RESTful Day #3: Resolve Dependency of Dependencies Using Inversion of Control & Dependency Injection in ASP.Net Web APIs with Unity Container and Managed Extensibility Framework (MEF)

container.RegisterType<IProductServices, ProductServices>()
         .RegisterType<UnitOfWork>(new HierarchicalLifetimeManager());

namespace Resolver
{
    /// <summary>
    /// Register underlying types with unity.
    /// </summary>
    public interface IComponent
    {
    }
}

namespace Resolver
{
    /// <summary>
    /// Responsible for registering types in Unity configuration by implementing IComponent
    /// </summary>
    public interface IRegisterComponent
    {
        /// <summary>
        /// Register type method
        /// </summary>
        /// <typeparam name="TFrom"></typeparam>
        /// <typeparam name="TTo"></typeparam>
        /// <param name="withInterception"></param>
        void RegisterType<TFrom, TTo>(bool withInterception = false) where TTo : TFrom;

        /// <summary>
        /// Register type with container controlled life time manager
        /// </summary>
        /// <typeparam name="TFrom"></typeparam>
        /// <typeparam name="TTo"></typeparam>
        /// <param name="withInterception"></param>
        void RegisterTypeWithControlledLifeTime<TFrom, TTo>(bool withInterception = false) where TTo : TFrom;
    }
}

void SetUp(IRegisterComponent registerComponent);

namespace Resolver
{
    /// <summary>
    /// Register underlying types with unity.
    /// </summary>
    public interface IComponent
    {
        void SetUp(IRegisterComponent registerComponent);
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Practices.Unity;
using System.ComponentModel.Composition.Hosting;
using System.ComponentModel.Composition.Primitives;
using System.Reflection;

namespace Resolver
{
    public static class ComponentLoader
    {
        public static void LoadContainer(IUnityContainer container, string path, string pattern)
        {
            var dirCat = new DirectoryCatalog(path, pattern);
            var importDef = BuildImportDefinition();
            try
            {
                using (var aggregateCatalog = new AggregateCatalog())
                {
                    aggregateCatalog.Catalogs.Add(dirCat);

                    using (var componsitionContainer = new CompositionContainer(aggregateCatalog))
                    {
                        IEnumerable<Export> exports = componsitionContainer.GetExports(importDef);

                        IEnumerable<IComponent> modules =
                            exports.Select(export => export.Value as IComponent).Where(m => m != null);

                        var registerComponent = new RegisterComponent(container);
                        foreach (IComponent module in modules)
                        {
                            module.SetUp(registerComponent);
                        }
                    }
                }
            }
            catch (ReflectionTypeLoadException typeLoadException)
            {
                var builder = new StringBuilder();
                foreach (Exception loaderException in typeLoadException.LoaderExceptions)
                {
                    builder.AppendFormat("{0}\n", loaderException.Message);
                }

                throw new TypeLoadException(builder.ToString(), typeLoadException);
            }
        }

        private static ImportDefinition BuildImportDefinition()
        {
            return new ImportDefinition(
                def => true, typeof(IComponent).FullName, ImportCardinality.ZeroOrMore, false, false);
        }
    }

    internal class RegisterComponent : IRegisterComponent
    {
        private readonly IUnityContainer _container;

        public RegisterComponent(IUnityContainer container)
        {
            this._container = container;
            // Register interception behaviour if any
        }

        public void RegisterType<TFrom, TTo>(bool withInterception = false) where TTo : TFrom
        {
            if (withInterception)
            {
                // register with interception   
            }
            else
            {
                this._container.RegisterType<TFrom, TTo>();
            }
        }

        public void RegisterTypeWithControlledLifeTime<TFrom, TTo>(bool withInterception = false) where TTo : TFrom
        {
            this._container.RegisterType<TFrom, TTo>(new ContainerControlledLifetimeManager());
        }
    }
}

Step 8. Now our Resolver wrapper is ready. Build the project and add its reference to the DataModel, BusinessServices, and WebApi projects like shown below.

