Question

I have a TwoWay binding that controls the value of a slider:

<Slider Orientation="Vertical" Height="200" Value="{Binding Path=MapScale, Mode=TwoWay}" Maximum="{Binding MaxScale}" Minimum="{Binding MinScale}" StepFrequency="0.1" />

The binding is in the ViewModel as a DependencyProperty:

    public static readonly DependencyProperty MapScaleProperty =
         DependencyProperty.Register("MapScale", typeof(Double?), typeof(MappingPageViewModel), new PropertyMetadata(0.0));

    public Double? MapScale
    {
        get { return GetValue(MapScaleProperty) as Double?; }
        set { SetValue(MapScaleProperty, value); OnPropertyChanged("MapScale"); }
    }

As the code is now, the slider updates properly when I update MapScale (e.g. MapScale += .1). But, if I remove the OnPropertyChanged method (which I was under the impression SetValue already calls), the slider doesn’t update properly.

What have I missed?

Answer 1

Slider.Value is of type double, not Nullable<double>. I’ve noticed that WinRT is very picky about matching binding types. It doesn’t auto-convert most things for you like WPF and Silverlight.

My answer is:

Use INotifyPropertyChanged as @Adi already mentioned AND use double as the property type.

If you’re stuck using Nullable<double> in your view-model, then create an IValueConverter that converts null to zero.