RuleMethod Class in VB.NET

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RuleMethod Class
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The ValidationRules class will maintain a list of rules for each property. This implies that ValidationRules has information about each rule method. This is the purpose of the RuleMethod class. It stores information about each rule, including the target object containing the data the rule should validate, a delegate reference to the rule method itself, a unique name for the rule, and any custom RuleArgs object that should be passed to the rule method. This information is stored in a set of fields with associated properties. The fields are declared like this: Private Private Private Private mTarget As Object mHandler As RuleHandler mRuleName As String = "" mArgs As RuleArgs
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The RuleMethod class is scoped as Friend, as it is used by other classes in the Csla.Validation namespace, but shouldn t be used by code outside the framework. The unique rule name associated with each rule is derived automatically by combining the name of the rule method with the string representation of the RuleArgs object. By default, this is the name of the property with which it is associated: mRuleName = mHandler.Method.Name & "!" & mArgs.ToString Because the rule name must be unique, any custom subclasses of RuleArgs should be sure to override ToString() to return a value that includes any custom data that is part of the arguments object. When the business developer associates a rule method with a property, ValidationRules creates a RuleMethod object to maintain all this information. This RuleMethod object is what s actually
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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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associated with the property, thus providing all the information needed to invoke the rule when appropriate. In fact, the RuleMethod object handles the invocation of the rule method itself by exposing an Invoke() method: Public Function Invoke() As Boolean Return mHandler.Invoke(mTarget, mArgs) End Function When ValidationRules is asked to check the business rules, it merely loops through its list of RuleMethod objects, asking each one to invoke the rule it represents. As you can see, the Invoke() method simply invokes the method via the delegate reference, passing in a reference to the object to be validated (the business object) and the RuleArgs object associated with the rule.
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ValidationRules Class
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The ValidationRules class is the primary class in the Csla.Validation namespace. Every business object that uses the validation rules functionality will contain its own ValidationRules object. ValidationRules relies on the other classes in Csla.Validation to do its work. Together, these classes maintain the list of rules for each property and the list of currently broken rules.
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Managing Rules for Properties
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You ve already seen how a business rule is defined based on the RuleHandler delegate. A key part of what ValidationRules does is keep a list of such rule methods for each of the business object s properties.
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Referencing the Business Object
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Remember that each rule method accepts a target parameter, which is the object containing the data to be validated. This target is always the business object, so ValidationRules keeps a reference to the business object. This reference is provided via the constructor and can be reset through the SetTarget() method both of which you ve seen in the implementation of Csla.Core. BusinessBase: <NonSerialized()> _ Private mTarget As Object Friend Sub New(ByVal businessObject As Object) SetTarget(businessObject) End Sub Friend Sub SetTarget(ByVal businessObject As Object) mTarget = businessObject End Sub Notice that the mTarget field is marked as <NonSerialized()>. This is important because otherwise the BinaryFormatter would trace the circular reference between the business object and the ValidationRules object, causing a bloated serialization byte stream. No failure would result, but the size of the byte stream would be larger than needed, which may cause a performance issue in some cases.
CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
Associating Rules with Properties
To provide good performance in managing the list of rules for each property, ValidationRules uses an optimal data structure. Specifically, it has a dictionary with an entry for each property. Each entry in the dictionary contains a list of the rules for that property. This provides for very fast lookup to get the list of rules for a specific property, since the dictionary can jump right to the property s entry. The dictionary is strongly typed, keyed by the property name, and used for storing strongly typed lists of RuleMethod objects: <NonSerialized()> _ Private mRulesList As _ Generic.Dictionary(Of String, List(Of RuleMethod)) The business developer calls an AddRule() method to associate a rule method with a property on the business object. There are two versions of this method, the simplest accepting just a rule method delegate and the name of the property: Public Sub AddRule( _ ByVal handler As RuleHandler, ByVal propertyName As String) ' get the list of rules for the property Dim list As List(Of RuleMethod) = GetRulesForProperty(propertyName) ' we have the list, add our new rule list.Add(New RuleMethod(mTarget, handler, propertyName)) End Sub The GetRulesForProperty() method returns the list of RuleMethod objects associated with the property. If such a list doesn t already exist, it creates an empty list and adds it to the dictionary. This is another example of lazy object creation. If there are no rules for a property, no list object is ever added to the dictionary, thus reducing the overhead of the whole process. In fact, the dictionary object itself is created on demand as well, so if no business rules are ever associated with properties for an object, even that little bit of overhead is avoided. The other AddRule() implementation provides an increased level of control. Its method signature is as follows: Public Sub AddRule(ByVal handler As RuleHandler, ByVal args As RuleArgs) This overload allows the business developer to provide a specific RuleArgs object that will be passed to the rule method when it is invoked. This is required for any rule methods that require custom RuleArgs subclasses, so it will be used any time extra information needs to be passed to the rule method. The combination of the RuleMethod class, the dictionary and list object combination, and the AddRule() methods covers the management of the rules associated with each property.
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