Cannot bind a negative Integer to an Entry Text - xaml

Question

I have an Entry on a Xamarin Forms ContentPage that I have bound to a ViewModel property QtyIn which is an Int32:
private int _qtyIn;
public int QtyIn
{
get { return _qtyIn; }
set
{
if (_qtyIn != value)
{
_qtyIn = value;
RaisePropertyChanged("QtyIn");
}
}
}
Here is my XAML:
<Entry Text="{Binding Path=Source.QtyIn, Mode=TwoWay, Converter={StaticResource intToStringConverter}}" />
And my IValueConverter:
public class IntToStringConverter : IValueConverter
{
// from Int32 to String
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value.ToString();
}
// String to Int
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
int parsedInt = 0;
if (int.TryParse(value.ToString(), out parsedInt))
{
return parsedInt;
}
return value;
}
}
The problem is when I try and enter a negative number, starting with the '-' minus sign, the binding fails because it can't convert '-' to an Int32. This is the error I see in the mono output when running this app on an Android device:
05-03 15:19:27.923 I/mono-stdout(19384): Binding: - can not be converted to type 'System.Int32'
Does anyone know how to bind an integer to an Text property that allows negative numbers? I can't find any documentation about this on Xamarin's website or forums.
In WPF, I would use the UpdateSourceTrigger=LostFocus property to only do the conversion after the whole number is entered, but Xamarin Forms doesn't have this property available.

Answer 1

Have you tried returning a 0 if the value passed is "-"?
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
int parsedInt = 0;
if (value == "-") return 0;
if (int.TryParse(value.ToString(), out parsedInt))
{
return parsedInt;
}
return value;
}

Answer 2

Your problem is, you return value, which is a string with - and try to pass it to an Int32 (back in your ViewModel).
Instead put second state into your logic. Return a null if parsing did fail - this also helps avoiding problems with any other character input.
public class IntToStringConverter : IValueConverter
{
// from Int32 to String
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value?.ToString() ?? "";
}
// String to Int
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
int parsedInt = 0;
if (int.TryParse(value.ToString(), out parsedInt))
{
return parsedInt;
}
return null;
}
}
And here is the ViewModel:
private int? _qtyIn;
public int? QtyInNullable // you should bind on this one in your entry
{
get { return _qtyIn; }
set
{
if (_qtyIn != value)
{
_qtyIn = value;
if (value != null)
OnPropertyChanged("QtyInNullable");
OnPropertyChanged("QtyIn");
}
}
}
public int QtyIn // you should bind on this one in your entry
{
get { return _qtyIn ?? 0; }
}
And bind it:
<Entry Text="{Binding Path=Source.QtyInNullable, Mode=TwoWay, Converter={StaticResource intToStringConverter}}" />