s BUSINESS OBJECT IMPLEMENTATION in Visual Basic .NET

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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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on the collection. The NewProjectResources() method creates an empty collection, ensuring that child objects can be added as required. The result is that the instance fields are declared and initialized so the object is immediately useful for things like data binding, setting property values, or adding child objects.
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The bulk of the code in the Business Methods region for most objects will be the properties. Some objects may include complex methods implementing business logic, but virtually all objects include properties to allow the UI to view or update the object s values. The Id property of the Project is read-only. It also represents the object s unique primary key value in the database: <System.ComponentModel.DataObjectField(True, True)> _ Public ReadOnly Property Id() As Guid Get CanReadProperty(True) Return mId End Get End Property Since this is the primary key for the data in the database, the value can also be considered to be a unique identifier for the object itself. The DataObjectField attribute is used to specify that the property is both a primary key and an identity value. This attribute is used by data binding, and in particular by the CslaDataSource ASP.NET control created in 5. The attribute is optional, but is useful for helping to identify the nature of primary key properties. Notice the use of the CanReadProperty() method in the get block. This code uses the overload created in 3, telling the method to throw a System.Security.SecurityException if the current user is not authorized to read the property. This is the simplest way to use the authorization functionality built into CSLA .NET. You could also opt to manually check the result with code like this: If CanReadProperty() Then Return mId Else ' take appropriate action End If This approach allows you to do something other than throw the default exception. You would write your code in the Else clause to cover the case in which the user isn t authorized to read the property. A third approach, which avoids the use of System.Diagnostics to determine the name of the property, is as follows: If CanReadProperty("Id") Then Return mId Else ' take appropriate action End If Notice that in this case, the name of the property is specified as literal text. This reduces the maintainability of the code, but has a marginal performance benefit by avoiding the System. Diagnostics call used by the previous overloads. You can determine whether the performance gain is worth the maintainability loss for your particular application.
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CHAPTER 8 s BUSINESS OBJECT IMPLEMENTATION
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s If you are using code generation or code snippets to create your business classes, there s no real cost to Tip using a literal value here. Since the code generator creates the code automatically, the likelihood of bugs due to typos is very small, and you may opt to use the literal in order to gain optimal performance.
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The Id property illustrates several things: a read-only property, a primary identity key value, and the use of the CanReadProperty() calling options.
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Now let s try something a bit more interesting by creating a read-write property, Name: Public Property Name() As String Get CanReadProperty(True) Return mName End Get Set(ByVal Value As String) CanWriteProperty(True) If mName <> Value Then mName = Value PropertyHasChanged() End If End Set End Property Since this is neither a primary key nor an identity value, there s no immediate need to use the DataObjectField attribute. You may still opt to use this attribute on your properties to provide this extra information for other purposes, such as automated unit testing. The Get block is virtually identical to that in the Id property. In fact, the Get block for properties will always be the same the only difference being the name of the instance field that s returned. The Set block deserves some discussion, however. First, notice the CanWriteProperty() method call. The options for calling CanWriteProperty() are the same as for CanReadProperty(), so you can take more control or use a literal name for the property if you so desire. Regardless, the idea is that the object s property value is only changed if the user is authorized to write to this property. Assuming the user is authorized to change the property value, the code checks to see if the provided value is actually new. If it s the same as the value already in the object, then there s no sense in any work being done. So, if the user is authorized to change the value, and the value is different from what is already in the object, then the new value is stored in the object. It is important to realize that this occurs before any validation code runs. This means that the object could end up storing invalid values. That s OK, though, because the object has an IsValid property that can be used to determine whether any validation rules are currently being violated by values in the object. The PropertyHasChanged() method is where the validation rules are actually invoked. This method performs a sequence of steps: 1. It checks the validation rules for the property. 2. It sets the object s IsDirty flag to True. 3. It raises a PropertyChanged event for data binding.
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