Question

i’m looking for a larger example of dependency injection and how it can be implemented. If class A depends on class B and passes a reference of class C to B’s constructor, must not class A also take a reference to class C in it’s constructor? This means that the main method in the application should create all classes really, which sounds wierd?

I understand that using DI frameworks we can have it in XML files somehow, but that sounds like it could be hard to quickly see what type that really is instanciated? Especially if it a very large application.

Answer 1

You are correct and each DI framework has a different way of managing it.

Some use attributes on the properties etc to denote dependency and then “automagically” supply an instance of the correct type, while others (say castle windsor for .net) allow xml configuration, fluent or other methods for “wiring up” the dependency graph.

Also no, class A takes a built reference to an instance of B which was built using an instance of C. A needs to know nothing about C unless exposed via B.

    public class C { }

    public class B { public B(C c) { ... }}

    public class A { public A(B b) { ... }}

    // manual wireup
    C c = new C();
    B b = new B(c);
    A a = new A(b);

   // DI framework
   InversionOfControlContainer container = new InversionOfControlContainer(... some configuration);
   A a = container.ResolveInstanceOf<A>();
   // container dynamically resolves the dependencies of A. 
   // it doesnt matter if the dependency chain on A is 100 classes long or 3.
   // you need an instance of A and it will give you one. 

Hope that helps.