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The Csla.Validation namespace contains types that assist the business developer in implementing and enforcing business rules. This includes managing a list of business rules for each of the object s properties and maintaining a list of currently broken business rules. Obviously, the framework can t implement the actual business rules and validation code that will vary from application to application. However, business rules follow a very specific pattern in that they are either broken or not. The result of a rule being checked is a Boolean value and a humanreadable description of why the rule is broken. This makes it possible to check the rules and then maintain a list of broken rules, including human-readable descriptions of each rule.
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Given that rules follow a specific pattern, it is possible to define a method signature that covers virtually all business rules. In .NET, a method signature can be formally defined using a delegate; here s the definition for a rule method: Public Delegate Function RuleHandler(ByVal target As Object, _ ByVal e As RuleArgs) As Boolean Every rule is implemented as a method that returns a Boolean result: True if the rule is satisfied, False if the rule is broken. The object containing the data to be validated is passed as the first argument, and the second argument is a RuleArgs object that you can use to pass extra rule-specific information. This means that a business rule in a business class looks like this: Private Shared Function CustNameRequired(ByVal target As Object, _ ByVal e As RuleArgs) As Boolean If String.IsNullOrEmpty(DirectCast(target, Customer).Name) Then e.Description = "Customer name required" Return False Else Return True End If End Function If the length of the target object s Name property is zero, then the rule is not satisfied, so it returns False. It also sets the Description property of the RuleArgs object to a human-readable description of why the rule is broken. This illustrates a rule that you could implement within a single business class. By using reflection, you could write entirely reusable rule methods that any business class can use. You ll see some
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CHAPTER 11 BUS INE SS AND V ALIDA TION RULES
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examples of this in the Common Validation Rules section later in this chapter, when I discuss the CommonRules class. Rule methods can also be generic and, thus, strongly typed. In that case, they follow this signature: Public Delegate Function RuleHandler(Of T, R As RuleArgs)(ByVal target As T, _ ByVal e As R) As Boolean There are a couple of variations on how to use this delegate type to create a rule method. The most common is to provide only the type of T: Private Shared Function CustNameRequired(Of T As Customer)( _ ByVal target As T, ByVal e As RuleArgs) As Boolean If String.IsNullOrEmpty(target.Name) Then e.Description = "Customer name required" Return False Else Return True End If End Function The highlighted lines show the differences from the previous implementation. Notice how the method is generic and how the generic type T is constrained to the type of the business object. This allows the compiler to realize that the target parameter is of type Customer (or a subclass), so you re able to use all the properties of that class in the method implementation. You can also supply both generic type parameters: Private Shared Function CustNameRequired(Of T As Customer, _ R As MyRuleArgsSubclass)(ByVal target As T, ByVal e As R) As Boolean If target.Name.Length > e.MaxLength Then e.Description = "Customer name too long" Return False Else Return True End If End Function This approach is less common, but it allows you to have strongly typed access to a custom subclass of RuleArgs that you ve created for your rule method.
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