C# 4 and .NET 4

1058 ❘ ChaPTer 36 business ApplicAtiOns with wpf
In the constructor of the MainWindow class, a new object of the class SomeData is initialized and passed to
the DataContext for data binding:
public partial class MainWindow: Window
{
private SomeData p1 = new SomeData { Value1 = 11 };
public MainWindow()
{
InitializeComponent();
this.DataContext = p1;
}
code snippet ValidationDemo/MainWindow.xaml.cs
The event handler method OnShowValue displays a message box to show the actual value of the SomeData
instance:
}
private void OnShowValue(object sender, RoutedEventArgs e)
{
MessageBox.Show(p1.Value1.ToString());
}
With simple data binding, here the Text property of a TextBox is bound to the Value1 property. If you run
the application now and try to change the value to one that is not valid, you can verify that the value never
changed by clicking the Submit button. WPF catches and ignores the exception thrown by the set accessor
of the property Value1:
Value1:
code snippet ValidationDemo/MainWindow.xaml
To display an error as soon as the context of the input field changes, you can set the ValidatesOnException
property of the Binding markup extension to True. With an invalid value (as soon as the exception is thrown
when the value should be set), the TextBox is surrounded by a red line, as shown in Figure 36-11.
Value1:
To return the error information in a different way to the user, you can assign
the attached property ErrorTemplate that is defined by the Validation
class to a template defining the UI for errors. The new template to mark the error
is shown here with the key validationTemplate. The ControlTemplate puts a
red exclamation point in front of the existing control content:
!
Setting the validationTemplate with the Validation.ErrorTemplate attached
property activates the template with the TextBox:
Value1:
The new look of the application is shown in Figure 36-12.
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figure 36-11
figure 36-12