Checking Validation Rules in VB.NET

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Checking Validation Rules
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Once a set of rule methods have been associated with the properties of a business object, there needs to be a way to invoke those rules. Typically, when a single property is changed on a business object, only the rules for that property need to be checked. At other times, the rules for all the object s properties need to be checked. This is true when an object is first created, for instance, since multiple properties of the object could start out with invalid values. To cover these two cases, ValidationRules implements two CheckRules() methods. The first checks the rules for a specific property:
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CHAPTER 3 s BUSINESS FRAMEWORK IMPLEMENTATION
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Public Sub CheckRules(ByVal propertyName As String) Dim list As List(Of RuleMethod) ' get the list of rules to check If RulesList.ContainsKey(propertyName) Then list = RulesList.Item(propertyName) If list Is Nothing Then Exit Sub ' now check the rules Dim rule As RuleMethod For Each rule In list If rule.Invoke() Then BrokenRulesList.Remove(rule) Else BrokenRulesList.Add(rule) End If Next End If End Sub This method checks to see if the RulesList (the dictionary) contains an entry for the specified property. If so, it retrieves the list of RuleMethod objects and loops through them, asking each one to invoke its underlying rule method. If a rule returns True, then BrokenRulesList.Remove() is called to ensure that the rule isn t listed as a broken rule. If the rule returns False, then BrokenRulesList.Add() is called to ensure that the rule is listed as a broken rule. The BrokenRulesList class is part of the Csla.Validation namespace, and will be discussed shortly. The other CheckRules() implementation checks all the rules that have been added to the ValidationRules object: Public Sub CheckRules() ' get the rules for each rule name Dim de As Generic.KeyValuePair(Of String, List(Of RuleMethod)) For Each de In RulesList Dim list As List(Of RuleMethod) = _ de.Value ' now check the rules Dim rule As RuleMethod For Each rule In list If rule.Invoke() Then BrokenRulesList.Remove(rule) Else BrokenRulesList.Add(rule) End If Next Next End Sub This method simply loops through all items in the RulesList dictionary. Every entry in the dictionary is a list of RuleMethod objects, so it then loops through each list, invoking all the rules. The rule is then added or removed from BrokenRulesList based on the result. At this point, it should be clear how ValidationRules associates rule methods with properties and is then able to check those rules for a specific property or for the business object as a whole.
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Maintaining a List of Broken Rules
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The ValidationRules object also maintains a list of currently broken validation rules. This list was used in the CheckRules() methods, and is declared as follows: Private mBrokenRules As BrokenRulesCollection Private ReadOnly Property BrokenRulesList() As BrokenRulesCollection Get If mBrokenRules Is Nothing Then mBrokenRules = New BrokenRulesCollection End If Return mBrokenRules End Get End Property Notice that the mBrokenRules field is not adorned with either the <NotUndoable()> or <NonSerialized()> attributes. The list of currently broken rules is directly part of a business object s state, and so it is subject to n-level undo operations and to being transferred across the network along with the business object. This way, if a business developer transfers an invalid object across the network or makes a clone, the object remains invalid, with its list of broken rules intact. The BrokenRulesList value is also exposed via a Public method. To any external consumer, such as code in the UI, this is a read-only collection: Public Function GetBrokenRules() As BrokenRulesCollection Return BrokenRulesList End Function The reason the collection is exposed publicly is to allow UI developers to use the list of broken rules as they see fit. Remember that a broken rule includes a human-readable description of the rule, and so it is perfectly reasonable to display this list to the end user in some circumstances.
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