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Nigel Sampson

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Using LINQ to initialize DI Containers

09 Mar 2008 by Nigel Sampson

Recently there have been some discussion on how to help initialize dependency injection containers. Ayende call this "Setting up Zero Fiction Projects" and uses the Boo language to create a DSL (Domain Specific Language) to specify components to be registered into the container. From hammet comes news around a fluent interface for adding components to Castle Windsor. The essence of both is to find all types in a given assembly that either implement a certain a certain interface or are a subclass of a layer supertype (I tend to have a ServiceBase that contains things Log and so on). We can use LINQ to achieve something similar to both the above approaches.

public static void RegisterControllers(params string[] assemblies)
{
    foreach(string assembly in assemblies)
    {
        IEnumerable<Type> controllers = from t in Assembly.Load(assembly).GetTypes()
                                        where t.Implements<IController>()
                                        select t;
 
        foreach(Type controller in controllers)
        {
            Container.AddComponentWithLifestyle(controller.FullName, controller, LifestyleType.Transient);
        }
    }
}

The above code iterates a list of assembly names and finds all types that implement the ASP.NET MVC IController interface. To increase the readability of the LINQ query I've used an extension method listed below.

public static bool Implements<T>(this Type type)
{
    return typeof(T).IsAssignableFrom(type);
}

The below code locates the first interface for the service and uses that as the service to register the type. Depending on how you create your types this may or may not be a good method. Using the Repository class I've described before doesn't allow this approach as it is the last interface that becomes the important one, however I've used this code to show some similar functionality as the examples above. Hope this helps someone.

public static void RegisterServices(params string[] assemblies)
{
    foreach(string assembly in assemblies)
    {
        var services = from t in Assembly.Load(assembly).GetTypes()
                       where t.GetInterfaces().Length > 0 && t.IsSubclassOf(typeof(ServiceBase))
                       let i = t.GetInterfaces()[0]
                       where !Container.Kernel.HasComponent(i)
                       select new
                       {
                           Interface = i.IsGenericType ? i.GetGenericTypeDefinition() : i,
                           Component = t
                       };
 
        foreach(var service in services)
        {
            Container.AddComponent(service.Interface.FullName, service.Interface, service.Component);
        }
    }
}

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