A very, very, very quick introduction to dependency injection. Examples use C# and .NET, written using Visual Studio for Mac.

1. …and you: So happy together 

2.  Here’s the absolute worst explanation ever:
 ”Instead of newing up objects, you just call ‘container.resolve’”

3.  With the definition of, “just call container.Resolve instead of newing an
object up”, when you go to a job interview and whiteboard some pseudo-
code, they might say something like:
Pretty sure this
guy voted ‘NO’
on hiring me…

4.  Merriam Webster Dictionary: “something that is dependent on something else”
 For example: “A time-traveling DeLorean’s dependency on plutonium was a real
problem in 1955.”
 Wikipedia’s definition relative to computer science: “[A dependency is a] coupling,
a state in which one object uses a function of another object.”
 For example: My app needs to make calls to a web service to get and store data, so my
business logic has a dependency on the web service.
 When you have a “coupling” of two classes, a loose coupling is the way to go in order to
make your code more testable and maintainable for the future.

5.  3 common types of dependency injection:
 Setter Injection
 Dependencies are injected through specific property setters. I have never seen this in the wild.
 Interface Injection
 Classes with dependencies implement an interface that exposes a setter method, and
dependencies use this to inject themselves into any class passed to it. I have never seen this in
the wild.
 Constructor Injection
 Yaaaaaaay! Winner winner!! Dependencies are injected through the class constructor. This is the
only method I’ve seen in the wild.

6. Interface: A
contract or
specification
Concrete
implementation of
the interface

7.  We want ALL of our dependencies to be based on an interface.
 Any service code you write should have an interface. Other classes in your app
will rely only on the interface… Not the “concrete” implementation of the
interface.
 You will definitely run into situations where some 3rd party code does not provide
an interface (Microsoft’s HttpClient class, as a ubiquitous for-instance).
 Never give up… Never surrender! There are ways to create wrappers for such classes
which keep the vast majority of your code testable and loosely coupled. True story!

8.  Before DI  After DI
 You’ll notice you use the “new”
keyword a WHOLE lot less when
using Dependency Injection.

9.  When I finally learned how to use dependency injection, I still had no clue WHY
we were using it. All I knew was that some highly paid consultants said to do it,
and that was that.
 The two most important reasons to use “DI” (Dependency Injection):
 Makes it super-mega-monster-easy to unit test code without insanely complicated
setups.
 Caveat: As long as you learn how to use mocks when unit testing. On Luxor, we use MOQ, which
I would absolutely recommend.
 Another popular mocking framework for .NET: NSubstitute
 Promotes writing code where various services (aka ‘dependencies’) your code requires
can be loosely coupled with your code.

10.  The container is at the very heart of your DI implementation.
 Interfaces are mapped to concrete types, so that the container can provide the
correct implementation when an interface is requested.
 Containers typically allow you to specify the “lifetime” of any object it knows
about. You can have a singleton, one instance per thread, and more.
 Containers map views to viewModels in the MVVM design pattern (used in UWP,
WPF, and Xamarin.Forms apps, for example).
 The container is the thing that will new up all of your services, views, and
viewModels as they are needed.

11.  The magic: When you need some service in a class, you just add the interface to
its constructor, and the container magically will provide it!
 No… SERIOUSLY. It’s kind of amazing!!

12.  How does the
container know what
concrete types to
provide for your
interfaces?
 How does the
container know which
views go with which
viewModels?
 Answer:
Registration!

13.  Notice that ALL of the constructor parameters are interface types.
 When we discover we need a new service, we can just add it to the constructor,
and our DI container takes care of injecting it (thanks to the registration).
 Note: Yes, there might be a code smell of too many dependencies in this ViewModel, but
that’s not important right now. 

14.  The ViewModel from the previous slide has SEVEN dependencies. That would be
a TON of overhead if they were all concrete classes, and LOTS of integration
concerns to worry about when it came to testing!
 Instead, we just mock, mock, mock all our problems away!

15. Arrange
Act
Assert

16.  Infinitely easier (and possible!) unit testing of code.
 Loosely coupled software.
 While DI is complex, the alternative is even MORE complex!
 Just ask for whatever services you want in a constructor, and your DI container
will “make it so”!
 No complications about whether a class is a singleton or not… You already took
care of that when you registered the class with the container.
 As long as the container knows about the interface and the concrete class you
want to implement it with, you’re all set.
 Swap out implementations of interfaces (demo mode, anyone??).