Setup Business Services

We have already added a reference for Resolver in the BusinessServices project. We agreed to implement the IComponent interface in each of our projects.

So create a class named DependencyResolver and implement an IComponent interface into it. We use reflection too to import the IComponent type. So add a class and add the following code to that DependencyResolver class.

using System.ComponentModel.Composition;
using DataModel;
using DataModel.UnitOfWork;
using Resolver;

namespace BusinessServices
{
    [Export(typeof(IComponent))]
    public class DependencyResolver : IComponent
    {
        public void SetUp(IRegisterComponent registerComponent)
        {
            registerComponent.RegisterType<IProductServices, ProductServices>();
        }
    }
}

Note that we have implemented the SetUp method and in the same method we registered the type for my ProductService.

All of the existing codebases remain the same. We don't need to touch the IProductServices interface or ProductServices class.

Setup DataModel

We have added a Resolver project reference to the DataModel project as well. So we'll try to register the type of UnitOfWork in this project. We proceed in the same fashion, just add a DependencyResolver class and implement its Setup method to register a type of UnitOfWork. To make the code more readable and standard, I made a change. I just added an interface for UnitOfWork and named it IUnitOfWork. Now my UnitOfWork class derives from this, you can do this exercise in earlier versions of the projects we discussed in the first two articles.

So my IUnitOfWork contains a declaration of a single public method in UnitOfWork as in the following.

namespace DataModel.UnitOfWork
{
    public interface IUnitOfWork
    {
        /// <summary>
        /// Save method.
        /// </summary>
        void Save();
    }
}

Now register the type for UnitOfWork in the DepenencyResolver class. Our class becomes as shown below.

using System.ComponentModel.Composition;
using System.Data.Entity;
using DataModel.UnitOfWork;
using Resolver;

namespace DataModel
{
    [Export(typeof(IComponent))]
    public class DependencyResolver : IComponent
    {
        public void SetUp(IRegisterComponent registerComponent)
        {
            registerComponent.RegisterType<IUnitOfWork, UnitOfWork.UnitOfWork>();
        }
    }
}

Again, there is no need to touch any existing code of this project.

Setup REST endpoint / WebAPI project

90% of the job is done.

We now need to set up the WebAPI project. We'll not add any DependencyResolver class to this project. We'll invert the calling mechanism of layers in the Bootstrapper class that we already have, so when you open your bootstrapper class, you'll get the code, something like.

using System.Web.Http;
using System.Web.Mvc;
using BusinessServices;
using DataModel.UnitOfWork;
using Microsoft.Practices.Unity;
using Unity.Mvc3;

namespace WebApi
{
    public static class Bootstrapper
    {
        public static void Initialise()
        {
            var container = BuildUnityContainer();

            DependencyResolver.SetResolver(new UnityDependencyResolver(container));

            // register dependency resolver for WebAPI RC
            GlobalConfiguration.Configuration.DependencyResolver = new Unity.WebApi.UnityDependencyResolver(container);
        }

        private static IUnityContainer BuildUnityContainer()
        {
            var container = new UnityContainer();

            // register all your components with the container here
            // it is NOT necessary to register your controllers
            
            // e.g. container.RegisterType<ITestService, TestService>();
            container.RegisterType<IProductServices, ProductServices>().RegisterType<UnitOfWork>(new HierarchicalLifetimeManager());

            return container;
        }
    }
}

Now, we need to change the codebase a bit to make our system loosely coupled. Just remove the reference of DataModel from the WebAPI project.

Let me revise what is the job of "HierarchicalLifetimeManager",

"HierarchicalLifetimeManager" , for this lifetime manager, as for the ContainerControlledLifetimeManager, Unity returns the same instance of the registered type or object each time you call the Resolve or ResolveAll method or when the dependency mechanism injects instances into other classes. The distinction is that when there are child containers, each child resolves its own instance of the object and does not share one with the parent. When resolving in the parent, the behavior is like a container-controlled lifetime; when resolving the parent and the child you have different instances with each acting as a container-controlled lifetime. If you have multiple children, each will resolve its own instance.

