Is this dependency injection? [duplicate]

Question

Is this dependency injection if I change the below code

class Needer
{
    Needed obj;
    AnotherNeeded obj2;

    public Needer()
    {
        obj = new Neede();
        obj2 = new AnotherNeede();
    }
}

To this code

class Needer
{
    Needed obj;
    AnotherNeeded obj2;

    public Needer(Needed param1, AnotherNeeded param2)
    {
        obj = param1;
        obj2 = param2;
    }
}

Answer 1

Robert C. Martin described Dependency Injecton in his SOLID design proposal. It basically states that:

• High-level modules should not depend on low-level modules. Both should depend on abstractions.

• Abstractions should not depend on details. Details should depend on abstractions.

Notice a word being used a lot in that description? "Abstraction".

You get part of the problem right in your second example, you no longer manually instantiate instances of the class, you instead pass them through the constructor. This leads into a new potential problem though, what if you need a different implementation of some class (e.g a "mock" and "real" services). If your constructor took the abstractions instead of the concretes you could change the implementations in your IoC configuration.

Any sort of service or functional class should typically have an abstraction behind it. This allows your code to be more flexible, extendable and easier to maintain. So to make your second example use true dependency injection:

class Needer
{
    public INeeded obj { get; set; }
    public IAnotherNeeded obj2 { get; set; }

    public Needer(INeeded param1, IAnotherNeeded param2)
    {
        obj = param1;
        obj2 = param2;
    }
}

Now you can have all sorts of implementations:

public class MockNeeded : INeeded

public class ApiNeeded : INeeded

etc, etc