public class TotalSumConverter : IValueConverter 
    { 
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
        { 
            var users = value as IEnumerable<object>;  
            if (users == null) 
                return "$0.00"; 

            double sum = 0; 
             
            foreach (var u in users)  
            {  
                sum += ((User)u).Total; 
            } 
            return sum.ToString("c");  
        } 

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
        { 
            throw new NotImplementedException(); 
        } 
    }