Concepts like the Dependency Inversion Principle, Inversion of Control containers and Dependency Injection libraries are often mixed up, but they are not the same. You shouldn’t use a DI library until you understand the problems your solving, nor is it always the best solution. Since I’ve felt this pain many times myself, I’d like to use this session to create some clarity. First, I’ll demonstrate the reason why people typically use libraries like Unity and Autofac in their code bases. Then I’ll show you that those same people often use the Dependency Inversion Principle in the wrong way (and need to revert to mocking libraries to compensate). Then I’ll show you how to apply the principles properly by carefully designing the seams in your architecture. And finally, I’ll show you how an advanced library can connect the dots in a very elegant way.

1. Dennis Doomen | @ddoomen | Aviva Solutions | The Continuous Improver

4. Order Processing
IOrderRepository
Nhibernate
OrderRepository
Storage
RavenDB
OrderRepository
Storage Storage
In-memory
Repository
Bookkeeping
Diagnostics
Decorator
Diagnostics
Decorator
Strategy Selection
Allow Test-time
dependencies
Inject
dependencies
from different
modules
Control lifecycle
(e.g. per http request,
singleton, etc)
Inject Cross-cutting
Concerns
Promote depending
on abstractions

14. Order Processing
IStoreOrders<T>
+ CreateQuery<T>();
+ Add<T>();
+ Delete<T>();
NHibernate
Repository
Order Processing
IStoreOrders
+ GetIncompletedOrders(minValue);
+ StoreOrder();
+ CompleteOrder();
OrderRepository
VS

16. • Don’t use the Service Locator pattern
• Use container only at the root…
• …so don’t use the container as a dependency
• Constructor injection only
• Pass local dependencies to methods
• Avoid container-specific dependencies
• Use delegates for factories, indexes, etc.
• Include registrations in at least one test
• Use types explicitly in registrations
• Let the container handle the life cycle
• Access date & time through delegate
• Avoid configuration-based setup (e.g. XML).

18. System
Package
Package
Package
PackagePackage
Package
Maybe
Probably not
Probably yes
TinyIoc
Autofac