WPF 4 - DatePicker - How to set DisplayMode to Decade?

Question

This code makes the background of the calendar orange, but it does not default the DisplayMode to Decade. Does anybody know why? What can I do to default it to "Decade"?

<DatePicker SelectedDate="{Binding TheDate}">
    <DatePicker.CalendarStyle>
        <Style TargetType="Calendar">
            <Setter Property="DisplayMode" Value="Decade"/>
            <Setter Property="Background" Value="Orange"/>
        </Style>
    </DatePicker.CalendarStyle>
</DatePicker>

Answer 1

The DatePicker control sets CalendarMode to month explicitly when the popup opens, which overrides the value from your style. From Reflector:

private void PopUp_Opened(object sender, EventArgs e)
{
    if (!this.IsDropDownOpen)
    {
        this.IsDropDownOpen = true;
    }
    if (this._calendar != null)
    {
        this._calendar.DisplayMode = CalendarMode.Month;
        this._calendar.MoveFocus(
            new TraversalRequest(FocusNavigationDirection.First));
    }
    this.OnCalendarOpened(new RoutedEventArgs());
}

I don't think you will be able to override that in XAML because it is setting a value explicitly. You could add a handler CalendarOpened="DatePicker_CalendarOpened" and set it back to Decade in the code behind by doing something like this:

private void DatePicker_CalendarOpened(object sender, RoutedEventArgs e)
{
    var datepicker = sender as DatePicker;
    if (datepicker != null)
    {
        var popup = datepicker.Template.FindName(
            "PART_Popup", datepicker) as Popup;
        if (popup != null && popup.Child is Calendar)
        {
            ((Calendar)popup.Child).DisplayMode = CalendarMode.Decade;
        }
    }
}

(I tried this with the WPF Toolkit DatePicker in 3.5, so I don't promise it works in 4.0.)