Constructor Injection has the advantage that it makes the dependency explicit and forces the client to provide an instance. It can also guarantee that the client cannot change the instance later. One (possible) downside is that you have to add a parameter to your constructor.
Setter Injection has the advantage that it doesn't require adding a parameter to the constructor. It also doesn't require the client to set the instance. This is useful for optional dependencies. This may also be useful if you want the class to create, for example, a real data repository by default, and then in a test you can use the setter to replace it with a testing instance.
Interface Injection, as far as I can tell, is not much different than setter injection. In both cases you are (optionally) setting a dependency that can be changed later.
Ultimately it is a matter of preference and whether or not a dependency is required. Personally, I use constructor injection almost exclusively. I like that it makes the dependencies of a class explicit by forcing the client to provide an instance in the constructor. I also like that the client cannot change the instance after the fact.
Often times, my only reason for passing in two separate implementations is for testing. In production, I may pass in a
DataRepository, but in testing, I would pass in a
FakeDataRepository. In this case I'll usually provide two constructors: one with no parameters, and another that accepts a
IDataRepository. Then, in the constructor with no parameters, I will chain a call to the second constructor and pass in a
Here's an example in C#:
public class Foo
private readonly IDataRepository dataRepository;
public Foo() : this(new DataRepository())
public Foo(IDataRespository dataRepository)
this.dataRepository = dataRepository;
This is known as Poor Man's Dependency Injection. I like it because in production client code, I don't need to repeat myself by having several repeated statements that look like
var foo = new Foo(new DataRepository());
However, I can still pass in an alternate implementation for testing. I realize that with Poor Man's DI I'm hardcoding my dependency, but that's acceptable for me since I mostly use DI for testing.