oop - C# code for association, aggregation, composition

Question

I am trying to confirm my understanding of what the code would look like for association, aggregation & composition. So here goes.

Aggregation: Has-a. It has an existing object of another type

public class Aggregation
{
    SomeUtilityClass objSC
    public void doSomething(SomeUtilityClass obj)
    {
      objSC = obj;
    }
}

Composition: Is composed of another object

public class Composition
{
    SomeUtilityClass objSC = new SomeUtilityClass();
    public void doSomething()
    {
        objSC.someMethod();
    }
}

Association: I have two views on this.

1. When one class is associated with another. Hence both the above are examples of association.

2. Association is a weaker form of Aggregation where the class doesn't keep a reference to the object it receives.

   public class Association
   {
       //SomeUtilityClass objSC   /*NO local reference maintained */
       public void doSomething(SomeUtilityClass obj)
       {
          obj.DoSomething();
       }
   }
   

Is my understanding correct? I have read conflicting articles here and here and so I am really not sure which one to follow. My understanding seems to be in line with the first link. I feel the second link is wrong, or maybe perhaps I haven't understood it properly.

What do you think?

Answer 1

For two objects Foo and Bar we have:

Dependency:

class Foo
{
    ...
    fooMethod(Bar bar){}
    ...
}

Association:

class Foo
{
    private Bar bar;
    ...
}

Composition: (When Foo dies so does Bar)

class Foo
{
    ...
    private Bar bar = new Bar();
    ...
}

Aggregation: (When Foo dies, Bar may live on)

class Foo 
{
    private List<Bar> bars;
}