加强不同对象之间的数据完整性

Question

I am learning OOP through developing a C# music management software. So far, I have laid out the interaction of different classes and object as shown on this class diagram http://img811.imageshack.us/img811/7624/classdiagramh.png

I'm however very confused on making my design solid as I am finding a myriad of possibility of how things can be done but I don't want to end up with a bad design. The main problem I'm having is to make sure that I enforce data integrity between different object (To my understanding, I am supposing that in OOP, like in database design, there is a way one can provide an accuracy, consistency, and reliability of data store in different object as it is done in database design (For example the use of foreign key constraint). For example, in my application, I have Artist, Song, and Album objects that should interact together. I do not want for example to have a song associated with the wrong album by mistake. I have been told that I should do something like:

class Album
{
  public String Name{ get; set;}

  List<Track> tracks;

  public void AddTrack(Track t)
  {
    tracks.Add(t);
    t.Album = this;
  }
} 
class Track
{
  public Album Album{ get; set;}
  public String AlbumName
  { 
    get{ return this.Album.Name}
  }

  public String ArtistName{ get; set}
}

However this is not working for me. Can someone suggest how I can actually make this work. Specifically, how I can add a new song with a title, an album and artist and make sure that If for some strange reason the name of the album is changed, all of the tracks will still return the correct album name, since they're getting the data from the album itself?

Also, can someone suggest a better design. Should I be using inheritance or any polymorphism to make my design robust?

Answer 1

As in DB design you have to make your choices based on your business requirements. Means: You have to ask yourself questions of possible constellations - are all the songs on a CD from the same artist? No -> artist field on track, yes -> artist field on album - do you have to be able to get the album when you know a track, or just all the tracks from a specific album - ...

What you are doing is correct in the way that you reference from the Track object the name of the referenced Album. What is to mention: they do return the albums name, but just the name the album has in the moment of the GET. So if you assigned the AlbumName property to a visual control or whatever it will not update automatical when you change the Albums name.

if thats not what you want to hear ask more specific.

Answer 2

Actually you're not doing associations.

In object-oriented programming, you associate objects instead of "keys". There's no concept of "foreign key".

If an object B has a parent A, and A has an unique identifier , you won't associate B's parent by setting this unique identifier but the object itself:

B has a parent A :

public class A { public Guid ID { get; set; } }
public class B { public A Parent { get; set; } }

A someA = new A();
B someB = new B();
someB.Parent = someA;

Working this way, as "instance of A" may be associated with many other objects, changing the same object from any reference, will result in modifying that object .

How to achieve integrity constraints? There're some design patterns, but one of most common one is Specification .

That's creating a common interface "ISpecification" which has a method that may be called "Check", which accepts an argument - the object to check for an specification -. In implementations, this method will check that some object conforms some specification.

public interface ISpecification { bool Check(object some); }

public class Person { public string Name { get; set; } }

public class PersonSpecification : ISpecification
{
     public bool Check(object some) 
     {
          // Checks that some person won't be null and his/her name isn't null or empty
          return some != null && !string.IsNullOrEmpty(((Person)some).Name);
     }
} 

This is a very simple and generic sample of how to implement business rules and/or constraints.

There're many frameworks which may help you in this area:

• Microsoft Patterns & Practices Validation Block: http://msdn.microsoft.com/en-us/library/ff648831.aspx

• NHibernate Validator: http://nhforge.org/wikis/validator/nhibernate-validator-1-0-0-documentation.aspx (it's part of NHibernate project, but it can be consumed standalone without NHibernate).

In some projects I've integrated NHibernate Validator, and others, a custom solution based on Specification pattern, but in a more complex and flexible way than the sample.

About Specification pattern:

• http://en.wikipedia.org/wiki/Specification_pattern