WPF Cyclic Converter

Question

When you drag the slider, you get a ConvertBack (expected), but why do I then get a "Convert" straight after that? I'd only expect Convert to be called when its first initialized, or if it was raising a property change notification, but it doesn't.

<Window x:Class="WpfApplication10.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:l="clr-namespace:WpfApplication10"
        Title="MainWindow">
    <Slider Value="{Binding Value, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay, Converter={l:Converter}}"/>
</Window>


public class Converter : MarkupExtension, IValueConverter
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value;
    }

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

public partial class MainWindow : Window
{
    public double Value { get; set; }

    public MainWindow()
    {
        DataContext = this;
        InitializeComponent();
    }
}

Answer 1

This is likely occurring because you're using .NET 4, which changed the way bindings work a little bit : http://karlshifflett.wordpress.com/2009/05/27/wpf-4-0-data-binding-change-great-feature/

Whereas prior to 4 the binding wouldn't update back (assuming that it didn't need to), the new behavior is to do that by default. Rationale is explained in the linked blog.

Edit: I suppose I should ask if there is a reason you don't want it to convert back, or if it's just curiosity? If you need to prevent converting back, the clearest strategy is probably to keep track of your last converted value to parrot back :

public class Converter : MarkupExtension, IValueConverter
{
public override object ProvideValue(IServiceProvider serviceProvider)
{
    return this;
}

public object lastValue;

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    return lastValue;
}

public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    lastValue = value;
    return value;
}
}

There's also the option of setting the binding to OneWayToSource, which would keep it from setting back to the slider value... assuming you don't need to push data back to the slider.