C# – Prism ServiceLocator GetInstance and MEF

cmefprism

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

Best Answer

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);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – MVC4 MEF-based dynamically loaded plugins

I'm currently working on the same issue. I've found this solution:

Blog post: http://blog.longle.net/2012/03/29/building-a-composite-mvc3-application-with-pluggable-areas/
Source code: https://bitbucket.org/darincreason/mvcapplication8.web

Basically it loads assemblies from specified location and with some name pattern on web application startup:

AssemblyInfo.cs:

[assembly: PreApplicationStartMethod(
  typeof(PluginAreaBootstrapper), &quot;Init&quot;)]

PluginAreaBootstrapper.cs:

public class PluginAreaBootstrapper
{
    public static readonly List<Assembly> PluginAssemblies = new List<Assembly>();

    public static List<string> PluginNames()
    {
        return PluginAssemblies.Select(
            pluginAssembly => pluginAssembly.GetName().Name)
            .ToList();
    }

    public static void Init()
    {
        var fullPluginPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Areas");

        foreach (var file in Directory.EnumerateFiles(fullPluginPath, "*Plugin*.dll", SearchOption.AllDirectories))
            PluginAssemblies.Add(Assembly.LoadFile(file));

        PluginAssemblies.ForEach(BuildManager.AddReferencedAssembly);

        // Add assembly handler for strongly-typed view models
        AppDomain.CurrentDomain.AssemblyResolve += AssemblyResolve;
    }

    private static Assembly AssemblyResolve(object sender, ResolveEventArgs resolveArgs)
    {
        var currentAssemblies = AppDomain.CurrentDomain.GetAssemblies();
        // Check we don't already have the assembly loaded
        foreach (var assembly in currentAssemblies)
        {
            if (assembly.FullName == resolveArgs.Name || assembly.GetName().Name == resolveArgs.Name)
            {
                return assembly;
            }
        }

        return null;
    }
}

But I believe you can create some directory observer which can dynamically load assemblies, so you don't even need to restart your web application.

In my opinion it meets your 1, 2 and 4 needs. It's very simple, doesn't require any frameworks, has minimal configuration, allows dynamic loading of the plugins and works with MVC 4.

This solution plugs assemblies into Area directory, but I believe you can quite easily tune it to play as you like using routing.

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – MVC4 MEF-based dynamically loaded plugins

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