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?
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; }
}