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Dependency Injection in SignalR

By Mike Wasson and Patrick Fletcher|

Dependency injection is a way to remove hard-coded dependencies between objects, making it easier to replace an object's dependencies, either for testing (using mock objects) or to change run-time behavior. This tutorial shows how to perform dependency injection on SignalR hubs. It also shows how to use IoC containers with SignalR. An IoC container is a general framework for dependency injection.

What is Dependency Injection?

Skip this section if you are already familiar with dependency injection.

Dependency injection (DI) is a pattern where objects are not responsible for creating their own dependencies. Here is a simple example to motivate DI. Suppose you have an object that needs to log messages. You might define a logging interface:

interface ILogger 
{
    void LogMessage(string message);
}

In your object, you can create an ILogger to log messages:

// Without dependency injection.
class SomeComponent
{
    ILogger _logger = new FileLogger(@"C:\logs\log.txt");

    public void DoSomething()
    {
        _logger.LogMessage("DoSomething");
    }
}

This works, but it’s not the best design. If you want to replace FileLogger with another ILogger implementation, you will have to modify SomeComponent. Supposing that a lot of other objects use FileLogger, you will need to change all of them. Or if you decide to make FileLogger a singleton, you’ll also need to make changes throughout the application.

A better approach is to “inject” an ILogger into the object—for example, by using a constructor argument:

// With dependency injection.
class SomeComponent
{
    ILogger _logger;

    // Inject ILogger into the object.
    public SomeComponent(ILogger logger)
    {
        if (logger == null)
        {
            throw new NullReferenceException("logger");
        }
        _logger = logger;
    }

    public void DoSomething()
    {
        _logger.LogMessage("DoSomething");
    }
}

Now the object is not responsible for selecting which ILogger to use. You can swich ILogger implementations without changing the objects that depend on it.

var logger = new TraceLogger(@"C:\logs\log.etl");
var someComponent = new SomeComponent(logger);

This pattern is called constructor injection. Another pattern is setter injection, where you set the dependency through a setter method or property.

Simple Dependency Injection in SignalR

Consider the Chat application from the tutorial Getting Started with SignalR. Here is the hub class from that application:

public class ChatHub : Hub
{
    public void Send(string name, string message)
    {
        Clients.All.addMessage(name, message);
    }
}

Suppose that you want to store chat messages on the server before sending them. You might define an interface that abstracts this functionality, and use DI to inject the interface into the ChatHub class.

public interface IChatRepository
{
    void Add(string name, string message);
    // Other methods not shown.
}

public class ChatHub : Hub
{
    private IChatRepository _repository;

    public ChatHub(IChatRepository repository)
    {
        _repository = repository;
    }

    public void Send(string name, string message)
    {
        _repository.Add(name, message);
        Clients.All.addMessage(name, message);
    }

The only problem is that a SignalR application does not directly create hubs; SignalR creates them for you. By default, SignalR expects a hub class to have a parameterless constructor. However, you can easily register a function to create hub instances, and use this function to perform DI. Register the function by calling GlobalHost.DependencyResolver.Register.

    protected void Application_Start()
    {
        GlobalHost.DependencyResolver.Register(
            typeof(ChatHub), 
            () => new ChatHub(new ChatMessageRepository()));

        RouteTable.Routes.MapHubs();

        // ...
    }

Now SignalR will invoke this anonymous function whenever it needs to create a ChatHub instance.

IoC Containers

The previous code is fine for simple cases. But you still had to write this:

... new ChatHub(new ChatMessageRepository()) ...

In a complex application with many dependencies, you might need to write a lot of this “wiring” code. This code can be hard to maintain, especially if dependencies are nested. It is also hard to unit test.

One solution is to use an IoC container. An IoC container is a software component that is responsible for managing dependencies.You register types with the container, and then use the container to create objects. The container automatically figures out the dependency relations. Many IoC containers also allow you to control things like object lifetime and scope.

