OOP Design and Practices in Business Systems
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by Brian Mains
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How Much Exposure?

When developing classes, the question often arises how much of the internal workings should be exposed.  For instance, when developing classes, how much of the class should be exposed?  One of the cases we'll talk about is repository-based classes, where the repository works like a list of domain objects.

One of the common ways to store domain objects is in a collection of some sort, where the objects can be added, removed, or inserted.  A list also has the capabilities to perform find/contain operations, searching for a specific item.  However, should the list be exposed directly?  If the list is exposed as a public read-only property, any object can add items to the list.  Most collection classes also don't expose events for when an object is added or removed, meaning that the source object doesn't know when these events may occur, which is one of the reasons I created a collection that raises events upon adding/removing items to/from the list in my Nucleo framework.

However, is that a bad thing?  Maybe not.  If your class doesn't need to keep track of state, new objects being added or existing objects being removed isn't that big of a deal.  However, there are times when it is a very big deal, especially when these objects represent data in a data store, and there should be some correlation between the two.

In addition, what about creating the object?  Should the object be directly instantiable?  Should it be created by a factory or another object?  In situations where you are tracking state of an object, I would highly recommend making the constructor internal, so that your code can control when the object is instantiated.  In situations where the class may be a helper object or utility, it doesn't really make a difference to me.  However, when you read about this subject on the web, there are a variety of topics.  My viewpoint is I make the constructor internal, especially if I feel I need an extra level of control over it, unless I don't care about what the object does.

For instance, imagine a Comment class that represents a comment generated from a web form.  The factory that would work with this object would look like the following:

Listing 11

public static class CommentFactory
{
  public static Comment CreateComment(string name, string email, string subject,
    string message, string source)
  {
    CommentsDataGateway gateway = new CommentsDataGateway();
    Guid id = gateway.AddComment(name, email, subject, message, source);
 
    return new Comment(id, name, email, subject, message, source);
  }
 
  private static Comment CreateCommentObject(DataRow row)
  {
    return new Comment((Guid)row["CommentID"], (string)row["Name"], (string)
      row["Email"], (string)row["Subject"], (string)row["Message"], !row.IsNull
      ("Source") ? (string)row["Source"]: string.Empty);
  }
 
  public static void DeleteCommentsPast(DateTime pastDate)
  {
    CommentsDataGateway gateway = new CommentsDataGateway();
    gateway.DeleteOldComments(pastDate);
  }
 
  public static CommentCollection GetComments(TimeSpan period)
  {
    CommentsDataGateway gateway = new CommentsDataGateway();
    DataTable commentsTable = gateway.GetComments(period);
    CommentCollection commentsList = new CommentCollection();
 
    foreach (DataRow commentRow in commentsTable.Rows)
      commentsList.Add(CreateCommentObject(commentRow));
 
    return commentsList;
  }
}

In the above factory, the factory instantiates the Comment object, as the comment's constructor is internal to the project.  It's solely responsible for creating the object, and meets a principle discussed in the next section.


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