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private int _tempLevel; private int _level; public int Level
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get { return _level; } set { _tempLevel = value; ValidateSalaryAndLevelAsync(value, Salary); } } private decimal _tempSalary; private decimal _salary; public decimal Salary { get { return _salary; } set { _tempSalary = value; ValidateSalaryAndLevelAsync(Level, value); } }
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Note the _tempSalary and _tempLevel variables. Due to the asynchronous nature of the validation, I needed some place to store the possibly invalid values; otherwise, you could never jump out of the validation error condition (one of the properties would always be the default value during validation) and correct the data. These properties are where I choose to store the temporary values. Consider them draft or unverified values. Some caveats to the code I presented here. I don t propose that these are best practices or even stable for production code. The code has been simplified to show the core concepts and stay within the reasonable bounds of a chapter. For example, in the preceding code, you may run into race conditions for multiple property changes that happen during a slow-running web service call. If they happen to get queued out of order, you can end up with skewed validation and entry. The INotifyDataErrorInfo class implementation is more complex than the other methods presented here. For that reason, you may prefer to implement it only on some fields and use either exception-based or IDataErrorInfo on the remaining fields. The choice is up to you; all of the methods coexist nicely in the same class. The binding system will know which ones to use based on the properties of the binding statement. INotifyDataErrorInfo fills in the missing gap left by the other methods by enabling you to provide asynchronous validation error reporting and supply multiple validation messages for a single field. IDataErrorInfo and INotifyDataErrorInfo are the premier ways of handling validation in Silverlight but they require a fair bit of code to implement. What if you want to do something more lightweight Do you need to turn to exceptions No, if you re using the DataForm or DataGrid, you can annotate your data using validation attributes.
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13.5 Annotating for validation
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There are innumerable ways to validate data and an equally diverse number of ways in which to store that validation information including the rules logic and the messages. In section 13.2, we saw how you can use exceptions in property setters to expose validation information to the user interface. In sections 13.3 and 13.4, we saw how to use specialized interfaces to support additional forms of validation. Though you can continue to code validation directly into properties setters or use interfaces, the System.ComponentModel.DataAnnotations assembly and namespace found in the Silverlight SDK provide a number of attributes designed to make data validation and display hinting easier for controls such as the DataForm, DataGrid, and some third-party controls. The approach taken by these attributes is to mark up the properties in your entities using attributes in code rather than require code within the properties or external to your entities. If your scenario supports their use, validation attributes are simple to implement and easy to use. In our discussion of these attributes, we ll first go over the available set of attributes and how to implement a select set of them in your own classes. Then, we ll extend the reach of the attributes to call out to external validation functions in your code. Finally, we ll create our own custom validators to handle situations not easily handled by the built-in set. All of these techniques help us create validation code that s cleaner and easier to read than many of the other methods.
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13.5.1 Validation attributes
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Previously we saw how to provide property-level validation using exceptions and synchronous and asynchronous interfaces. Though those work in almost any situation, they re not a very clean approach and lead to a significant amount of branch/check code inside the property setters in your entities. They also require significant code modification to your entities a luxury we don t always have. One better way to tackle basic validation is to use attributes tied to the properties in the class. To support attribute or annotation-based validation, the DataAnnotations namespace includes the validation attributes shown in table 13.5.
Table 13.5 Validation attributes in System.ComponentModel.DataAnnotations Validation capabilities It specifies that the value must be parsable as one of the members of a specified enum. It specifies that the value must be between two other values. The type can be any type that implements IComparable. It enables you to associate a regular expression to validate a value. This is useful for things such as phone numbers and email addresses, as well as any other data that must adhere to one or more specific formats.
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