Question

I’m using mef to create WCF web services. This is how service looks:

[Export]
    [ServiceContract]
    [AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
    public class MobileService
    {
        [Import] 
        public IEmailService EmailService { get; set; }

        [Import] 
        public ILoggerService LoggerService { get; set; }

        [Import] 
        public IContextManager ContextManager { get; set; }

This is how code to retreive service instance looks:

// Get Service instace via MEF        
    public object GetInstance(InstanceContext instanceContext, Message message)
    {
        var lazyInstance = Container.GetExports(ServiceType, null, null).FirstOrDefault();
        var instance = lazyInstance.Value;

        return instance;
    } 

MEF creates EmailService, LoggerService and this if fine, they live happily while service lives.

Now, ContextManager is different animal. In GetInstance – I really like to “kill” it and re-create. ContextManager studies URL and headers during construction and populates “context”. With code like I have – it is created first time and never get’s destroyed. How to change this behavior?

Thanks!

Answer 1

On your export of the implementation of IContextManager, you need to mark the export with the non-shared part creation policy. For example:

[Export(typeof(IContextManager)]
[PartCreationPolicy(CreationPolicy.NonShared)]
public class ContextManager : IContextManager
{
   ...
}

This will inform MEF that it should create a new instance of the export every time it satisfies an import. By default MEF uses CreationPolicy.Shared which will create just a single exported value (a singleton), which is probably what you want for the Email and Logging implementations.