Value Converter does not apply the desired string format

Question

I have been trying to implement a simple value converter I just can't figure out why the StringFormat is not being applied. The positive values turn green and the negatives red but instead of getting the desired result (+15.74), I end up with: 15.74747474.

How can I get this to work? Can someone please give me an idea of what I need to change?

XAML

<Window.Resources>

    <local:PositiveConverter x:Key="PositiveConverter"/>

    <Style TargetType="TextBlock" x:Key="NumericTextBlock">
        <Style.Triggers>
            <DataTrigger Binding="{Binding Text, RelativeSource={RelativeSource Self}, Converter={StaticResource PositiveConverter}}" Value="True">
                <Setter Property="Foreground" Value="Green"/>
                <Setter Property="Text" Value="{Binding StringFormat='({0:+0.0})'}"/>
            </DataTrigger>

            <DataTrigger Binding="{Binding Text, RelativeSource={RelativeSource Self}, Converter={StaticResource PositiveConverter}}" Value="False">
                <Setter Property="Foreground" Value="Red"/>
                <Setter Property="Text" Value="{Binding StringFormat='({0:-0.0})'}"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>

</Window.Resources>

    <Grid>
        <StackPanel>
            <TextBlock Text="{Binding ActualValue}" Style="{StaticResource NumericTextBlock}"/>
        </StackPanel>   
    </Grid>

ViewModel

namespace My.ViewModel
{
public class LocationViewModel : UserControl, INotifyPropertyChanged
{
public MyViewModel()
{
        SetActualValue();
}

public DateTime mDate = DateTime.Now;
public DateTime MDate
{  
get { return mDate; }
set
    {
        if (value == mDate)
        {
            return;
        }
        else
        {
            mDate = value;
            OnPropertyChanged("MDate");
            SetActualValue();
        }

    }

}

double _actualValue;
public double ActualValue
{
    get { return _actualValue; }
    set
    {
    _actualValue = value;
    OnPropertyChanged("ActualValue");
    }
}

ValueConverter

public class PositiveConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var number = 0.0;
        var isNumber = double.TryParse(value.ToString(), out number);

        return isNumber && number >= 0.0;
    }

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

Answer 1

Setting

Text="{Binding ActualValue}"

directly on the TextBlock has higher value precedence than a Style Setter like

<Setter Property="Text" Value="{Binding StringFormat='({0:+0.0})'}"/>

So the Bindings from the Setter will never be applied. They are also wrongly written, because they miss the property path ActualValue .

Your approach won't work. You should instead have DataTriggers with Bindings to the view model property like this:

<Style TargetType="TextBlock" x:Key="NumericTextBlock">
    <Setter Property="Text"
            Value="{Binding ActualValue, StringFormat='({0:+0.0})'}"/>

    <Style.Triggers>
        <DataTrigger Binding="{Binding ActualValue, Converter={StaticResource PositiveConverter}}"
                     Value="True">
            <Setter Property="Foreground" Value="Green"/>
            <Setter Property="Text"
                    Value="{Binding ActualValue, StringFormat='({0:+0.0})'}"/>
        </DataTrigger>

        <DataTrigger Binding="{Binding ActualValue, Converter={StaticResource PositiveConverter}}"
                     Value="False">
            <Setter Property="Foreground" Value="Red"/>
            <Setter Property="Text"
                    Value="{Binding ActualValue, StringFormat='({0:-0.0})'}"/>
        </DataTrigger>
    </Style.Triggers>
</Style>

and remove Text="{Binding ActualValue}" from the TextBlock.