Dependancy injection in web applications

I saw a funny description of Dependency Injection as a "25-dollar term for a 5-cent concept".

Basically, it's just passing references to dependencies to an object constructor rather than referencing global resources or constructing them internally.

Without dependency injection:

public SomeClass() {
    myObject = GlobalService.getObject();  // a library service is used
}

With dependency injection:

public SomeClass (ServiceClass theService) {
    myObject = theService.getObject();     // the injected service is used
}

You use dependency injection to split up your codes into separate testable parts. When you instantiate an object, the constructor can have parameters that are interfaces. The dependency injection manager takes a look at what interfaces the new object is asking for and then gives it the proper objects that those interfaces represent.

public class MainClass
{
     private readonly IMyService _myService;

     public MainClass(IMyService myService)
     {
            _myService = myService;
      }
}

Interface IMyService
{
     public void DoSomething()
}

public class MyService : IMyService
{
     public MyService()
     {

      }

     public void DoSomething()
     {
         //put code to do something here
     }
}

In this example we have an Interface called IMyService which tells us that a class that implements this interface has to have a void method called "DoSomething". The MyService class inherits from IMyService so it has the method DoSomething(). Now when MainClass is instantiated, it is going to ask for an object that implements the interface IMyService. In the background, your dependency injection tool (autofac, etc) would then give MainClass an instance of MyService. You could also have other classes that implement the class IMyService and inject those into the constructor in other situations.

The simplest example

public int AddThis(int a, int b)
{
    return a + b;
}

In this case you injected a and b to the method AddThis.

Take a look at this for a more complex example

https://github.com/nopSolutions/nopCommerce/blob/develop/src/Presentation/Nop.Web/Controllers/CatalogController.cs