Inversion of Control in ASP.NET Web API

Adding Inversion of Control to ASP.NET Web API provides tremendous value. Our ApiControllers become decoupled from the rest of our codebase, no longer have to manage their dependencies, and become easier to test. Adding Inversion of Control is simple and only takes a few lines of code.

IDependencyResolver

ASP.NET Web API provides an abstraction for dependency injection containers with the IDependencyResolver & IDependencyScope interfaces.

using System;

namespace System.Web.Http.Dependencies
{
    // Summary:
    //     Represents a dependency injection container.
    public interface IDependencyResolver : IDependencyScope, IDisposable
    {
        // Summary:
        //     Starts a resolution scope.
        //
        // Returns:
        //     The dependency scope.
        IDependencyScope BeginScope();
    }
}

using System;
using System.Collections.Generic;

namespace System.Web.Http.Dependencies
{
    // Summary:
    //     Represents an interface for the range of the dependencies.
    public interface IDependencyScope : IDisposable
    {
        // Summary:
        //     Retrieves a service from the scope.
        //
        // Parameters:
        //   serviceType:
        //     The service to be retrieved.
        //
        // Returns:
        //     The retrieved service.
        object GetService(Type serviceType);
        //
        // Summary:
        //     Retrieves a collection of services from the scope.
        //
        // Parameters:
        //   serviceType:
        //     The collection of services to be retrieved.
        //
        // Returns:
        //     The retrieved collection of services.
        IEnumerable<object> GetServices(Type serviceType);
    }
}

Implementing IDependencyResolver

There are many IoC containers out there. Any will do. In this example, we will use my favorite, TinyIoC.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http.Dependencies;
using TinyIoC;

public class TinyIoCDependencyResolver : IDependencyResolver
{
    private TinyIoCContainer _container;

    public TinyIoCDependencyResolver(TinyIoCContainer container)
    {
        if (container == null)
            throw new ArgumentNullException("container");

        _container = container;
    }

    public IDependencyScope BeginScope()
    {
        if (_disposed)
            throw new ObjectDisposedException("this", "This scope has already been disposed.");

        return new TinyIoCDependencyResolver(_container.GetChildContainer());
    }

    public object GetService(Type serviceType)
    {
        if (_disposed)
            throw new ObjectDisposedException("this", "This scope has already been disposed.");

        try
        {
            return _container.Resolve(serviceType);
        }
        catch (TinyIoCResolutionException)
        {
            return null;
        }
    }

    public IEnumerable<object> GetServices(Type serviceType)
    {
        if (_disposed)
            throw new ObjectDisposedException("this", "This scope has already been disposed.");

        try
        {
            return _container.ResolveAll(serviceType);
        }
        catch (TinyIoCResolutionException)
        {
            return Enumerable.Empty<object>();
        }
    }

    #region IDisposable

    bool _disposed;

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed)
            return;

        if (disposing)
            _container.Dispose();

        _disposed = true;
    }

    #endregion IDisposable
}

Using IDependencyResolver

Using our IDependencyResolver is simple, all we have to do is to register it with our ASP.NET Web API configuration.

var container = new TinyIoCContainer();
container.Register<IMyDependency>(new MyDependency());

GlobalConfiguration.Configuration.DependencyResolver = new TinyIoCDependencyResolver(container);

Now, the instantiation of our ApiControllers pass through our IoC container, injecting our dependencies along the way. Here is an example ApiController with a dependency:

using System;
using System.Net;
using System.Net.Http;
using System.Web.Http;

public class ExampleController : ApiController
{
    private IMyDependency _myDependency;

    public ExampleController(IMyDependency myDependency)
    {
        if (myDependency == null)
            throw new ArgumentNullException("myDependency");

        _myDependency = myDependency;
    }

    [HttpGet]
    [Route("example")]
    public HttpResponseMessage Test()
    {
        var model = new { Message = "dependency injected!" };
        return Request.CreateResponse(HttpStatusCode.OK, model);
    }
}

Conclusion

In a few lines of code, we have added tremendous value to our ASP.NET Web API. Our ApiControllers are decoupled from the rest of our codebase, no longer have to manage their dependencies, and are easier to test!