C# 4 and .NET 4

1060 ❘ ChaPTer 36 business ApplicAtiOns with wpf
With a .NET entity class, it would not be clear what an indexer would return; for
example, what would you expect from an object of type Person calling an indexer
That’s why it is best to do an explicit implementation of the interface IDataErrorInfo.
This way this indexer can be accessed only by using the interface, and the .NET class
could do a different implementation for other purposes.
If you set the property ValidatesOnDataErrors of the Binding class to true , the interface
IDataErrorInfo is used during binding. Here, when the TextBox is changed, the binding mechanism
invokes the indexer of the interface and passes Value2 to the columnName variable:
< Label Margin="5" Grid.Row="1" Grid.Column="0" > Value2: < /Label >
< TextBox Margin="5" Grid.Row="1" Grid.Column="1"
Text="{Binding Path=Value2, ValidatesOnDataErrors=True}" / >
code snippet ValidationDemo/MainWindow.xaml
Custom Validation rules
To get more control of the validation you can implement a custom validation rule. A class implementing
a custom validation rule needs to derive from the base class ValidationRule . With the previous
two examples, validation rules have been used as well. Two classes that derive from the abstract
base class ValidationRule are DataErrorValidationRule and ExceptionValidationRule .
DataErrorValidationRule is activated by setting the property ValidatesOnDataErrors and uses the
interface IDataErrorInfo ; ExceptionValidationRule deals with exceptions and is activated by setting
the property ValidatesOnException .
Here a validation rule is implemented to verify for a regular expression. The class
RegularExpressionValidationRule derives from the base class ValidationRule and overrides the
abstract method Validate() that is defi ned by the base class. With the implementation, the RegEx class
from the namespace System.Text.RegularExpressions is used to validate the expression defi ned by the
Expression property:
public class RegularExpressionValidationRule: ValidationRule
{
public string Expression { get; set; }
public string ErrorMessage { get; set; }
}
public override ValidationResult Validate(object value,
CultureInfo cultureInfo)
{
ValidationResult result = null;
if (value != null)
{
Regex regEx = new Regex(Expression);
bool isMatch = regEx.IsMatch(value.ToString());
result = new ValidationResult(isMatch, isMatch
null: ErrorMessage);
}
return result;
}
Regular expressions are explained in Chapter 9.
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