1.

What is the difference between Composition and Aggregation?

Answer»

Composition

Like Inheritance gives us an 'is-a' relationship. Composition gives us a 'part-of' relationship. Composition is shown on a UML diagram.

If we were going to model a car, it would make sense to say that an engine is part-of a car. Within composition, the LIFETIME of the part (Engine) is managed by the WHOLE (Car), in other WORDS, when Car is destroyed, Engine is destroyed ALONG with it.

public class Engine {  . . . } public class Car {    Engine e = new Engine();    ....... }

As you can see in the example code above, Car manages the lifetime of Engine.

Aggregation

As Inheritance gives us 'is-a' and Composition gives us 'part-of', Aggregation gives us a 'has-a' relationship. Within aggregation, the lifetime of the part is not managed by the whole.

For Example:

Aggregation would make sense in this situation, as a Person 'has-a' Address. It wouldn't make sense to say that an Address is 'part-of' the Person, because it isn't. Consider it this way, if the person ceases to exist, does the address

So how do we express the CONCEPT of aggregation in C#? Well, it's a little different to composition. Consider the following code:

public class Address {  . . . } public class Person {     private Address address;     public Person(Address address)     {         this.address = address;     }     . . . }

Person would then be used as follows:

Address address = new Address(); Person person = new Person(address);

or

Person person = new Person( new Address() );

Here  Person does not manage the lifetime of Address. If Person is destroyed, the Address still exists.


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