I'm trying to learn implementing TDD with mocking/fake objects. One of the questions I have is how to initialize a dependency in an application which implements TDD? An example from this article Beginning Mocking With Moq 3 shows:

public class OrderWriter
    private readonly IFileWriter fileWriter;

    public OrderWriter(IFileWriter fileWriter)
        this.fileWriter = fileWriter;

    public void WriteOrder(Order order)
        fileWriter.WriteLine(String.Format("{0},{1}", order.OrderId, order.OrderTotal));

In this example, the constructor takes an IFileWriter parameter, I suppose because you want to supply the real file writer in case of the actual application, and the fake one for unit test. My question is, in the real application, who will supply this parameter? I suppose it will be the caller of this application. What if it has dependency as well in the constructor? Will the caller code be responsible for that too?

Maybe the better way is to use factory. How would this factory work? And how will the factory be distributed? Will it be in the constructor parameter like in the above manner?


What you're looking for is an IoC container to autowire all your objects at application startup. Take a look at Ninject, it has a very simple example on the front page. (It's also a good product and ... well, ninjas!)

As a general rule, you should attempt to resolve all of your top-level objects (eg. Page in ASP.NET, Controller in MVC for ASP.NET, Form in Winforms) directly from the IoC container and let it wire up ALL your dependencies through constructor injection. There will be times when you have to force it to resolve a lower-level item -- this is known as using it as a Service Locator -- but this should generally be avoided as they are tricky (but not impossible) to test, and create an API that can be confusing for the consumer, if that isn't you.

ASP.NET for MVC has, since v3, been specifically designed to abstract away the IoC container from the rest of your code while allowing you to auto-inject into any top-level class (Controller, View, Filter, etc) through the DependencyResolver class. Other .NET frameworks take a bit more effort, but it's possible if you Google around.

There is a book on the subject called Dependency Injection in .NET. I haven't read it personally, but I've heard good things.


I have seen the approach of adding a default constructor that calls the parameterized constructor with default instances, i.e. "poor man's dependency injection".

public OrderWriter()
    : this(new TextFileWriter())

The regular code path calls the default constructor, whereas the unit test initializes using the custom constructor.

If anyone could elaborate on the pros and cons of this approach, I would much appreciate it.

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