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Protected Function GetPropertyConvert(Of F, P)(_)( _ ByVal propertyInfo As PropertyInfo(Of F), _ ByVal noAccess As Security.NoAccessBehavior) As P Return Utilities.CoerceValue(Of P)(GetType(F), Nothing, _ GetProperty(Of F)(propertyInfo, noAccess)) End Function As with the earlier overload, this one gets the value and then passes it to Utilities.CoerceValue() to coerce the value to a different type.
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Business objects can contain other business objects. As discussed in 4, the containing object is a parent object, and the contained object is a child object. In this case, the parent object maintains a reference to the child object. I ll discuss the issues around parent-child relationships in 9. For now, you should know that the normal way to create a property that references a child object is to write code like this in your parent object: Private Shared ChildProperty As PropertyInfo(Of ChildType) = _ RegisterProperty(New PropertyInfo(Of ChildType)("Child")) Public ReadOnly Property Child() As ChildType Get If Not FieldManager.FieldExists(ChildProperty) Then LoadProperty(ChildProperty, ChildType.NewChild()) End If Return GetProperty(ChildProperty) End Get End Property This stores the child reference in a managed backing field, which allows the field manager to automatically take care of all the housekeeping details involved with a child reference. The RegisterProperty() and GetProperty() calls should be familiar by this point. But these two lines are new: If Not FieldManager.FieldExists(ChildProperty) Then LoadProperty(ChildProperty, ChildType.NewChild()) The first line uses the FieldManager to determine whether this child object has been created. If it has not, then the second line uses LoadProperty() to add a new instance of the object as a child. The call to ChildType.NewChild() is invoking the child object s factory method, which is a concept I discussed in s 4 and 5. This relatively simple-looking code is hiding some fairly complex object interactions, and I ll discuss them in 9 when I cover parent-child relationships.
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If you look closely at the way RegisterProperty() is called in the example code in this chapter, you ll see that it is called while initializing a Shared field: Private Shared NameProperty As PropertyInfo(Of String) = _ RegisterProperty(New PropertyInfo(Of String)("Name")) It makes sense that your properties need to be registered with CSLA .NET before they can be used, and if your business class inherits directly from BusinessBase or ReadOnlyBase, then this will happen
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automatically. The reason is that any attempt to get or set a property will call the GetProperty() or SetProperty() method, which accepts the PropertyInfo field as a parameter for example: Return GetProperty(NameProperty) Any attempt to access a Shared field forces .NET to initialize all the Shared fields declared in that class. However, if you use inheritance such that your business class inherits from a class that, in turn, inherits from BusinessBase or ReadOnlyBase, then things will get more complex. This is because of the way .NET initializes Shared fields, which turns out to be complex and counterintuitive. The .NET runtime initializes the Shared fields for a class only when one of the Shared fields on that class is accessed (read or changed) or when the class has a Shared constructor. To be very clear, this means that instance methods and properties can be called before the Shared fields are initialized. Unless you are absolutely sure that a Shared field from every class in the inheritance hierarchy has been accessed, you can t be sure that all the Shared fields have been initialized. The end result is that your properties might be accessed before they have all been registered, which will ultimately cause the field manager to throw an exception.
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Remember that this is an issue only if your business classes don t inherit directly from a CSLA .NET base class.
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You can use one of two techniques to prevent this issue. You can add a Shared constructor to each of your custom base classes, or you can ensure that some Shared field is initialized as each object instance is created.
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