DateTimeEditBox - Focused calendar date when value is null

using System;
using System.Windows;
using System.Windows.Controls.Primitives;
using System.Windows.Media;
using System.Windows.Threading;
using ActiproSoftware.Windows.Controls.Editors;
using ActiproSoftware.Windows.Media;

namespace SampleCode {

    /// <summary>
    /// Provides attached behaviors for <see cref="DateTimeEditBox"/> controls to flash the associated tab.
    /// </summary>
    public static class CustomActiveDateBehavior {

        #region Dependency Properties

        /// <summary>
        /// Identifies the <c>ActiveDate</c> attached dependency property.  This field is read-only.
        /// </summary>
        /// <value>The identifier for the <c>ActiveDate</c> attached dependency property.</value>
        public static readonly DependencyProperty ActiveDateProperty = DependencyProperty.RegisterAttached("ActiveDate",
            typeof(DateTime?), typeof(CustomActiveDateBehavior), new FrameworkPropertyMetadata(null, OnActiveDatePropertyValueChanged));

        #endregion // Dependency Properties

        /////////////////////////////////////////////////////////////////////////////////////////////////////
        #region NON-PUBLIC PROCEDURES
        /////////////////////////////////////////////////////////////////////////////////////////////////////

        /// <summary>
        /// Called when <see cref="ActiveDateProperty"/> is changed.
        /// </summary>
        /// <param name="d">The dependency object that was changed.</param>
        /// <param name="e">The <see cref="DependencyPropertyChangedEventArgs"/> instance containing the event data.</param>
        private static void OnActiveDatePropertyValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
            DateTimeEditBox control = d as DateTimeEditBox;
            if (null == control)
                return;

            if (null != e.NewValue)
                control.DropDownOpened += new RoutedEventHandler(OnDateTimeEditBoxDropDownOpened);
            else
                control.DropDownOpened -= new RoutedEventHandler(OnDateTimeEditBoxDropDownOpened);
        }

        /// <summary>
        /// Handles the <c>DropDownOpened</c> event of the <c>DateTimeEditBox</c> control.
        /// </summary>
        /// <param name="sender">The source of the event.</param>
        /// <param name="e">The <see cref="System.Windows.RoutedEventArgs"/> instance containing the event data.</param>
        private static void OnDateTimeEditBoxDropDownOpened(object sender, RoutedEventArgs e) {
            DateTimeEditBox control = sender as DateTimeEditBox;
            if (null == control || null != control.Value)
                return;

            DateTime? activeDate = GetActiveDate(control);
            if (null == activeDate)
                return;

            control.Dispatcher.BeginInvoke(DispatcherPriority.DataBind, (DispatcherOperationCallback)delegate {
                Popup popup = VisualTreeHelperExtended.GetFirstDescendant(control, typeof(Popup)) as Popup;
                if (null != popup) {
                    Visual visual = popup.Child as Visual;
                    if (null != visual) {
                        MonthCalendar monthCalendar = VisualTreeHelperExtended.GetFirstDescendant(visual, typeof(MonthCalendar)) as MonthCalendar;
                        if (null != monthCalendar) {
                            monthCalendar.AreTransitionAnimationsEnabled = false;
                            try {
                                monthCalendar.ActiveDate = activeDate.Value;
                            }
                            finally {
                                monthCalendar.AreTransitionAnimationsEnabled = true;
                            }
                        }
                    }
                }
                return null;
            }, null);
        }

        #endregion // NON-PUBLIC PROCEDURES

        /////////////////////////////////////////////////////////////////////////////////////////////////////
        #region PUBLIC PROCEDURES
        /////////////////////////////////////////////////////////////////////////////////////////////////////

        /// <summary>
        /// Gets the value of the <see cref="ActiveDateProperty"/> attached property for a specified <see cref="DateTimeEditBox"/>.
        /// </summary>
        /// <param name="obj">The object to which the attached property is retrieved.</param>
        /// <returns>
        /// The desired active date.
        /// </returns>
        public static DateTime? GetActiveDate(DateTimeEditBox obj) {
            if (null == obj) throw new ArgumentNullException("obj");
            return (DateTime?)obj.GetValue(CustomActiveDateBehavior.ActiveDateProperty);
        }
        /// <summary>
        /// Sets the value of the <see cref="ActiveDateProperty"/> attached property to a specified <see cref="DateTimeEditBox"/>.
        /// </summary>
        /// <param name="obj">The object to which the attached property is written.</param>
        /// <param name="value">
        /// A value indicating whether the desired active date.
        /// </param>
        public static void SetActiveDate(DateTimeEditBox obj, DateTime? value) {
            if (null == obj) throw new ArgumentNullException("obj");
            obj.SetValue(CustomActiveDateBehavior.ActiveDateProperty, value);
        }

        #endregion // PUBLIC PROCEDURES
    }
}