MEF Recomposition of child containers

I've built a MEF container that creates a service. I build another MEF container with these services from the first container. The problem is those services get recomposed when being added to the second container.

[Export(typeof(IFoo))]
public class Foo : IFoo
{
    [Import(typeof(IServiceA))]
    public IServiceA A { get; set; }
}

[Export(typeo开发者_开发百科f(IServiceA))]
public class ServiceAImpl : IServiceA
{
    public ServiceAImpl()
    {
        Console.Out.WriteLine("Service A Created");
    }
}

//Create the parent container
var parentContainer = new CompositionContainer(Composer.AggregateCatalog);
IFoo foo = parentContainer.GetExportedValue<IFoo>();


//..... some work


//Create a child container providing it an existing instance of IFoo
var childContainer = new CompositionContainer(Composer.AggregateCatalog);
var batch = new CompositionBatch();
batch.AddPart(foo); //Add existing IFoo

//This causes a recomposition of IFoo resulting in 
//a new instance of IServiceA to be created and injected
childContainer.Compose(batch);

"Service A Created" gets called twice on the last line above when the container creates the composition because it's trying to recompose Foo, which I don't want it to.

Does anyone have a solution to this? I've tried explicitly specifying AllowRecomposition=false too but that doesn't work either.

What are you trying to achieve with parent/child containers? In the code you show, you're not actually creating a parent and a child, you're creating two unrelated containers. To actually create a child container, do the following:

var childContainer = new CompositionContainer(childCatalog, parentContainer);

Then anything in the parent will automatically be available in the child (so you won't have to add it via a batch). You'll want a different catalog for the child container with the parts you want to be in the child.