Prism ServiceLocator GetInstance and MEF

c#, mef, prism

Solution

Prism uses `MefServiceLocatorAdapter` to adapt MEF's `CompositionsContainer` to the `IServiceLocator` interface. This is the actual code that is used by it to get your part:

protected override object DoGetInstance(Type serviceType, string key)
{
    IEnumerable<Lazy<object, object>> exports = this.compositionContainer.GetExports(serviceType, null, key);
    if ((exports != null) && (exports.Count() > 0))
    {
        // If there is more than one value, this will throw an InvalidOperationException, 
        // which will be wrapped by the base class as an ActivationException.
        return exports.Single().Value;
    }

    throw new ActivationException(
        this.FormatActivationExceptionMessage(new CompositionException("Export not found"), serviceType, key));
}

So as you can see, you're exporting and calling `GetInstance` correctly. However, I believe your service locator isn't set up correctly, and that's why you're getting this exception. If you're using Prism's `MefBootstrapper` to initialize your application, this should already be done for you. Otherwise, you'll need to use this code to initialize it:

IServiceLocator serviceLocator = new MefServiceLocatorAdapter(myCompositionContainer);
ServiceLocator.SetLocatorProvider(() => serviceLocator);

Problem

I'm trying to use Microsoft.Practices.ServiceLocation.ServiceLocator and MEF. Interface IServiceLocator defines method GetInstance with two parameters. First parameter is serviceType and second paremeter is key. I have two classes which implement interface IMyInterface and they have Export attribute: ``` [Export(typeof(IMyInterface)), PartCreationPolicy(CreationPolicy.Shared)] public class Class1 : IMyInterface {} [Export(typeof(IMyInterface)), PartCreationPolicy(CreationPolicy.Shared)] public class Class2: IMyInterface {} ``` I want to get instance of Class1 via Prism ServiceLocator GetInstance method: ``` ServiceLocator.Current.GetInstance(typeof(IMyInterface),"Key"); ``` But I have no idea how and where should be "key" defined. I tried to define key in export attribute: ``` [Export("Key1",typeof(IMyInterface)), PartCreationPolicy(CreationPolicy.Shared)] public class Class1 : IMyInterface {} [Export("Key2",typeof(IMyInterface)), PartCreationPolicy(CreationPolicy.Shared)] public class Class2: IMyInterface {} ``` When I call method GetInstance with key parameter ``` ServiceLocator.Current.GetInstance(typeof(IMyInterface),"Key1"); ``` I get Microsoft.Practices.ServiceLocation.ActivationException (Activation error occured while trying to get instance of type IMyInterface, key "Key1"). Does anybody know how can be the export key defined? Thanks

Original source