WPF TextBox setting Text using Trigger during validation

Question

I am having a requirement where in I have to revert the values of a TextBox to old value when the user enters a wrong input. I am using MVVM framework so I dont want to write any codebehind.

The Text and Tag of TextBox is databound from ViewModel variable. So my Tag field of TextBox will always have old value. I want to use the Tag field value to revert my Text value.

  <Style TargetType="{x:Type TextBox}">
        <Setter Property="Validation.ErrorTemplate">
            <Setter.Value>
                <ControlTemplate>
                    <DockPanel LastChildFill="True">
                        <TextBlock DockPanel.Dock="Right" 
                    Foreground="Orange"
                    FontSize="12pt">

                </TextBlock>
                        <Border BorderBrush="Red" BorderThickness="1">
                            <AdornedElementPlaceholder />
                        </Border>
                    </DockPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="Validation.HasError" Value="true" >
                <Setter Property="ToolTip" 
                        Value="{Binding Path=Tag,RelativeSource={RelativeSource Self}}">
                </Setter>
                <Setter Property="Text"
                            Value="{Binding Path=Tag,RelativeSource={RelativeSource Self}}">
                </Setter>
            </Trigger>
        </Style.Triggers>
    </Style>


  <TextBox Width="68" Tag="{Binding SampleText}" Height="23" HorizontalAlignment="Left" Margin="39,37,0,0" VerticalAlignment="Top" >
        <TextBox.Text>
            <Binding Path="SampleText"  NotifyOnValidationError="True" ValidatesOnDataErrors="True" ValidatesOnExceptions="True">
                <Binding.ValidationRules>
                    <val:SampleTextValidator></val:SampleTextValidator>
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>        
 </TextBox>

Now When an error happens, the TextBox is highlighted red.I have written a Trigger to revert the value back to original value (value stored in Tag field). Tt is not working. But Tooltip part is working. I am confused fully. Please help where am I doing wrong!!!. Correct me with a sample code if possible!!!!

Answer 1

My first guess is that when you made your text input invalid (eg. delete all values), you cause the tag to bind to the same value, hence, it will reflect an empty string.

What you need is a separate property for your original value to bind your tag to.

private string _oldValue;
public string OldValue
{
    get {...}
    set {... NotifyPropertyChanged()...}
}

private string _sampleText;
public string SampleText
{
    get { return _sampleText; }
    set {
            OldValue = _sampleText;
            _sampleText = value;
            NotifyPropertyChanged(...);
        }
}

<TextBox Width="68" Tag="{Binding OldValue}" ... >

Don't forget to implement INotifyPropertyChanged.