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Programming WPF, 2nd Edition
book

Programming WPF, 2nd Edition

by Chris Sells, Ian Griffiths
August 2007
Intermediate to advanced content levelIntermediate to advanced
864 pages
25h 52m
English
O'Reilly Media, Inc.
Content preview from 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' ...
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Publisher Resources

ISBN: 9780596510374Supplemental ContentErrata Page