namespace TestApp
{
    class Program
    {
        [Import(RequiredCreationPolicy = CreationPolicy.NonShared)]
        ITest Instance { get; set; }

        static void Main(string[] args)
        {
            CompositionContainer container = new CompositionContainer();
            CompositionBatch batch = new CompositionBatch();
            AddExportedValue<ITest>(batch, () =>
            {
                Console.WriteLine("Create new");
                return new Haha();
            }, CreationPolicy.NonShared);

            container.Compose(batch);

            Program p1 = new Program();
            container.SatisfyImportsOnce(p1);
            // Why do I see only one "Create new" print although CreationPolicy on import and export level is NonShared?
            container.SatisfyImportsOnce(p1);
        }

        interface ITest
        {}

        class Haha : ITest
        {}

        static ComposablePart AddExportedValue<iT>(CompositionBatch batch, Func<iT> factory, CreationPolicy policy)
        {
            string contractNameAndTypeIdentity = AttributedModelServices.GetContractName(typeof(iT));
            IDictionary<string, object> metadata = new Dictionary<string, object>();
            metadata.Add(CompositionConstants.ExportTypeIdentityMetadataName, contractNameAndTypeIdentity);
            metadata.Add(CompositionConstants.PartCreationPolicyMetadataName, policy); // <--- the policy seems to be ignored by MEF although incompatible policies are errored out with an exception.
            return batch.AddExport(new Export(contractNameAndTypeIdentity, metadata, () => factory()));
        }
    }
}

Expected Output:

• Create new
• Create new

Actual Output

• Create new

Answer 1

you can use ExportFactory with .NET4.0 too. Glenn Block posted a version a while ago. i use it in my projects too.