What is the MEF method to get an existing exported value / object (not get or create)?

Question

This is how I want it to work

container.GetButDoNotCreate<T>(); // should throw (or can return null) if container doesn't contain an instance matching the contract

var x = container.GetExportedValue<T>();

var y = container.GetButDoNotCreate<T>(); // should return the object created in previous step

Assert.That(x).IsSameAs(y);

The difference is that this method will not instantiate if the container does not contain it. It is pure-get ie get me this object if it exists in the container. I need this for my tests, where I do not want the test code to create objects in the working container (if they are not created), just use the existing ones. Only production code should add / remove objects to the container.

Submitted to the MEF codeplex forum , but no replies. So hoping that someone from SO can get an answer. Also, if I need to write this function as an extension method ... it will be fine too, as an answer.

Answer 1

. . , , , .

, , , - , CompositionContainer, GetButDoNotCreate.

: , , , GetExportedValue . , [Import].

:

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

namespace GetButNotCreate
{
    public class CustomContainer : CompositionContainer
    {
        private List<Type> composedTypes = new List<Type>();

        public CustomContainer(ComposablePartCatalog catalog)
            : base(catalog) 
        {
        }

        public new T GetExportedValue<T>()
        {
            if (!composedTypes.Contains(typeof(T)))
                composedTypes.Add(typeof(T));

            return base.GetExportedValue<T>();
        }

        public T GetButDoNotCreate<T>()
        {
            if (composedTypes.Contains(typeof(T)))
            {
                return base.GetExportedValue<T>();
            }

            throw new Exception("Type has not been composed yet.");
        }
    }
}

, GetExportedValue, , , GetButNotCreate. , .

, GetExportedValue ( , ) , , , . , .

, :

using System.ComponentModel.Composition;
using System.ComponentModel.Composition.Hosting;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;

namespace GetButNotCreate
{
    public interface IInterface { }

    [Export(typeof(IInterface))]
    public class MyClass : IInterface 
    {
    }

    [TestClass]
    public class UnitTest1
    {      
        [TestMethod]
        [ExpectedException(typeof(Exception), "Type has not been composed yet.")]
        public void GetButNotCreate_will_throw_exception_if_type_not_composed_yet()
        {
            var catalog = new AssemblyCatalog(typeof(UnitTest1).Assembly);
            CustomContainer container = new CustomContainer(catalog);
            container.ComposeParts(this);

            var test = container.GetButDoNotCreate<IInterface>();
        }

        [TestMethod]
        public void GetButNotCreate_will_return_type_if_it_as_been_composed()
        {
            var catalog = new AssemblyCatalog(typeof(UnitTest1).Assembly);
            CustomContainer container = new CustomContainer(catalog);
            container.ComposeParts(this);

            var x = container.GetExportedValue<IInterface>();
            var y = container.GetButDoNotCreate<IInterface>();

            Assert.IsNotNull(y);
            Assert.AreEqual(x, y);
        }
    }
}

, GetButNotCreate , , .

, ( ), , MEF , CustomContainer .

Answer 2

, . - IContainerWrapper .

:

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