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DataAnnotations Attributes In addition to using the built-in validation attributes, the Validation block will perform validation defined in the vast majority of the validation attributes in the System. ComponentModel.DataAnnotations namespace. These attributes are typically used by frameworks and object/relational mapping (O/RM) solutions that auto-generate classes that represent data items. They are also generated by the ASP.NET validation controls that perform both client-side and server-side validation. While the set of validation attributes provided by the Validation block does not map exactly to those in the DataAnnotations namespace, the most common types of validation are supported. A typical use of data annotations is shown here.
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[System.ComponentModel.DataAnnotations.Required( ErrorMessage = "You must specify a value for the product ID.")] [System.ComponentModel.DataAnnotations.StringLength(6, ErrorMessage = "Product ID must be 6 characters.")] [System.ComponentModel.DataAnnotations.RegularExpression("[A-Z]{2}[0-9]{4}", ErrorMessage = "Product ID must be 2 capital letters and 4 numbers.")] public string ID { get; set; }
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In reality, the Validation block validation attributes are data annotation attributes, and can be used (with some limitations) whenever you can use data annotations attributes for example, with ASP.NET Dynamic Data applications. The main difference is that the Validation block attribute validation occurs only on the server, and not on the client. Also keep in mind that, while DataAnnotations supports most of the Validation block attributes, not all of the validation attributes provided with the Validation block are supported by the built-in .NET validation mechanism. For more information, see the documentation installed with Enterprise Library, and the topic System.Component Model.DataAnnotations Namespace at http://msdn.microsoft.com/en-us/library/system. componentmodel.dataannotations.aspx. Self-validation might sound as though you should be congratulating yourself on your attractiveness and wisdom, and your status as fine and upstanding citizen. However, in Enterprise Library terms, self-validation is concerned with the use of classes that contain their own validation logic. For example, a class that stores spare parts for aircraft might contain a function that checks if the part ID matches a specific format containing letters and numbers. You add the HasSelfValidation attribute to the class, add the SelfValidation attribute to any validation functions it contains, and optionally add attributes for the built-in Validation block validators to any relevant properties. Then you can validate an instance of the class using the Validation block. The block will execute the self-validation method. Self-validation cannot be used with the UI validation integration features for Windows Forms, WPF, or ASP.NET. Self-validation is typically used where the validation rule you want to apply involves values from different parts of your class or values that are not publicly exposed by the class, or when the validation scenario requires complex rules that even a combination of composed validators cannot achieve. For example, you may want to check if the sum of the number of products on order and the number already in stock is less than a certain value before allowing a user to order more. The following extract from one of the examples you ll see later in this chapter shows how self-validation can be used in this case.
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[HasSelfValidation] public class AnnotatedProduct : IProduct ... ... code to implement constructor and properties goes here ... [SelfValidation] public void Validate(ValidationResults results) { string msg = string.Empty; if (InStock + OnOrder > 100) { msg = "Total inventory (in stock and on order) cannot exceed 100 items."; results.AddResult(new ValidationResult(msg, this, "ProductSelfValidation", "", null)); } }
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The Validation block calls the self-validation method when you validate this class instance, passing to it a reference to the collection of ValidationResults that it is populating with any validation errors found. The code above simply adds one or more new Validation Result instances to the collection if the self-validation method detects an invalid condition. The parameters of the ValidationResult constructor are: The validation error message to display to the user or write to a log. The ValidationResult class exposes this as the Message property. A reference to the class instance where the validation error was discovered (usually the current instance). The ValidationResult class exposes this as the Target property. A string value that describes the location of the error (usually the name of the class member, or some other value that helps locate the error). The ValidationResult class exposes this as the Key property. An optional string tag value that can be used to categorize or filter the results. The ValidationResult class exposes this as the Tag property. A reference to the validator that performed the validation. This is not used in self-validation, though it will be populated by other validators that validate individual members of the type. The ValidationResult class exposes this as the Validator property.
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