WPF TextBox Binding Percentage

Question

I have to bind a TextBox to a % value. So I set the StringFormat Property on the Binding like this:

<TextBox Text="{Binding Path=BewertungsFaktore.Gewinn, StringFormat=P2, UpdateSourceTrigger=PropertyChanged}" Style="{StaticResource TextBoxStyle}" HorizontalAlignment="Left"/>

That seems to work quite fine. But if I edit the value there are issues. As example if I enter a value of 1 the textbox should format this in 1%. But the main problem is it formats into 100.00%. Another question is if I am using a German Sys do I have to enter ',' instaed of a '.'?

Answer 1

If you just use a string format like {0:F2}% then you will have to put up with database values like 4.23 to represent 4.23% which is unacceptable to me (until they introduce a "percentage" data type into SQL Server).

I created the following value converter to map percentage values in the TextBox like 4.2367% into database values like 0.042367:

public class PercentageConverter : IValueConverter
{
    //E.g. DB 0.042367 --> UI "4.24 %"
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var fraction = decimal.Parse(value.ToString());
        return fraction.ToString("P2");
    }

    //E.g. UI "4.2367 %" --> DB 0.042367
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        //Trim any trailing percentage symbol that the user MAY have included
        var valueWithoutPercentage = value.ToString().TrimEnd(' ', '%');
        return decimal.Parse(valueWithoutPercentage)/100;
    }
}

Note how the formatting string (in this case "P2") only limits how many decimal places are displayed, not how many decimal places are passed to the underlying binding source.

Answer 2

The StringFormat property controls only the output string of the bound value. "P2" simply multiplies the value by 100 and formats it with two decimal digits and a trailing " %". On input, this format is ignored.

If you also need to input percent value you would have to use a binding converter. Such a converter could also parse the input string in a culture-invariant way like this:

double fraction;
double percentage;
if (double.TryParse(value, NumberStyles.AllowDecimalPoint, CultureInfo.InvariantCulture, out percentage))
{
    fraction = percentage / 100d;
}