MEF Hacks: Constructors with parameters

Often, there’s a need to register an object into your dependency container that takes constructor arguments. There are a few ways to accomplish this with MEF.

You’re mileage will vary, but my favorite approach is a bit of a hack that leverages a lesser-known feature of MEF: Property Exports.

public class ComplexObjectExportFactory
{
    [Export]
    public MyComplexObject ComplexObject
    {
        get
        {
            return new MyComplexObject("config.xml");
        }
    }
}

This "simply works" and doesn't require you to do anything intrusive like sub-classing a dependency or registering named parameters on application start-up. The technique is especially useful if you need to provide any kind of conditional logic based on settings and it can even build upon other MEF registered parts, for example:

internal class ComplexObjectExportFactory
{
    private readonly IApplicationSettingsProvider _settings;
    private readonly IEventAggregator _eventAggregator;

    [ImportingConstructor]
    public ComplexObjectExportFactory(
            IApplicationSettingsProvider settings, 
            IEventAggregator eventAggregator)
    {
        _settings = settings;
        _eventAggregatore = eventAggregator;
    }

    [Export]
    public MyComplexObject ComplexObject
    {
        get
        {
            string configFile = _settings.GetSetting("ComplexObjectConfigFile");
            int timeout = _settings.GetSetting<int>("ComplexObjectTimeoutSeconds");
            
            return new MyComplexObject(_eventAggregator, configFile, timeout);
        }
    }
}

The only real down-fall to this overall approach is that the ComplexObjectExportFactory will never have any usages, so tools like Resharper will think this is dead-code, but this can be solved with a few well placed #pragmas and some comments. The other reason this feels like a hack, and this is more of an esthetics or stylistic difference, is that this feels like it should be a method (eg, factory.Create()) but MEF treats exported methods very differently.

Despite the awkwardness of this class, I’d prefer this to sub-classing third-party dependencies or bloating the bootstrapper with additional initialization logic. Maybe you think so too?

Happy coding.