Correct implementation for readonly DependcyProperties

Author: timunie

src/MahApps.Metro/Controls/MultiSelectionComboBox/MultiSelectionComboBox.cs

@@ -138,40 +138,47 @@ private static bool IsValidSelectionMode(object o)
                 || value == SelectionMode.Extended;
         }
 
-        /// <summary>Identifies the <see cref="SelectedItems"/> dependency property.</summary>
-        public static readonly DependencyProperty SelectedItemsProperty = 
-            DependencyProperty.Register(
+
+        internal static readonly DependencyPropertyKey SelectedItemsPropertyKey = 
+            DependencyProperty.RegisterReadOnly(
                 nameof(SelectedItems),
                 typeof(IList),
                 typeof(MultiSelectionComboBox),
                 new PropertyMetadata((IList)null));
 
+        /// <summary>Identifies the <see cref="SelectedItems"/> dependency property.</summary>
+        public static readonly DependencyProperty SelectedItemsProperty = SelectedItemsPropertyKey.DependencyProperty;
+            
+
         /// <summary>
         /// The currently selected items.
         /// </summary>
         [Bindable(true), Category("Appearance"), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
         public IList SelectedItems
         {
-            get
-            {
-                return PART_PopupListBox?.SelectedItems;
-            }
+            get { return (IList)GetValue(SelectedItemsProperty); }
+            protected set { SetValue(SelectedItemsPropertyKey, value); }
         }
 
-        /// <summary>Identifies the <see cref="DisplaySelectedItems"/> dependency property.</summary>
-        public static readonly DependencyProperty DisplaySelectedItemsProperty =
-            DependencyProperty.Register(
+
+
+        internal static readonly DependencyPropertyKey DisplaySelectedItemsPropertyKey =
+            DependencyProperty.RegisterReadOnly(
                 nameof(DisplaySelectedItems),
                 typeof(IEnumerable),
                 typeof(MultiSelectionComboBox),
                 new PropertyMetadata((IEnumerable)null));
 
+        /// <summary>Identifies the <see cref="DisplaySelectedItems"/> dependency property.</summary>
+        public static readonly DependencyProperty DisplaySelectedItemsProperty = DisplaySelectedItemsPropertyKey.DependencyProperty;
+
         /// <summary>
         /// Gets the <see cref="SelectedItems"/> in the specified order which was set via <see cref="OrderSelectedItemsBy"/>
         /// </summary>
         public IEnumerable DisplaySelectedItems
         {
             get { return (IEnumerable)GetValue(DisplaySelectedItemsProperty); }
+            protected set { SetValue(DisplaySelectedItemsPropertyKey, value); }
         }
 
         /// <summary>Identifies the <see cref="OrderSelectedItemsBy"/> dependency property.</summary>
@@ -246,20 +253,25 @@ public string Separator
             set { SetValue(SeparatorProperty, value); }
         }
 
-        /// <summary>Identifies the <see cref="HasCustomText"/> dependency property.</summary>
-        public static readonly DependencyProperty HasCustomTextProperty = 
-            DependencyProperty.Register(
+
+        internal static readonly DependencyPropertyKey HasCustomTextPropertyKey = 
+            DependencyProperty.RegisterReadOnly(
                 nameof(HasCustomText),
                 typeof(bool),
                 typeof(MultiSelectionComboBox),
                 new PropertyMetadata(false));
 
+
+        /// <summary>Identifies the <see cref="HasCustomText"/> dependency property.</summary>
+        public static readonly DependencyProperty HasCustomTextProperty = HasCustomTextPropertyKey.DependencyProperty;
+
         /// <summary>
         /// Indicates if the text is userdefined
         /// </summary>
         public bool HasCustomText
         {
             get { return (bool)GetValue(HasCustomTextProperty); }
+            protected set { SetValue(HasCustomTextPropertyKey, BooleanBoxes.Box(value)); }
         }
 
         /// <summary>Identifies the <see cref="TextWrapping"/> dependency property.</summary>
@@ -351,7 +363,7 @@ public IParseStringToObject StringToObjectParser
         /// </summary>
         public void ResetEditableText()
         {
-            SetCurrentValue(HasCustomTextProperty, BooleanBoxes.FalseBox);
+            HasCustomText = false;
             UpdateEditableText();
         }
 
@@ -580,18 +592,18 @@ private void UpdateHasCustomText(string selectedItemsText)
 
             bool hasCustomText = !((string.IsNullOrEmpty(selectedItemsText) && string.IsNullOrEmpty(Text)) || string.Equals(Text, selectedItemsText, EditableTextStringComparision));
 
-            SetCurrentValue(HasCustomTextProperty, BooleanBoxes.Box(hasCustomText));
+            HasCustomText = hasCustomText;
         }
 
         private void UpdateDisplaySelectedItems(OrderSelectedItemsBy orderBy)
         {
             if (orderBy == OrderSelectedItemsBy.SelectedOrder)
             {
-                SetCurrentValue(DisplaySelectedItemsProperty, SelectedItems);
+                DisplaySelectedItems = SelectedItems;
             }
             else if (orderBy == OrderSelectedItemsBy.ItemsSourceOrder)
             {
-                SetCurrentValue(DisplaySelectedItemsProperty, ((IEnumerable<object>)PART_PopupListBox.SelectedItems).OrderBy(o => Items.IndexOf(o)));
+                DisplaySelectedItems= ((IEnumerable<object>)SelectedItems).OrderBy(o => Items.IndexOf(o));
             }
         }
 
@@ -860,6 +872,7 @@ public override void OnApplyTemplate()
             {
                 PART_PopupListBox.SelectionChanged -= PART_PopupListBox_SelectionChanged;
                 PART_PopupListBox.SelectionChanged += PART_PopupListBox_SelectionChanged;
+                SelectedItems = PART_PopupListBox.SelectedItems;
             }
             else
             {