We don't want our DataModel to be exposed to the WebAPI project. That was our intent though, so we cut down the dependency of the DataModel project now.

Add the following code of the Bootstrapper class to the existing Bootstrapper class.

using System.Web.Http;
//using DataModel.UnitOfWork;
using Microsoft.Practices.Unity;
using Resolver;
using Unity.Mvc3;

namespace WebApi
{
    public static class Bootstrapper
    {
        public static void Initialise()
        {
            var container = BuildUnityContainer();

            System.Web.Mvc.DependencyResolver.SetResolver(new UnityDependencyResolver(container));

            // register dependency resolver for WebAPI RC
            GlobalConfiguration.Configuration.DependencyResolver = new Unity.WebApi.UnityDependencyResolver(container);
        }

        private static IUnityContainer BuildUnityContainer()
        {
            var container = new UnityContainer();

            // register all your components with the container here
            // it is NOT necessary to register your controllers

            // e.g. container.RegisterType<ITestService, TestService>();
            // container.RegisterType<IProductServices, ProductServices>().RegisterType<UnitOfWork>(new HierarchicalLifetimeManager());

            RegisterTypes(container);

            return container;
        }

        public static void RegisterTypes(IUnityContainer container)
        {

            // Component initialization via MEF
            ComponentLoader.LoadContainer(container, ".\\bin", "WebApi.dll");
            ComponentLoader.LoadContainer(container, ".\\bin", "BusinessServices.dll");

        }
    }
}

It is kind of redefining the Bootstrapper class without touching our existing controller methods. We now don't even need to register the type for ProductServices as well, we already did this in the BusinessServices project.

Note that in the RegisterTypes method, we load the components/DLLs using reflection using ComponentLoader.We wrote two lines, first to load the WebAPI.dll and another one to load Business Services.dll.

Had the name of the BusinessServicess.dll been WebAPI.Services.dll then we would have only written one line of code to load both the WebAPI and the BusinessService DLL as shown below.

ComponentLoader.LoadContainer(container, ".\\bin", "WebApi*.dll");

Yes, we can use a Regex.

Running the application

Just run the application. We get:

We already have our test client added, but for new readers, just go to Manage Nuget Packages by right-clicking the WebAPI project and type WebAPITestClient into the searchbox in online packages as in the following:

You'll get “A simple Test Client for ASP.NET Web API”, just add it. You'll get a help controller in Areas -> HelpPage as shown below.

I have already provided the database scripts and data in my previous article, you can use that.

Append “/help” to the application URL and you'll get the test client as in the following.

You can test each service by clicking on it.

Service for GetAllProduct.

To create a new product.

In the database, we get a new product as in the following.

Update product.

We get into the database.

Delete product.

In the database.

Advantages of this design

In my earlier articles I focussed more on design flaws, but our current design has emerged with a few added advantages.

We have an extensible and loosely coupled application design that can go far with more new components added in the same way.

Registering types automatically using reflection. Assume we want to register any Interface implementation to our REST endpoint, we just need to load that DLL into our Bootstrapper class, or if the DLLs share a common suffix of the names then we just need to place that DLL in the bin folder and that will automatically be loaded at run time.

Image source: http://a.pragprog.com/magazines/2013-07/images/iStock_000021011143XSmall__1mfk9b__.jpg

Database transactions or any of such module is now not exposed to the service endpoint, this makes our service more secure and maintains the design structure too.

Conclusion

We now know how to use a Unity container to resolve dependency and perform inversion of control using MEF too. In my next article, I'll try to explain how to open multiple endpoints to our REST service and create custom URLs in the true REST fashion in my WebAPI. Until then Happy Coding. You can also download the source code from GitHub. Add the required packages, if they are missing in the source code.

Read more

• C# and ASP.NET Questions (All in one)
• MVC Interview Questions
• C# and ASP.NET Interview Questions and Answers
• Web Services and Windows Services Interview Questions

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My other series of articles.

• Learning MVC
• Learning OOP concepts with C#