WORKING WITH DATA in Visual Basic .NET

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CHAPTER 6 WORKING WITH DATA
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Binding.NotifyOnValidationError Property and FrameworkElement.BindingValidationError Event
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Extending the simple data validation process, as we saw in the last section, to perform some additional actions when the value entered in the bound control raises an exception, you can set NotifyOnValidationError to true, along with ValidatesOnExceptions also set to true. Turning both properties to true will fire the FrameworkElement.BindingValidationError event when the data is not entered correctly. The FrameworkElement.BindingValidationError event is a routed bubbling event. So it can be handled either at the control that raises this or can be bubble up to each successive parent control level, such as the layout container of that control, like Grid or StackPanel. In an n-tier Silverlight application scenario with pattern implementation such as Model-View-View-Model (MVVM), if you catch such exceptions at the boundary of that particular layer (at a higher level), it will give you an opportunity to use the same logic to handle different kinds of errors raised by various controls. However, if you want more precise error handling for particular types of errors, you should catch such errors where they happen and not at the boundary of that layer. To demonstrate this, let s extend our previous example and add one TextBlock as a last element and a NotifyOnValidationError event handler as shown in the following code snippet. Note that the updated part of the code is in bold font. <StackPanel x:Name="LayoutRoot" Background="White"> <TextBlock Height="23" HorizontalAlignment="Left" Text="Enter Value:" VerticalAlignment="Top" Width="120" /> <TextBox x:Name="txtAnswer" Height="23" HorizontalAlignment="Left" VerticalAlignment="Top" Width="120" > <TextBox.Text> <Binding Mode="TwoWay" Path="AnswerValue" ValidatesOnExceptions="True" NotifyOnValidationError="True" /> </TextBox.Text> </TextBox> <TextBlock x:Name="errMessage" /> </StackPanel > Here our idea is to display that error message in the errMessage TextBlock control. So in the BindingValidationError event handler, we will set errMessage s Text property to the content of the error message. Of course, as a better approach, you may want to use the Label control here instead, as we discussed in 3. Here we used TextBlock for keeping things simple for demonstration purposes. Note that here we defined the BindingValidationError event at the User Control level. To do that, in the UserControl constructor we wired this event handler as shown here: this.BindingValidationError += new EventHandler<ValidationErrorEventArgs> (SimpleDataValidationDemo_BindingValidationError); And the corresponding method SimpleDataValidationDemo_BindingValidationError is implemented as follows, which basically populates the added newly added TextBlock with the error message: void SimpleDataValidationDemo_BindingValidationError (object sender, ValidationErrorEventArgs e) {
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CHAPTER 6 WORKING WITH DATA
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errMessage.Text = e.Error.ErrorContent.ToString(); } We are all set now to run the sample. Input invalid data to cause the data error. You will see the additional error message right beneath the TextBox in the errMessage TextBlock control, as shown in Figure 6-4:
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Figure 6-4. Using DataBinding.ValidatesOnExceptions and DataBinding.NofifyOnValidationError properties with the FrameworkElement.BindingValidationError event to display a data validation error
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IDataErrorInfo Interface for Client-Side Validation
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If you have ever worked with WPF applications, you will find that it provides validation infrastructure for binding scenarios through the IDataErrorInfo interface. Silverlight 3 introduced support for validation by throwing exceptions, and you can catch them and convey the appropriate error message to the end user, which we covered in the previous section. With Silverlight 4 there is now support for the IDataErrorInfo interface (similar to WPF) that allows you to validate property values without throwing exceptions. The IDataErrorInfo interface enables client-side validation logic implementation reporting validation errors so that a user interface can bind to them. Table 6-3 shows the properties of this interface. Table 6-3. Properties of the IDataErrorInfo Interface
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Property
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Type
string
Description
Gets a message that describes any client-side validation error on the object. It contains a null reference or is Empty if no error is reported. Gets a message that describes a single validation error message for a specified property or column name. The message can represent multiple errors. If you want to display individual messages for each error, use INotifyDataErrorInfo instead.
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