Show WPF validation error message in a fixed place

Question

I got WPF validation running (added ValidationRules to the binding) and with the template I can create nice adorners. There are many posting out there.

But I cannot find a way to display the error message outside of the adorned control in a fixed place like a TextBlock in a corner of the window e.g.

How could I achieve this? Can I bind all of my validation error messages to my DataContext (ViewModel here)?

---

Update: Thanks to an answer I got it partly working. The validation messages are now displayed in another label. As all the textboxes with their validation rules are created on the fly by code, the binding to do so is done this way:

Binding bindSite = new Binding();
bindSite.Source = this.validationErrorDisplayLabel;
BindingOperations.SetBinding(textBox, Validation.ValidationAdornerSiteProperty, bindSite);

But the validation messages are only forwarded to the adornersite for the last textbox for which this code was executed.

---

I reproduced the problem in this small example.

XAML:

<Grid>
    <TextBox 
        Validation.ValidationAdornerSite="{Binding ElementName=ErrorDisplay}"
        HorizontalAlignment="Left" Height="23" Margin="10,10,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120">
        <TextBox.Text>
            <Binding>
                <Binding.Path>Box1</Binding.Path>
                <Binding.ValidationRules>
                    <local:RuleA />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <TextBox 
        Validation.ValidationAdornerSite="{Binding ElementName=ErrorDisplay}"
        HorizontalAlignment="Left" Height="23" Margin="10,38,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="120">
        <TextBox.Text>
            <Binding>
                <Binding.Path>Box2</Binding.Path>
                <Binding.ValidationRules>
                    <local:RuleA />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <TextBlock 
        x:Name="ErrorDisplay"
        Background="AntiqueWhite"
        Foreground="Red"
        Text="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.ValidationAdornerSiteFor).(Validation.Errors)[0].ErrorContent}"
        HorizontalAlignment="Left" Margin="230,10,0,0" TextWrapping="Wrap" VerticalAlignment="Top" RenderTransformOrigin="2.218,-4.577" Width="177" Height="51"/>
</Grid>

The class RuleA produces a validation error when the value equals the string "A". The errors in the 2nd textbox are displayed in the TextBlock, the errors of the first one not (instead it uses the default template and gets a red border).

How can it work for both? The textblock does not need to sum up all errors but display the very first error.

Answer 1

You can use a BindingGroup in combination with the Validation.ValidationAdornerSite and Validation.ValidationAdornerSiteFor properties.

This blog post shows you an example of how to do this.

---

<StackPanel x:Name="FormRoot"
            Validation.ValidationAdornerSite="{Binding ElementName=ErrorDisplay}">
  <FrameworkElement.BindingGroup>
    <BindingGroup Name="FormBindingGroup" />
  </FrameworkElement.BindingGroup>

  <TextBox>
    <TextBox.Text>
      <Binding BindingGroupName="FormBindingGroup"
               UpdateSourceTrigger="LostFocus"
               Path="Box1">
        <Binding.ValidationRules>
          <l:RuleA />
        </Binding.ValidationRules>
      </Binding>
    </TextBox.Text>
  </TextBox>

<TextBox>
    <TextBox.Text>
      <Binding BindingGroupName="FormBindingGroup"
               UpdateSourceTrigger="LostFocus"
               Path="Box2">
        <Binding.ValidationRules>
          <l:RuleA />
        </Binding.ValidationRules>
      </Binding>
    </TextBox.Text>
  </TextBox>

  <ItemsControl x:Name="ErrorDisplay"
                Background="AntiqueWhite"
                Foreground="Red"
                ItemsSource="{Binding RelativeSource={RelativeSource Self},
                                      Path=(Validation.ValidationAdornerSiteFor).(Validation.Errors)}"
                DisplayMemberPath="ErrorContent" />
</StackPanel>

To commit the values as the user types, change the UpdateSourceTrigger values to PropertyChanged. Note that it isn't strictly necessary to use ValidationAdornerSite here; you could simply point the ErrorDisplay binding directly to the owner of the BindingGroup:

ItemsSource="{Binding ElementName=FormRoot, Path=(Validation.Errors)}"