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The EvaluateIsValid() override of the LengthValidatorAttribute illustrates a case where you delegate work to the Validation class: You must specify a maximum length, and can optionally specify a minimum length: You can only apply the LengthValidatorAttribute to string properties. Similar to the ASP.NET RequiredFieldValidator, the RequiredValidatorAttribute can specify an InitialValue property optionally. For example, a string property is invalid if it's null or an empty string, whereas an integer property is invalid if it has the default value of zero. The behavior of the RequiredFieldValidator adjusts depending on the datatype of the property. The RequiredValidatorAttribute is a basic attribute that allows the user to mark a property as required: You can apply multiple validator attributes to a single property. Also, all attributes must define an ErrorMessage property in the constructor and whatever customized arguments each validator control requires. Note that the ASP.NET validator controls follow a similar pattern internally. The key is that all subclasses must implement the EvaluateIsValid() method, where the data evaluation takes place. The six built-in attributes all derive from a common abstract base class: ValidatorAttribute (see Figure 1). You can build your validator attributes that consume the Validation class once you have the Validation class in place. Therefore, the generic validation method for comparisons can be reused in a type-safe manner for Int32, DateTime, Decimal, Guid, and so on, due to the generic constraints: For example, Int32 implements the IComparable interface.

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These generic methods enable type-safe code reuse across disparate types. The Validation class includes several generic methods. The validators consume these methods, but you can use the methods anywhere in your solution. The Validation class's sole job is to provide methods for verifying data.

This prevents direct instantiation or inheritance. The C# compiler enforces that all members of the class are static and marks the class as sealed and abstract at compile time. The third component is the ValidationManager, which is responsible for interpreting and executing all validator attributes.

The second component is the collection of validation attributes that all derive from a common base class: ValidatorAttribute. The first is the static Validation class that contains all data checking methods. The Declarative Validation framework consists of three major components.

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This means you will never have to write another line of validation code in the UI layer again.

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You can also incorporate the framework's attributes automatically in Windows applications by leveraging the interface. This saves you significant time as a developer, while reducing duplicate code logic. The Declarative Validation framework can take advantage of the attributes defined on class properties to generate ASP.NET validator controls for Web applications automatically. Or, consider what happens if you develop a Windows Forms application where you cannot utilize the handy ASP.NET validator controls. You cannot blindly trust the UI developer to perform all necessary validation-even when you are the UI developer! For example, assume a malicious user finds a way to bypass the UI constraints. However, best practices dictate that you perform data checking on your business objects.

For example, you might want to perform immediate checks after every property is set in a Windows application, but check only in the postback when writing a Web application.ĪSP.NET introduced validation controls that provide rich client-side and server-side validation. You can incorporate the framework in different ways. It comes with two abstract base classes that you can use to make implementation trivial by querying the IsValid property. You can plug the framework into any architecture. The framework consists of six built-in validator attributes: RequiredValidator, LengthValidator, RegexValidator, RangeValidator, CompareValidator, and CustomValidator.

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I'll show you how to construct the Declarative Validation framework to validate business objects by decorating class properties with attributes. Otherwise, you aren't doing much to 'protect' your object's data. However, it isn't enough to encapsulate the field you must also perform validation in your property setters. One of the many benefits that object-oriented programming gives you is the ability to protect an object's private members by encapsulating them inside public properties. Technology Toolbox: C#, ASP.NET, Windows Forms, Reflection NET attributes to provide robust validation for your business objects, while generating user-interface validation automatically. In-Depth Validate Business Objects Declaratively

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