Dependency injection with multiple-layered application - dependency-injection

Question

I'm currently struggling a bit to understand how I can use the dependency injection pattern outside of my controller classes.
Say for instance that I have the following controller:
public class TestController : Controller {
ILog logger;
public TestController(ILog log) {
logger = log;
}
public string TestMethod() {
businessLayer businessLayer = new businessLayer();
return businessLayer.DoSomethingAndLogIt();
}
}
I understand that in most cases it is not possible to use constructor injection outside of the controller-classes. So it would not be possible to directly use the ILog-implementation insdie the "businesslayer"-class.
One simple solution I could imagine is the following:
public class TestController : Controller {
ILog logger;
public TestController(ILog log) {
logger = log;
}
public string TestMethod() {
businessLayer businessLayer = new businessLayer(logger);
return businessLayer.DoSomethingAndLogIt();
}
}
So passing on the dependencies from the controller to the underlaying layers. But is this the best way? Are there better solutions to have my businessLayer-class access to the ILog-implementation?
Thx!

Answer 1

I understand that in most cases it is not possible to use constructor
injection outside of the controller-classes.
This is incorrect. You should use constructor injection for all your components (every class in your application that contains behavior).
Dependency injection is about injecting dependent services/components into consuming components. So this means that you should not new up the businesslayer class in your controler; you should inject it using the constructor. By newing up this dependency you are violating the Dependency Inversion Principle, which causes high coupling. This again makes your code harder to test and makes it harder to change the controller and makes it much harder to apply cross-cutting concerns (such as logging, audit trailing, transaction management, etc) to the system.
So it would not be possible to directly use the ILog-implementation
insdie the "businesslayer"-class.
Incorrect. The ILog implementation should be injected into the constructor of your businesslayer class.
Long story short, your controller should look like this:
public class TestController : Controller {
IBusinessLayer businessLayer;
public TestController(IBusinessLayer bl) {
this.businessLayer = bl;
}
public string TestMethod() {
return businessLayer.DoSomethingAndLogIt();
}
}
Since TestController doesn't seem to use ILog directly, it should not be injected into its constructor. ILog is an implementation detail of the business layer class and implementation details should not leak to the consumer (this would again be a violation of the Dependency Inversion Principle).