Question

Ok, I thought this would be a no-brainer, but evidently I’m doing something wrong. The problem is that when clicking on the “Up” and “Down” buttons of the Extended WPF toolkit DoubleUpDown control, the values do not get updated correctly. When I click Up, the value in the control changes, but the view model does not get updated. Only when I change from clicking Up to clicking Down, does the model get updated, but with the then previous value.

To reproduce, I used a simple view model like so:

public class ViewModel : INotifyPropertyChanged
{

    public ViewModel()
    {
        MyValue = 0.5;
    }

    private double _myValue;
    public double MyValue
    {
        get { return _myValue; }
        set
        {

            _myValue = value;
            PropertyChanged(this, new PropertyChangedEventArgs("MyValue"));

        }
    }

    public event PropertyChangedEventHandler PropertyChanged = delegate { };
}

And my MainWindow.xaml looks like the code below, where the DoubleUpDown control and the label are both bound in TwoWay fashion to the ViewModel’s MyValue property:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
        Title="MainWindow" Height="100" Width="200">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <xctk:DoubleUpDown
            Value="{Binding MyValue, Mode=TwoWay}"
            Increment="0.5"
            Minimum="0.0"
            Maximum="10"
            ValueChanged="DoubleUpDown_ValueChanged"
            />
        <Label Grid.Column="1" Content="{Binding MyValue, Mode=TwoWay}"/>
    </Grid>
</Window>

And in the code-behind, I set the DataContext in the MainWindow constructor to be an instance of ViewModel:

    public MainWindow()
    {
        DataContext = new ViewModel();
        InitializeComponent();
    }

Answer 1

Default binding update logic for DoubleUpDown control is LostFocus. Try setting explicitly UpdateSourceTrigger=PropertyChanged in your binding like this –

<xctk:DoubleUpDown
      Value="{Binding MyValue, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
      Increment="0.5"
      Minimum="0.0"
      Maximum="10"
      ValueChanged="DoubleUpDown_ValueChanged"/>