Note: “IoC” stands for “inversion of control”, which is a general pattern where a framework calls into application code. An IoC container constructs your objects for you, which “inverts” the usual flow of control.

Using IoC Containers in SignalR

The Chat application is probably too simple to benefit from an IoC container. Instead, let’s look at the StockTicker sample.

The StockTicker sample defines two main classes:

  • StockTickerHub: The hub class, which manages client connections.
  • StockTicker: A singleton that holds stock prices and periodically updates them.

StockTickerHub holds a reference to the StockTicker singleton, while StockTicker holds a reference to the IHubConnectionContext for the StockTickerHub. It uses this interface to communicate with StockTickerHub instances. (For more information, see Server Broadcast with ASP.NET SignalR.)

We can use an IoC container to untangle these dependencies a bit. First, let's simplify the StockTickerHub and StockTicker classes. In the following code, I've commented out the parts that we don't need.

Remove the parameterless constructor from StockTicker. Instead, we will always use DI to create the hub.

[HubName("stockTicker")]
public class StockTickerHub : Hub
{
    private readonly StockTicker _stockTicker;

    //public StockTickerHub() : this(StockTicker.Instance) { }

    public StockTickerHub(StockTicker stockTicker)
    {
        if (stockTicker == null)
        {
            throw new ArgumentNullException("stockTicker");
        }
        _stockTicker = stockTicker;
    }

    // ...

For StockTicker, remove the singleton instance. Later, we'll use the IoC container to control the StockTicker lifetime. Also, make the constructor public.

public class StockTicker
{
    //private readonly static Lazy<StockTicker> _instance = new Lazy<StockTicker>(
    //    () => new StockTicker(GlobalHost.ConnectionManager.GetHubContext<StockTickerHub>().Clients));

    // Important! Make this constructor public.
    public StockTicker(IHubConnectionContext clients)
    {
        if (clients == null)
        {
            throw new ArgumentNullException("clients");
        }

        Clients = clients;
        LoadDefaultStocks();
    }

    //public static StockTicker Instance
    //{
    //    get
    //    {
    //        return _instance.Value;
    //    }
    //}

Next, we can refactor the code by creating an interface for StockTicker. We’ll use this interface to decouple the StockTickerHub from the StockTicker class.

Visual Studio makes this kind of refactoring easy. Open the file StockTicker.cs, right-click on the StockTicker class declaration, and select Refactor ... Extract Interface.

In the Extract Interface dialog, click Select All. Leave the other defaults. Click OK.

Visual Studio creates a new interface named IStockTicker, and also changes StockTicker to derive from IStockTicker.

Open the file IStockTicker.cs and change the interface to public.

public interface IStockTicker
{
    void CloseMarket();
    IEnumerable<Stock> GetAllStocks();
    MarketState MarketState { get; }
    void OpenMarket();
    void Reset();
}

In the StockTickerHub class, change the two instances of StockTicker to IStockTicker:

[HubName("stockTicker")]
public class StockTickerHub : Hub
{
    private readonly IStockTicker _stockTicker;

    public StockTickerHub(IStockTicker stockTicker)
    {
        if (stockTicker == null)
        {
            throw new ArgumentNullException("stockTicker");
        }
        _stockTicker = stockTicker;
    }

Creating an IStockTicker interface isn’t strictly necessary, but I wanted to show how DI can help to reduce coupling between components in your application.

Add the Ninject Library

There are many open-source IoC containers for .NET. For this tutorial, I’ll use Ninject. (Other popular libraries include Castle Windsor, Spring.Net, Autofac, Unity, and StructureMap.)

