Automatic input validation in WPF with data binding and Enterprise Library Validation Application Block

WPF provides validation infrastructure for databinding scenarios through ValidationRule objects. Only one built-in rule is provided, and you're expected to write most of these yourself. The way they work is by assigning one or more explicit rules to a binding:

<TextBox ...>
  <TextBox.Text>
    <Binding .>
      <Binding.ValidationRules>
        <local:JpgValidationRule/> 
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
</TextBox> 

[Example from Adam Nathan's excelent book on WPF]

Note that the syntax is much more involved than the simple non-validating binding:...

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