Building a Base Business Object Class in VS .NET

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Building a Base Business Object Class
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This code sets the state appropriately This is not as simple as just assigning the state to the value passed in to the method, because some states cannot be changed For example, if the state is already de ned to be Added, further changes to the object leave the state as Added And if the state is Deleted, it does not matter which other state it was; it needs to be deleted In your code, call DataStateChanged with a state of Added when the user creates a new item Call DataStateChanged with a state of Deleted when the user deletes an item Call DataStateChanged with a state of Modified whenever the user changes any of the data associated with an object Because you de ned all your object data with properties, you can add the call to DataStateChanged to the setter for each property, as described in the next section Building a base business object class keeps the majority of the housekeeping code out of the business object class itself and lets you focus on the unique business rules and business processing code required for the speci c business object
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For the Purchase Tracker sample application: Add a class project item to the business object Class Library project (PTBO) using the Class template If you created your own class template using steps from 3, you can use your class template here Name the class PTBOBase If not already added by the selected template, add the standard set of regions to the class as described earlier in this chapter Add documentation to the class using XML comments Add the code defined in this section You now have an operational base business object class that ensures the business objects from all derived classes consistently handle their state But at this point, the base class does not actually do anything To make use of the base class, you need to inherit from it, as described in the next section
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5 BUILDING THE BUSINESS LOGIC LAYER
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Building the Business Logic Layer
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Inheriting from the Base Business Object Class
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After you create a base business object class, you use it by inheriting from it Each business object class that needs to manage its state can inherit from the base business object class The business object then has access to the properties and methods from the base business object class The Inherits keyword speci es that a class inherits from another class Add the Inherits keyword to any business object class as follows:
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Public Class Product Inherits PTBOBase
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The class, in this case Product, then has all the properties and methods from the base business object class You can easily see this by typing Me somewhere within a property or method of the Product class The Intellisense List Members box displays properties and methods of both the base class (PTBOBase) and the derived class (Product in this case) To take advantage of the code in the base business object class, the derived classes can use the properties and methods of the base class For example, when a property in the business object is changed, the code calls the DataStateChanged method in the base business object class to correctly set the business object state The code in the ProductName property provides an example:
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Public Property ProductName() As String Get Return _ProductName End Get Set(ByVal value As String) If _ProductName <> value Then Dim propertyName As String = "ProductName" MeDataStateChanged(EntityStateEnumModified) _ProductName = value End If End Set End Property
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NOTE: This code does not currently use the propertyName variable It is used later when validation code is added in 7
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Building a Base Business Object Class
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The Dim statement declaring the propertyName variable could be a Const statement instead since the property name does not change within the property
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The setter code rst determines whether the value is the same as it was If so, it does not reset it If the value is indeed changed, the setter sets a variable for the property s name The DataStateChanged method in the base business object class is then called and passed a state of Modified Finally, the property value is changed to the passed-in value In every derived class, modify each updatable property to include similar code When any property value changes, the object is marked as modi ed This ensures that each object is aware of its state so that it can react accordingly
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