7: Constraints and Validation in Visual C#

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7: Constraints and Validation
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Invoking Validation Having marked validation methods with the correct attribute, let s look in a little more detail at the mechanism by which they get called A fa ade class called a ValidatonController is used to invoke validation methods This fa ade is able to call the appropriate set of validation methods using the NET re ection mechanism to look for the correct attributes and re ectively invoke the methods We ll see more detail on the ValidationController shortly; however, in the typical case it is not necessary to use this class directly, because the DSL model and code generator will create a basic validation system for you In the DSL designer, under the Editor node of the DSL explorer window you ll nd a Validation node, as shown in Figure 7-1
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Figure 7-1: Setting values in the Validation node
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This node controls generation of code to make calls to the validation framework in response to user actions, with each property causing validation methods with the matching ValidationCategory attribute to be called The generated code is in the form of extra code in the le open and save mechanisms in the DocData class and an extra command in the CommandSet class to provide the Validate menu item There is no requirement to use this
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facility to invoke validation in a designer; it is simply provided as a convenience It s quite possible to set all of the properties of the Validation node in the DSL explorer to be False and then set up the validation infrastructure yourself We ll see how to do that when we discuss using the infrastructure outside of the IDE a little later in the chapter
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Custom Validation Categories You ll notice that there is a Uses Custom property on the Validation node If Open, Save, or Menu are not suitable times to run your particular validation, then you might wish to run validation as part of some other custom code in your designer, perhaps as part of some larger custom command The Uses Custom property is a marker to signal your intent to do this and generates just enough infrastructure code in your designer to make it easy Typically, you d use custom validation categories when some set of validation functions are only true in the context of some speci c operation For example, imagine a database designer with a set of validations working at le open and save times and also checkable from a validate menu item These validations check that the database model could generate a working database schema However, perhaps this tool also has a second code generator to create a data access layer (DAL) targeting the schema The DAL generator doesn t support arbitrary database schemas in its current version because it hasn t been coded to cope with large binary elds such as pictures, even though they form part of a perfectly valid database schema Using custom validation groups, the tool authors can add a set of validation methods that disallow large binary elds Then, when the DAL code generator is invoked, they can run both the regular validation methods and the custom set, ensuring that the model will generate a valid schema and a working DAL on top of it However, users who never use the DAL generator aren t encumbered with validations that they have no interest in If a tool had many different code generators (or other tool chains attached to it), it might have many sets of custom validation methods that might well overlap To see how to accomplish custom validation, let s take a look at the overall architecture of the validation framework in the DSL Tools in Figure 7-2 As you can see, the ValidationController is the main entry point for validation To initiate validation, make a call to one of the overrides of either Validate() or ValidateCustom() on the controller Validate() takes
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