WPF UserControl: setter not called on binding one property in multiple controls and back

Question

I've got a problem with my WPF UserControl binding one property in multiple controls and back. The setters of the business object will not be called. I searched for hours now and tried serveral things which did not work.

My Code

Can be download here: WpfApplicationUserControlProblem.zip

My Business object has 2 DateTime Values to be bound.

public class BusinessObject
{
    private DateTime _value1 = DateTime.Today.AddHours(10);
    public DateTime Value1
    {
        get { return _value1; }
        set { _value1 = value; }        // will never be called but why??
    }

    private DateTime _value2 = DateTime.Today.AddDays(1).AddHours(15);
    public DateTime Value2
    {
        get { return _value2; }
        set { _value2 = value; }        // will never be called but why??
    }
}

My Main-Window has 2 user controls to bind the 2 values of my object

<Window x:Class="WpfApplicationUserControlProblem.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfApplicationUserControlProblem"
    mc:Ignorable="d"
    Title="MainWindow" Height="120.961" Width="274.489">
<Grid>
    <local:DateTimeUserControl DateTimeValue="{Binding Value1, UpdateSourceTrigger=PropertyChanged}" Margin="10,10,0,0" VerticalAlignment="Top" HorizontalAlignment="Left" Width="241"/>
    <local:DateTimeUserControl DateTimeValue="{Binding Value2, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Margin="10,39,0,0" VerticalAlignment="Top" Width="241"/>
</Grid>

public partial class MainWindow : Window
{
    private BusinessObject _businessObject = new BusinessObject();
    public MainWindow()
    {
        InitializeComponent();
        DataContext = _businessObject;
    }
}

My UserControl DateTimeUserControl has one DependencyProperty "DateTimeValue" for receiving the bound business value from the Main-Window. With the "DateTimeValuePropertyChangedCallback" I redirect the received value into a DateValue for the DatePicker and HourValue for the HourTextBox. Changing the DatePicker or HourTextBox should update the DependencyProperty "DateTimeValue" and therefore also the bounded business object. That was my plan.

<UserControl x:Class="WpfApplicationUserControlProblem.DateTimeUserControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:WpfApplicationUserControlProblem"
         x:Name="_this"
         mc:Ignorable="d">
<Grid>
    <DatePicker SelectedDate="{Binding Path=DateValue, ElementName=_this, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Margin="0,0,61,0"/>
    <TextBox Text="{Binding Path=HourValue, ElementName=_this, UpdateSourceTrigger=PropertyChanged}" Height="24" VerticalAlignment="Top" HorizontalAlignment="Right" Width="56"/>
</Grid>

public partial class DateTimeUserControl : UserControl, INotifyPropertyChanged
{
    public DateTimeUserControl()
    {
        InitializeComponent();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public static readonly DependencyProperty DateTimeValueProperty = DependencyProperty.Register(nameof(DateTimeValue), typeof(DateTime), typeof(DateTimeUserControl), new PropertyMetadata(DateTimeValuePropertyChangedCallback));
    public DateTime DateTimeValue
    {
        get { return (DateTime)GetValue(DateTimeValueProperty); }
        set { SetValue(DateTimeValueProperty, value); }
    }

    private static void DateTimeValuePropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        DateTimeUserControl control = d as DateTimeUserControl;
        control.FirePropertyChanged(d, new PropertyChangedEventArgs(nameof(DateValue)));
        control.FirePropertyChanged(d, new PropertyChangedEventArgs(nameof(HourValue)));
    }

    private void FirePropertyChanged(object sender, PropertyChangedEventArgs args)
    {
        PropertyChanged?.Invoke(sender, args);
    }

    public DateTime DateValue
    {
        get { return DateTimeValue.Date; }
        set { DateTimeValue = value.Date.AddHours(DateTimeValue.Hour); }
    }

    public string HourValue
    {
        get { return DateTimeValue.Hour.ToString(); }
        set { DateTimeValue = DateTimeValue.Date.AddHours(int.Parse(value)); }
    }
}

I don't get it

Everything seems to work fine except that the setter of the business object is not called when the DependencyProperty is updated. But why? I also tried everything with DependencyProperties or MultiBindingConverters. I failed on every try.

Can anybody help?

Answer 1

The DateTimeValue Bindings should be declared as TwoWay , while UpdateSourceTrigger=PropertyChanged is certainly redundant:

<local:DateTimeUserControl DateTimeValue="{Binding Value1, Mode=TwoWay}" .../>

You could also declare your DateTimeValue dependency property to bind two-way by default:

public static readonly DependencyProperty DateTimeValueProperty =
    DependencyProperty.Register(
        nameof(DateTimeValue),
        typeof(DateTime),
        typeof(DateTimeUserControl),
        new FrameworkPropertyMetadata(
            default(DateTime),
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
            DateTimeValuePropertyChangedCallback));

---

You may ask why this isn't also necessary on the two "internal" bindings in the UserControl's XAML, but both the DatePicker.SelectedDate and the TextBox.Text property are already registered with BindsTwoWayByDefault .