Question

sometimes I see when the binding present an incorrect input shows a red border around the control. How can I modify it or where? And if it’s possible, put me an example.

Thanks in advance.

Answer 1

You can do something like this for a TextBox

<Style x:Key="TextBoxWithValidation" TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">
    <Setter Property="Validation.ErrorTemplate" Value="{StaticResource TextBoxValidationTemplate}"/>
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="true">
            <Setter Property="Background" Value="{StaticResource BackgroundValidationBrush}"/>
            <Setter 
                Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Mode=Self},
                Path=(Validation.Errors)[0].ErrorContent)}"
            />
        </Trigger>
    </Style.Triggers>
</Style>

Where BackgroundValidationBrush would be say Pink.

Note that the common solution of binding ToolTip to (Validation.Errors)[0].ErrorContent will result in lots of debug spew (technical term) when there are no errors, so you’re better off with a converter like this:

[ValueConversion(typeof(ReadOnlyObservableCollection<ValidationError>), typeof(string))]
public class ValidationErrorsToStringConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter,
        CultureInfo culture)
    {
        var errors = value as ReadOnlyObservableCollection<ValidationError>;

        // If there are no errors then return an empty string. 
        // This prevents debug exception messages that result from the usual Xaml of "Path=(Validation.Errors)[0].ErrorContent".
        // Instead we use "Path=(Validation.Errors), Converter={StaticResource ValidationErrorsConverter}".
        if (errors == null)
        {
            return string.Empty;
        }

        var errors2 = errors.Select(e => e.ErrorContent).OfType<string>().ToArray();

        return errors.Any() ? string.Join("\n", errors2) : string.Empty;
    }
    public object ConvertBack(object value, Type targetType, object parameter,
        CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

and the we can use

<converters:ValidationErrorsToStringConverter x:Key="ValidationErrorsConverter"/>

<!-- Style to be used as the base style for all text boxes -->
<Style x:Key="TextBoxWithValidation" TargetType="{x:Type TextBox}" BasedOn="{StaticResource {x:Type TextBox}}">
    <Setter Property="Validation.ErrorTemplate" Value="{StaticResource TextBoxValidationTemplate}"/>
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="true">
            <Setter Property="Background" Value="{StaticResource BackgroundValidationBrush}"/>
            <Setter 
                Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Mode=Self},
                Path=(Validation.Errors),
                Converter={StaticResource ValidationErrorsConverter}}"
            />
        </Trigger>
    </Style.Triggers>
</Style>