OOP Design and Practices in Business Systems
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by Brian Mains
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Determining Changes/Errors

When designing object-oriented systems, often the approach to designing the objects is to use domain or business objects containing properties and methods related to the underlying data.  There are various schools of thought about how to do this, and whether to use objects at all, instead of some of the built-in objects to .NET.  Rather than get into that discussion, this section will be using the domain object concept for designing application systems.

An object is created in some way.  For instance, an object is created new, meaning that a new instance of an object is created.  At this point, it would be considered "dirty", meaning it hasn't been uploaded to the underlying data source (database, XML, text file, or some other source).  The data source doesn't know anything about that data, and it shouldn't know anything about the data source (at any time).  Upon saving it to the repository, it would be considered "clean" in that it doesn't contain any changes from what the data source knows it to be.  However, upon changing the properties of an object, this object would be considered "dirty" again because it contains changes from what the underlying data source knows it to be.

What is the means to track changes?  As shown with the previous code example above, IDomainEntity describes an IsDirty property that tracks whether the object is dirty.  The methods change the value of that property internally based on whether any changes were made.  It may seem obvious to make IsDirty writable and assign the value directly, but I like this indirection better.  It makes the actions more clear, to set the dirty value of the property or to clear it.  Making the property writable could mean an accidental mistake of setting it to true when it was meant to be false, which is harder to do with method declarations.  The following is an example of a property:

Listing 6

public string Name
{
  get
  {
    return _name;
  }
  set
  {
    if (_name != value)
    {
      _name = value;
      this.SetDirty();
    }
  }
}

The property above only sets the dirty flag if the name property value changed from what it existed previously.  If you have a repository that is a collection of IDomainEntity objects, then in your repository, it is possible to have a method as below:

Listing 7

public virtual int SaveChanges()
{
  int savedItems = 0;
 
  foreach (T item in this.Items)
  {
    if (item.IsDirty)
    {
      this.SaveItem(item);
      item.ClearDirtyStatus();
      savedItems++;
    }
  }
 
  return savedItems;
}

This method would then call the SaveItem method to perform the actual storage of the item data.  It is possible to take this more in-depth in creating a repository that works with a business object.


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