Use NuGet Package Manager to install the Ninject library. In Visual Studio, from the Tools menu select Library Package Manager | Package Manager Console. In the Package Manager Console window, enter the following command:

    Install-Package Ninject -Version 3.0.1.10 

Replace the SignalR Dependency Resolver

To use Ninject within SignalR, create a class that derives from DefaultDependencyResolver.

internal class NinjectSignalRDependencyResolver : DefaultDependencyResolver
{
    private readonly IKernel _kernel;
    public NinjectSignalRDependencyResolver(IKernel kernel)
    {
        _kernel = kernel;
    }

    public override object GetService(Type serviceType)
    {
        return _kernel.TryGet(serviceType) ?? base.GetService(serviceType);
    }

    public override IEnumerable<object> GetServices(Type serviceType)
    {
        return _kernel.GetAll(serviceType).Concat(base.GetServices(serviceType));
    }
}

This class overrides the GetService and GetServices methods of DefaultDependencyResolver. SignalR calls these methods to create various objects at runtime, including hub instances, as well as various services used internally by SignalR.

  • The GetService method creates a single instance of a type. Override this method to call the Ninject kernel's TryGet method. If that method returns null, fall back to the default resolver.
  • The GetServices method creates a collection of objects of a specified type. Override this method to concatenate the results from Ninject with the results from the default resolver.

Configure Ninject Bindings

Now we’ll use Ninject to declare type bindings.

Open the file RegisterHubs.cs. In the RegisterHubs.Start method, create the Ninject container, which Ninject calls the kernel.

var kernel = new StandardKernel();

Create an instance of our custom dependency resolver:

var resolver = new NinjectSignalRDependencyResolver(kernel);

Create a binding for IStockTicker as follows:

kernel.Bind<IStockTicker>()
    .To<Microsoft.AspNet.SignalR.StockTicker.StockTicker>()  // Bind to StockTicker.
    .InSingletonScope();  // Make it a singleton object.

This code is saying two things. First, whenever the application needs an IStockTicker, the kernel should create an instance of StockTicker. Second, the StockTicker class should be a created as a singleton object. Ninject will create one instance of the object, and return the same instance for each request.

Create a binding for IHubConnectionContext as follows:

kernel.Bind<IHubConnectionContext>().ToMethod(context =>
        resolver.Resolve<IConnectionManager>().GetHubContext<StockTickerHub>().Clients
    ).WhenInjectedInto<IStockTicker>();

This code creatres an anonymous function that returns an IHubConnection. The WhenInjectedInto method tells Ninject to use this function only when creating IStockTicker instances. The reason is that SignalR creates IHubConnectionContext instances internally, and we don't want to override how SignalR creates them. This function only applies to our StockTicker class.

Pass the dependency resolver into the MapHubs method:

RouteTable.Routes.MapHubs(config);

Now SignalR will use the resolver specified in MapHubs, instead of the default resolver.

Here is the complete code listing for RegisterHubs.Start.

public static class RegisterHubs
{
    public static void Start()
    {
        var kernel = new StandardKernel();
        var resolver = new NinjectSignalRDependencyResolver(kernel);

        kernel.Bind<IStockTicker>()
            .To<Microsoft.AspNet.SignalR.StockTicker.StockTicker>()
            .InSingletonScope();

        kernel.Bind<IHubConnectionContext>().ToMethod(context =>
                resolver.Resolve<IConnectionManager>().
                    GetHubContext<StockTickerHub>().Clients
            ).WhenInjectedInto<IStockTicker>();

        var config = new HubConfiguration()
        {
            Resolver = resolver
        };

        // Register the default hubs route: ~/signalr/hubs
        RouteTable.Routes.MapHubs(config);
    }
}

To run the StockTicker application in Visual Studio, press F5. In the browser window, navigate to http://localhost:port/SignalR.Sample/StockTicker.html.

The application has exactly the same functionality as before. (For a description, see Server Broadcast with ASP.NET SignalR.) We haven't changed the behavior; just made the code easier to test, maintain, and evolve.

Author Information

Mike Wasson

Mike Wasson – Mike Wasson is a programmer-writer at Microsoft.

Patrick Fletcher

Patrick Fletcher – Patrick Fletcher is a programmer-writer on the ASP.NET team, currently working on SignalR.