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Published: 17 March 2015

IoC (see Inversion of Control on Wikipedia) is a pattern of best practice which is used to decouple a component that relies on other functionality to perform its task.

An example being an application which needs to access data storage be it a file, database, webservice or other mechanism. It requires this to do its job but doesn't need to know about the implementation details.

IoC allows you to decouple your program into separate parts. This is good because:

  • Components can be easily tested independently.
  • Program complexity can be reduced.
  • You can switch components to another implementation.
  • However in some cases, IoC can make code harder to understand.

If you want to see good example of real-world usage of IoC, have a look at Mircosoft Composite UI Application Block and CompositeWPF

Two types of IoC are Depedency Injection and Service Locator

Simple Example

To explain how IoC works I've created this simple class that either adds two numbers together or takes two numbers away. They look like this.

            public interface ICalculator
                string Calculate(decimal number, decimal number2);
            public abstract class Calculator : ICalculator
                public abstract string Calculate(decimal number, decimal number2);
            public class Add : Calculator
                public override string Calculate(decimal number, decimal number2)
                    return (number + number2).ToString(CultureInfo.InvariantCulture);
            public class Subtract : Calculator
                public override string Calculate(decimal number, decimal number2)
                    return (number - number2).ToString(CultureInfo.InvariantCulture);

Why IoC

In the usual way we would create any dependants we need in the constructor. Taking this code as an example, we create a calculator object. We could be creating a validator to validate the input on a fictional control. Or we need to access the database so we create a Repository to do so.

            public CalculatorController()
                var ICalculator _calculator = new Add();

you might rewrite it like the following code to use IoC. We say that whatever class is given to the constructor as long as it implements the same set of commands our code will work.

        public class AddressEditController : Controller
            private readonly ICalculator _calculator;
            private IRepository _repository;

            public AddressEditController(ICalculator calculator, IRepository repository)
                _calculator = calculator;

The biggest thing in my opinion to watch out for is were polymorphism is being broken. This usually happens without the coders knowledge, were swapping one class for another doesn't result in the same out come. For more information I recommend reading "Agile Principles, Patterns, and Practices in C#"

Service Locator

Service locator is a system were by a central store of 'services' is recorded, this central store is usually referred to as a 'registry'. When a component requires a particular service it calls the service locator to get an instance of what is required.

Microsoft MVC and WPF are built using the Service Locator pattern behind the scenes. They usually use the pattern as it's defined in Microsoft Patterns and Practices.

For instance in MVC you can use Microsoft.Practices.ServiceLocation.ServiceLocator as long as you have defined a ServiceLocator before hand.

You must also install the NuGet package Microsoft.Practices.ServiceLocation

The service locator is commonly referred to as an Anti Pattern due to the component parts needing to know about the service locator.

Dependency Injection

DI consists of several types these are:

  • Type 1 Interface injection
  • Type 2 Setter injection
  • Type 3 Constructor Injection

Interface injection

Interface injection is essentially the 'right' way of wiring applications together. That is you should define a contract between components. For instance in dependency injection you can wire it up like this.


What that means is that each component part of the system needs to know about the Calculator class and what operations it can perform. The way it should be done is like this.


So you define an interface and that interface is swapped out for a concrete class.

Constructor Injection

Constructor Injection as the name implies takes the thing you want and passes it automagically into the constructor. Its usually done by checking the interfaces on the constructor and passing in the things required.

        private readonly ICalculator _myCalculator;
        public ConstructorInjectionExampleController(ICalculator myCalculator)
            _myCalculator = myCalculator;

The above example with the correct dependency injection setup, the object will get automatically passed into the constructor for use by the rest of the class.

Setter injection

Setter injection will inject a class into a property on an object when its instantiated. So say your class had a property like this.

        public ICalculator Calculator { get; set; }

You can setup dependency injection to create a new Add or Subtract object and inject it straightinto the property when your class is created.

Don't use ObjectFactory.GetInstance

Most Inversion Of Control containers give you a method for returning an object. For instance in Structure Map you can use ObjectFactory.GetInstance. This shouldn't be called directly, because the whole point is not to litter you code with code to create objects. You might as well use new Account in this example as ObjectFactory.GetInstance.

            using System;
            using NBehave.Spec.NUnit;
            using StructureMap;
            namespace StructureMapTest {
                public interface ICustomer { string GetName(); }
                public class Customer : ICustomer
                    public string GetName(){return "T. Boone Pickens";}
                    public override string ToString() {return GetName();}
                public class Account
                    public ICustomer MyCustomer { get; set;}
                    public Account(ICustomer customer) {
                        MyCustomer = customer;
                class StructureMapRunner {
                    static void Main() {
                        ObjectFactory.Initialize(x => x.For<icustomer>().Use<customer>));
                        Account account = ObjectFactory.GetInstance<account>();
                        account.MyCustomer.GetName().ShouldEqual("T. Boone Pickens");

Beware the circular reference

Its quite easy to create a circular refernce for example:

        public class ClassA
            public ClassA(ClassB INeedClassB)

        public class ClassB
            public ClassB(ClassA INeedClassA)

Class A tries to inject Class B which in turn tries to inject Class A which in turn injects Class B which... You get the idea massive Stack Overflow exception.

That example is obviously pretty contrived but imagine several classes that are joined together in a chain its quite easy to accidentally do this.

Lifecycle aka Caching management

An insanely powerful feature in IoC is the ability to set a lifecycle also called caching in some systems. What this does is based on the type of lifecycle

  • PerRequest - The default operation. A new instance will be created for each request.
  • Singleton - A single instance will be shared across all requests
  • ThreadLocal - A single instance will be created for each requesting thread. Caches the instances with ThreadLocalStorage.
  • HttpContext - A single instance will be created for each HttpContext. Caches the instances in the HttpContext.Items collection.
  • HttpSession - A single instance will be created for each HttpSession. Caches the instances in the HttpContext.Session collection. Use with caution.
  • Hybrid - Uses HttpContext storage if it exists, otherwise uses ThreadLocal storage.

The objects that are being injected can be cached for a period of time. So say you have an object that takes a long time to create but can be reused over and over again. Well ideally you might want that object to last as long as the users session. Using lifecycle you can make an object stay in memory and be reused as long as you want it to.

If you would like to try this article out as an interactive inversion of control project then download it here.



Richard Brisley

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