Validation in MVVM-based applications

As mentioned in the introduction to this chapter, we have a lot of validation options to choose from in XAML-based technologies. Let's now take a look at the options that are best suited for use in MVVM applications.

Validation rules

This is the simplest and most commonly used validation technique. A WPF binding can be associated with a number of custom validation rules, each subclassed from ValidationRule. There are only two validation rules provided as part of the library: DataErrorValidationRule and ExceptionValidationRule.

Using validation rules

We can define additional custom validation rules by inheriting from ValidationRule. We can add them to the ValidationRules collection for a binding, as follows:

<TextBox Height="26" HorizontalAlignment="Left" 
         Margin="3,2,0,0"          VerticalAlignment="Top" Width="390"                 
         Grid.Column="1" Grid.Row="2" >
    <TextBox.Text>
        <Binding Path="ProductQuantity...