Convert and Convertback between double and "+" & " -" String - wpf

Question

I would like to create WPF converter that converts double number to "+" or "-" String based on the number Sign(positive or negative)
but I can't handle the converted back method because I don't have the number anymore.
The "+" and "-" signs bounded to a combobox at the xaml side
any ideas???
public class AmountToDebitCreditConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if ((value == null) || (System.Convert.ToDecimal(value) == 0))
return string.Empty;
return System.Convert.ToDecimal(value) > 0 ? "+" : "-";
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
KeyValue kv = value as KeyValue;
if ((String)value == "+")
return 1;
else
return -1;
}
}
Here's my Xaml code
<igDP:UnboundField Name="ActualAdjustmentAmount" Label="PlusMinusKey" Converter={StaticResource signConverter} >
<igDP:Field.Settings>
<igDP:FieldSettings EditorType="{x:Type igEditors:XamComboEditor}" LabelWidth="40" CellWidth="40">
<igDP:FieldSettings.EditorStyle>
<Style TargetType="{x:Type igEditors:XamComboEditor}">
<Setter Property="ItemsSource" Value="{Binding Path=Flags, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:MainWindow}}}" />
<Setter Property="FlowDirection" Value="LeftToRight"/>
<Setter Property="SelectedItem" Value="{Binding Path=ActualAdjustmentAmount }" />
</Style>
</igDP:FieldSettings.EditorStyle>
</igDP:FieldSettings>
</igDP:Field.Settings>
</igDP:UnboundField>

Answer 1

If the idea is not just to be able to use (and exploit) Converters, then I would rather suggest you to have a specific Sign property in your ViewModel and bind your view showing sign with that property.

Answer 2

It is probably best to do this sort of conversion in a view-model whenever possible.
Nonetheless, you've pinpointed the problem -- you no longer have the number after your conversion. The fact is that you don't need to return a string from your converter -- you can return any object that will return the desired string from its ToString() override, and that object can contain any other data and behaviour that you want.
Here is a converter that should accomplish what you're after, though I haven't actually tested this:
public class SignedDoubleConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return new DisplaySignedDouble(value);
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
var d = value as DisplaySignedDouble;
return (d == null || !d.Value.HasValue)
? 0d
: d.Value.Value;
}
private class DisplaySignedDouble
{
public DisplaySignedDouble(object value)
{
Value = value is double ? (double) value : (double?) null;
}
public double? Value { get; private set; }
public override string ToString()
{
if (!Value.HasValue || Value.Value == 0d)
{
return string.Empty;
}
return Value.Value > 0 ? "+" : "-";
}
}
}