MEF Constructor Parameters with Multiple Constructors

Question

I'm starting to use MEF, and I have a class with multiple constructors, like this:

[Export(typeof(ifoo))]
class foo : ifoo {
    void foo() { ... }
    [ImportingConstructor]
    void foo(object par1) { ... }
}

I am using catalog.ComposeExportedValue() when composing to supply the par1 value to second constructor:

...
catalog.ComposeExportedValue(par1Value);
catalog.ComposeParts(this);
...

To hold the components I'm using:

[ImportMany(typeof(ifoo))]
public List<Lazy<ifoo, ifoometadata>> FooList { get; set; }

And to create the foo instance I'm using the value property, FooList[0].Value.

Everthing works fine, except that the second constructor of the foo class is never called. What's wrong?

How do I select the constructor I want to use when MEF instantiates the class?

Answer 1

MEF should use the constructor you put the ImportingConstructorAttribute on. I'm not sure what is happening for you, I wasn't able to reproduce the issue. Here is a test which shows using an ImportingConstructor on a class that also has a default constructor:

[TestClass]
public class MefTest
{
    public const string ConstructorParameterContract = "FooConstructorParameterContract";

    [TestMethod]
    public void TestConstructorInjectionWithMultipleConstructors()
    {
        string ExpectedConstructorParameterValue = "42";

        var catalog = new TypeCatalog(typeof(Foo), typeof(FooImporter));
        var container = new CompositionContainer(catalog);

        container.ComposeExportedValue<string>(ConstructorParameterContract, ExpectedConstructorParameterValue);

        var fooImporter = container.GetExportedValue<FooImporter>();

        Assert.AreEqual(1, fooImporter.FooList.Count, "Expect a single IFoo import in the list");
        Assert.AreEqual(ExpectedConstructorParameterValue, fooImporter.FooList[0].Value.ConstructorParameter, "Expected foo's ConstructorParameter to have the correct value.");
    }
}

public interface IFoo
{
    string ConstructorParameter { get; }
}

[Export(typeof(IFoo))]
public class Foo : IFoo
{
    public Foo()
    {
        ConstructorParameter = null;
    }

    [ImportingConstructor]
    public Foo([Import(MefTest.ConstructorParameterContract)]string constructorParameter)
    {
        this.ConstructorParameter = constructorParameter;
    }


    public string ConstructorParameter { get; private set; }
}

[Export]
public class FooImporter
{
    [ImportMany]
    public List<Lazy<IFoo>> FooList { get; set; }
}