.net qr code generator BUS IN ESS AN D V ALIDA TION RULES in Visual Basic .NET

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CHAPTER 11 BUS IN ESS AN D V ALIDA TION RULES
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The ValidationException class allows CSLA.NET to throw a custom exception to indicate that a validation problem has occurred. This exception is thrown by the Save() method in BusinessBase. This exception class doesn t add any new information to the base Exception class from the .NET Framework. Thus, its code is very simple, since it merely declares a set of constructors, each of which delegates to the Exception base class. You can look at the class in the code download for the book. The reason ValidationException exists is to allow UI code to easily catch a ValidationException as being separate from other exceptions that might be thrown by the Save() method. For instance, UI code might look like this: Try customer = customer.Save() Catch ex As ValidationException ' handle validation exceptions Catch ex As Exception ' handle other exceptions End Try Even if they offer no extra information, custom exceptions are often valuable in this way. You should now have a high-level understanding of how ValidationRules consolidates the association of rules with properties, and the tracking of broken rules. It provides a single entry point for use of the business and validation rule subsystem. You ll see this used in 17, and you ll see examples of some rule methods in the next section of this chapter.
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Most applications use a relatively small, common set of validation rules such as that a String value is required or has a maximum length or that a numeric value has a minimum or maximum value. Using reflection, it is possible to create highly reusable rule methods which is the purpose behind the Csla.Validation.CommonRules class. Obviously, using reflection incurs some performance cost, so these reusable rule methods may or may not be appropriate for every application. However, the code reuse offered by these methods is powerful, and most applications won t be adversely affected by this use of reflection. In the end, whether you decide to use these rule methods is up to you.
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If reflection-based rules are problematic for your application, you can implement hard-coded rule methods on a per-type basis.
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If you find the idea of these reusable rules appealing and useful, you may opt to create your own library of reusable rules as part of your application. In that case, you can add a class to your project similar to CommonRules, and you can use the rule methods from CommonRules as a guide for building your own reusable rule methods.
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The RuleHandler delegate specifies that every rule method accepts two parameters: a reference to the object containing the data and a RuleArgs object that is used to pass extra information into and out of the rule method.
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CHAPTER 11 BUS INE SS AND V ALIDA TION RULES
The base RuleArgs object has a PropertyName property that provides the rule method with the name of the property to be validated. It also includes a Description property that the rule method should set for a broken rule to describe why the rule was broken. Table 11-3 lists the methods in the CommonRules module.
Table 11-3. Methods in the CommonRules Module
Method
StringRequired StringMinLength StringMaxLength IntegerMinValue IntegerMaxValue MinValue MaxValue RegEx
Description
Ensures a String value is not Nothing and has a length greater than zero Ensures a String value has a minimum length Ensures a String value doesn t exceed a maximum length Ensures an Integer value meets a minimum value Ensures an Integer value doesn t exceed a maximum value Ensures any numeric value meets a minimum value Ensures any numeric value doesn t exceed a maximum value Ensures a String value matches a regular expression
You can look at the code for each of these in the code download, but I do want to walk through a couple of them to explain how these methods work.
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