Design pattern strategy in MEF

Question

I have method what returns objects implementing particular interface. Depends on argument method return different object. All implement the same interface so I can use the same method like Execute() on interface outside the method. This solution force me to avoid of using MEF. How can I use both solution at once? Importing constructor from MEF and isolation of different strategies in separate classes?

Here is an example code:

[Export(typeof(ICrowdMessageProcessorFactory))]
public class CrowdMessageProcessorFactory : ICrowdMessageProcessorFactory
{
    private readonly IDefaultCrowdRequestAnalyzer _defaultProcessor;

    [ImportingConstructor]
    public CrowdMessageProcessorFactory(IDefaultCrowdRequestAnalyzer defaultProcessor)
    {
        _defaultProcessor = defaultProcessor;
    }

    public Metadata PayloadMetadata { get; private set; }

    public ICrowdMessageProcessor Create(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }

        PayloadMetadata = Metadata.Create(request);
        var marketRegion = PayloadMetadata?.GetMarketRegion();

        switch (marketRegion)
        {
            case MarketRegion.Uk:
                return new UkCrowdMessageProcessor();
        }

        return new DefaultCrowdMessageProcessorAdapter(request, fireUtcDateTime, _defaultProcessor);
    }
}

And here is use of the method

    [ImportingConstructor]
    public CrowdResponseAnalyzer(
        ICrowdMessageProcessorFactory processorFactory)
    {
             _processorFactory = processorFactory;
    }

    public void Execute(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        Guard.ArgumentNotNull(request, "request");

        try
        {
            ICrowdMessageProcessor processor = _processorFactory.Create(request, fireUtcDateTime);
            processor.Execute();
        }
        //(...)
    }  

In summary: I like to separate different strategies to different class, here in UkCrowdMessageProcessor and DefaultCrowdMessageProcessorAdapter. But in new this kind of class (ICrowdMessageProcessor) i need to use ImportingConstructor also. How can I do it?

Answer 1

Solution 1: Use CompositionContainer and it method GetExportedValue. when it looks:

public ICrowdMessageProcessor Create(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }

        PayloadMetadata = Metadata.Create(request);

        var marketRegion = PayloadMetadata?.GetMarketRegion();
        switch (marketRegion)
        {
            case MarketRegion.Uk:
                return _cc.GetExportedValue<UkCrowdMessageProcessor>();
        }

        return PrepareDefaultCrowdMessageProcessor(request, fireUtcDateTime);
    }

    private ICrowdMessageProcessor PrepareDefaultCrowdMessageProcessor(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        var processor = _cc.GetExportedValue<DefaultCrowdMessageProcessorAdapter>();
        processor.Initialize(request, fireUtcDateTime, _defaultProcessor);
        return processor;
    }

Solution 2: Use ServiceLocator and GetInstance

Solution 3: This one is the only one right from design point of view.

[Export(typeof(ICrowdMessageProcessorFactory))]
public class CrowdMessageProcessorFactory : ICrowdMessageProcessorFactory
{

    private readonly IDefaultCrowdRequestAnalyzer _defaultProcessor;

    private readonly UkCrowdMessageProcessor _ukCrowdMessageProcessor;

    private readonly DefaultCrowdMessageProcessorAdapter _defaultCrowdMessageProcessor;

    [ImportingConstructor]
    public CrowdMessageProcessorFactory(
        IDefaultCrowdRequestAnalyzer defaultProcessor, 
        UkCrowdMessageProcessor ukCrowdMessageProcessor,
        DefaultCrowdMessageProcessorAdapter defaultCrowdMessageProcessor)
    {
        _defaultProcessor = defaultProcessor;
        _ukCrowdMessageProcessor = ukCrowdMessageProcessor;
        _defaultCrowdMessageProcessor = defaultCrowdMessageProcessor;
    }

    public Metadata PayloadMetadata { get; private set; }

    public ICrowdMessageProcessor Create(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }

        PayloadMetadata = Metadata.Create(request);

        var marketRegion = PayloadMetadata?.GetMarketRegion();
        switch (marketRegion)
        {
            case MarketRegion.Uk:
                return _ukCrowdMessageProcessor;
        }

        return PrepareDefaultCrowdMessageProcessor(request, fireUtcDateTime);
    }

    private ICrowdMessageProcessor PrepareDefaultCrowdMessageProcessor(InsertCrowdsourcingEventRequest request, DateTime fireUtcDateTime)
    {
        _defaultCrowdMessageProcessor.Initialize(request, fireUtcDateTime, _defaultProcessor);
        return _defaultCrowdMessageProcessor;
    }
}