How to handle exception in Value converter so that custom error message can be displayed

Question

I have a textbox that is bound to a class with a property of type Timespan, and have written a value converter to convert a string into TimeSpan.

If a non number is

Answer 1

I would use a ValidationRule for that, this way the converter can be sure that the conversion works since it only is called if validation succeeds and you can make use of the attached property Validation.Errors which will contain the errors your ValidationRule creates if the input is not the way you want it.

e.g. (note the tooltip binding)

<TextBox>
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="True">
                    <Setter Property="Background" Value="Pink"/>
                    <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
    <TextBox.Text>
        <Binding Path="Uri">
            <Binding.ValidationRules>
                <vr:UriValidationRule />
            </Binding.ValidationRules>
            <Binding.Converter>
                <vc:UriToStringConverter />
            </Binding.Converter>
        </Binding>
    </TextBox.Text>
</TextBox>

Answer 2

I used validation and converter to accept null and numbers

XAML:

<TextBox x:Name="HeightTextBox" Validation.Error="Validation_Error">
    <TextBox.Text>
        <Binding Path="Height"
                 UpdateSourceTrigger="PropertyChanged" 
                 ValidatesOnDataErrors="True"
                 NotifyOnValidationError="True"
                 Converter="{StaticResource NullableValueConverter}">
            <Binding.ValidationRules>
                <v:NumericFieldValidation />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

Code Behind:

private void Validation_Error(object sender, ValidationErrorEventArgs e)
{
    if (e.Action == ValidationErrorEventAction.Added)
        _noOfErrorsOnScreen++;
    else
        _noOfErrorsOnScreen--;
}

private void Confirm_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = _noOfErrorsOnScreen == 0;
    e.Handled = true;
}

ValidationRule :

public class NumericFieldValidation : ValidationRule
{
    private const string InvalidInput = "Please enter valid number!";

    // Implementing the abstract method in the Validation Rule class
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        float val;
        if (!string.IsNullOrEmpty((string)value))
        {
            // Validates weather Non numeric values are entered as the Age
            if (!float.TryParse(value.ToString(), out val))
            {
                return new ValidationResult(false, InvalidInput);
            }
        }

        return new ValidationResult(true, null);
    }
}

Converter :

public class NullableValueConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (string.IsNullOrEmpty(value.ToString()))
            return null;
        return value;
    }
}

Answer 3

You shouldn't throw exceptions from the converter. I would implement IDataErrorInfo and implement the Error and String on that. Please check http://www.codegod.biz/WebAppCodeGod/WPF-IDataErrorInfo-and-Databinding-AID416.aspx. HTH daniell