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 entered into the textbox, I would like a custom error message to be displayed (rather than the default 'input string is in the wrong format').

The converter code is:

    public object ConvertBack(
        object value,
        Type targetType,
        object parameter,
        CultureInfo culture)
    {
        try
        {
            int minutes = System.Convert.ToInt32(value);
            return new TimeSpan(0, minutes, 0);
        }
        catch
        {
            throw new FormatException("Please enter a number");
        }
    }

I have set 'ValidatesOnExceptions=True' in the XAML binding.

However, I have come across the following MSDN article, which explains why the above will not work:

"The data binding engine does not catch exceptions that are thrown by a user-supplied converter. Any exception that is thrown by the Convert method, or any uncaught exceptions that are thrown by methods that the Convert method calls, are treated as run-time errors"

I have read that 'ValidatesOnExceptions does catch exceptions in TypeConverters, so my specific questions are:

• When would you use a TypeConverter over a ValueConverter
• Assuming a TypeConverter isn't the answer to the issue above, how can I display my custom error message in the UI

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>