Programming WPF, 2nd Edition

Debugging Data Binding

You may have noticed that our age text box's binding options have gotten fairly involved:

<TextBox ...
  Foreground="{Binding Path=Age,
                       Source={StaticResource Tom},
                       Converter={StaticResource ageConverter}}"
  ToolTip="{Binding RelativeSource={RelativeSource Self},
                    Path=(Validation.Errors)[0].ErrorContent}">
  <TextBox.Text>
    <Binding Path="Age" UpdateSourceTrigger="PropertyChanged">
      <Binding.ValidationRules>
        <local:NumberRangeRule Min="0" Max="128" />
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
</TextBox>

There's a lot going on here and it would be easy to get some of it wrong. For example, if we had a background in journalism, we might have used one-based indexing instead of zero-based indexing to access the first error in our list of validation errors when setting up the binding for the tool tip:

<TextBox ...
  ToolTip="{Binding RelativeSource={RelativeSource Self},
                    Path=(Validation.Errors)[1].ErrorContent}">
  ...
</TextBox>

In this case, as in most others, the WPF data binding engine will simply swallow the error so as not to disturb our user friends.^[34] So, how are we to find it? Well, you need only check the debug output to see the error shown in Example 6-30, and all will be revealed.

Example 6-30. Watch debug output for help debugging data binding problems

System.Windows.Data Error: 12 : Cannot get '' value (type 'ValidationError') from
'(Validation.Errors)' (type 'ReadOnlyObservableCollection'1') BindingExpression:Path=(0).[1].ErrorContent; DataItem='TextBox' ...