Dependency injection for beginners (like me)
Since starting to practise proper TDD, I started to learn dependency injection too. It's a simple pattern that makes a world of difference. The following are some resources which I found helpful when getting to grips with it:
- Misko Hevery's Testability Blog. This focuses on writing testable code, and is a fantastic resource. Everything is worth reading, but if you read only three posts then make them How to Think About the 'new' Operator, Where Have All The Singletons Gone?, and Dependency Injection Myth: Reference Passing.
- Ninject is a simple and clean dependency injection framework for C#. It doesn't require setting up XML configuration files like some other frameworks which is a big win in my book. It has a good tutorial too, which is very helpful when just getting started.
- Stack Overflow. Rapidly replacing google as my first stop for any programming questions.
class ShoppingCartItem
{
private ILogger logger;
private string name;
public ShoppingCartItem(ILogger logger, string name)
{
this.logger = logger;
this.name = name;
}
}
class ShoppingCart
{
private IKernel kernel;
public ShoppingCart(IKernel kernel)
{
this.kernel = kernel;
}
public void AddItem(string name)
{
ShoppingCartItem item = kernel.Get<ShoppingCartItem>(
new ConstructorArgument("name", name));
...
}
}This works, but it is wrong! Ninject will self-inject the kernel to the ShoppingCart, and then inject the ILogger to the ShoppingCartItem. But this is bad for a couple of reasons:
- The code is now coupled to Ninject. Changing the dependency injection framework now requires all the code which creates objects to be changed.
- Passing extra non-injected parameters to the ShoppingCartItem constructor is now difficult. It's possible in Ninject, but the syntax is horrible and brittle.
class ShoppingCartItemFactory
{
private ILogger logger;
public ShoppingCartItemFactory(ILogger logger)
{
this.logger = logger;
}
public void CreateItem(string name)
{
return new ShoppingCartItem(logger, name);
}
}
class ShoppingCart
{
public ShoppingCart(ShoppingCartItemFactory itemFactory)
{
this.itemFactory = itemFactory;
}
public void AddItem(string name)
{
ShoppingCartItem item = itemFactory.CreateItem(name);
...
}
}Now everything is decoupled, the syntax is clean, and we still have the ability to inject mock items by injecting a different factory implementation.