TimeSpanToXmlString.cs

using System;
using System.Globalization;
using System.Windows.Data;
using System.Windows.Markup;
 
namespace LegalLab.Converters
{
    public class TimeToXmlString : MarkupExtension, IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value?.ToString();
        }
 
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is string s && TimeSpan.TryParse(s, out TimeSpan TS) && TS >= TimeSpan.Zero && TS.TotalDays == 0)
                return TS;
            else
                return value;
        }
 
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return this;
        }
    }
}