Constructor Injection: How many dependencies is too many?

Question

I've been using manual constructor injection DI for a little bit now. One thing that I notice is that my constructors are starting to get rather long.

I have a class that depends on a bunch of little objects - anywhere between 6 and 10 sometimes. As I continue to break my application into smaller chunks, I could see this number increasing over time. Is this a common problem?

Obviously this is going to depend a great deal on the project. However, the basic question is this:

When do you start to get uncomfortable with the number of dependencies that a class has? What are some strategies that you use to reduce these dependencies?

Answer 1

This may be a sign that the class with the 6-10 dependencies itself needs to be refactored.

Answer 2

I would think no more than three or four. If you are getting more than that, I would start thinking about how well you are abstracting your concerns. A single repository object, for example, should fulfill all of your data retrieval needs within the class in question.

Answer 3

You may also want to see if any of the parameters to your constructor should be combined into a single class as well (assuming that the parameters make sense as a class).

It might also be that you want to look at using the ServiceLocator pattern for some of your dependencies. This is particularly true if you're having to pass the dependencies down a long chain of constructors.

Answer 4

Runcible,

Here is a link to the Castle Windsor project. It is an Inversion of Control container. These containers allow factory classes to collect your dependencies together and inject them as a single object into your constructor.

http://www.castleproject.org/container/index.html

I have heard good things about Windsor. Spring also makes an IoC container, and there are others.