Binding Errors

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Binding errors occur when a noncritical exception is raised after the ConvertBack method of the ValueConverter is called. The following is a list of the three types of binding errors that can occur when modifying the value of a target property:

Image The user enters a value that fails to be converted to the source binding type (syntactic validation).

Image An exception is raised within the property set accessor.

Image The property does not have a set accessor and is, therefore, read-only.

Note

When a binding error occurs, the target, a TextBox, for example, retains the entered value, but the source property is not updated.

By default, the XAML data-binding infrastructure fails silently when a binding error occurs. The NotifyOnValidationError and the ValidatesOnExceptions binding properties allow you to provide the user with feedback when a value is incorrectly set.

NotifyOnValidationError Binding Property

With the Binding.NotifyOnValidationError property set to true, the data binding system raises a BindingValidationError event when a binding error occurs. The following example demonstrates enabling validation on a TextBox element:

Click here to view code image

<TextBox Text="{Binding ValidatedString2, Mode=TwoWay,
        NotifyOnValidationError=True, ValidatesOnExceptions=True}"
        Style="{StaticResource ValidatingTextBoxStyle}" />

The BindingValidationError is a RoutedEvent, allowing it to be handled by an ancestor element in the visual tree. RoutedEvents bubble upward through the visual tree until they are either handled, indicated by the Handled property of the event arguments, or they reach the root element of the visual tree. Subscription to the BindingValidationError event at the page level, therefore, provides the opportunity to handle validation errors for all elements in the page. We leverage this later in the chapter to create a validation summary control.

ValidatesOnExceptions Property

The Binding class’s NotifyOnValidationError property works in unison with its ValidatesOnExceptions property. If the ValidatesOnExceptions property is not explicitly set to true and a binding error results from a noncritical exception being raised from a property accessor, the error is silently ignored. Furthermore, the VisualState of the FrameworkElement is not changed, and the BindingValidationError event is not raised.

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