Question

What is best practise for IValueConverter: Is it ok to put Exception in Convert method or should it return "something"?

Here is an example:

[ValueConversion(typeof(float), typeof(String))]
public class PercentConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || string.IsNullOrEmpty(value.ToString()))
            return string.Empty;

        if (value is float) //Edited to support CultureInfo.CurrentCulture,
            return string.Format(culture, "{0:n}{1}", ((float)value) * 100, "%");

        //** Is it ok to put Exception here or should I return "something" here? **
        throw new Exception("Can't convert from " + value.GetType().Name + ". Expected type if float.");
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException("Converting back is not implemented in " + this.GetType());
    }
}

Answer 1

If you fail to convert (malformed values, types, …), return DependencyProperty.UnsetValue.

It indicates that the converter produced no value and that the binding uses the FallbackValue, if available, or the default value instead.

Also, you should convert data with culture-specific conversion or invariant conversions to be on the safe side.