Tracking Whether the Object Has Changed in VB.NET

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Tracking Whether the Object Has Changed
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Another concept is that an object should keep track of whether its state data has been changed. This is important for the performance and efficiency of data updates. Typically, data should only be updated into the database if the data has actually changed. It s a waste of effort to update the database with values it already has! Although the UI developer could keep track of whether any values have changed, it s simpler to have the object take care of this detail, and it allows the object to better encapsulate its behaviors. This can be implemented in a number of ways, ranging from keeping the previous values of all fields (allowing comparisons to see if they ve changed), to saying that any change to a value (even changing it to its original value) will result in the object being marked as having changed. Rather than having the framework dictate one cost over the other, it will simply provide a generic mechanism by which the business logic can tell the framework whether each object has been changed. This scheme supports both extremes of implementation, allowing you to make a decision based on the requirements of a specific application.
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Strongly Typed Collections of Child Objects
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The .NET Framework includes the System.Collections.Generic namespace, which contains a number of powerful collection objects, including List(Of T), Dictionary(Of TKey, TValue), and others.
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CHAPTER 2 s FRAMEWORK DESIGN
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There s also System.ComponentModel.BindingList(Of T), which provides collection behaviors and full support for data binding.
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Generic types are a new feature in .NET 2.0. A generic type is a template that defines a set of behaviors, but the specific data type is specified when the type is used rather than when it is created. Perhaps an example will help. Consider the ArrayList collection type. It provides powerful list behaviors, but it stores all its items as type Object. While you can wrap an ArrayList with a strongly typed class, or create your own collection type in many different ways, the items in the list are always stored in memory as type object. The new List(Of T) collection type has the same behaviors as ArrayList, but it is strongly typed all the way to its core. The type of the indexer, enumerator, Remove(), and other methods are all defined by the generic type parameter, T. Even better, the items in the list are stored in memory as type T, not type Object. So what is T It is the type provided when the List(Of T) is created. For instance: Dim myList As New List(Of Integer) In this case, T is Integer, meaning that myList is a strongly typed list of Integer values. The public properties and methods of myList are all of type Integer, and the values it contains are stored internally as Integer values. Not only do generic types offer type safety due to their strongly typed nature, but they typically offer substantial performance benefits because they avoid storing values as type Object.
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Strongly Typed Collections of Child Objects
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Sadly, the basic functionality provided by even the generic collection classes isn t enough to integrate fully with the rest of the framework. As mentioned previously, the business objects need to support some relatively advanced features, such as undo capabilities. Following this line of reasoning, the n-level undo capabilities discussed earlier must extend into the collections of child objects, thereby ensuring that child object states are restored when an undo is triggered on the parent object. Even more complex is the support for adding and removing items from a collection, and then undoing the addition or the removal if an undo occurs later on. Also, a collection of child objects needs to be able to indicate if any of the objects it contains are dirty. Although the business object developer could easily write code to loop through the child objects to discover whether any are marked as dirty, it makes a lot more sense to put this functionality into the framework s collection object. That way, the feature is simply available for use. The same is true with validity: if any child object is invalid, then the collection should be able to report that it s invalid. If all child objects are valid, then the collection should report itself as being valid. As with the business objects themselves, the goal of the business framework will be to make the creation of a strongly typed collection as close to normal .NET programming as possible, while allowing the framework to provide extra capabilities common to all business objects. What I m defining here are two sets of behaviors: one for business objects (parent and/or child) and one for collections of business objects. Though business objects will be the more complex of the two, collection objects will also include some very interesting functionality.
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