Correct implementation for readonly DependcyProperties

Additionally make these minor improvements

- make Command fully static 
- restore selection after clear text
- better check if ItemsSource is in use (PopupListBox)

Author: punker76

src/MahApps.Metro.Samples/MahApps.Metro.Demo/MainWindowViewModel.cs

@@ -537,7 +537,7 @@ private void ToggleIconScaling(object obj)
 
         public bool IsToggleSwitchVisible { get; set; }
 
-        public ObservableCollection<string> Animals { get; } = new ObservableCollection<string>()
+        public ObservableCollection<string> Animals { get; } = new ObservableCollection<string>
         {
             "African elephant",
             "Ant",

src/MahApps.Metro.Samples/MahApps.Metro.Demo/Models/ObjectParser.cs

@@ -32,7 +32,7 @@ public object CreateObjectFromString(string input, CultureInfo culture, string s
                 return null;
             }
 
-            MetroDialogSettings dialogSettings = new MetroDialogSettings()
+            MetroDialogSettings dialogSettings = new MetroDialogSettings
             {
                 AffirmativeButtonText = "Yes",
                 NegativeButtonText = "No",

src/MahApps.Metro/Controls/Helper/BindingHelper.cs

@@ -64,9 +64,12 @@ public static object Eval(object source, string expression, string format)
         /// <returns></returns>
         public static object Eval(Binding binding, object source)
         {
-            if (binding is null) throw new ArgumentNullException(nameof(binding));
+            if (binding is null)
+            {
+                throw new ArgumentNullException(nameof(binding));
+            }
 
-            Binding newBinding = new Binding()
+            Binding newBinding = new Binding
             {
                 Source = source,
                 AsyncState = binding.AsyncState,
@@ -91,7 +94,7 @@ public static object Eval(Binding binding, object source)
         /// <param name="binding">The <see cref="Binding"/> to evaluate</param>
         /// <param name="dependencyObject">optional: The <see cref="DependencyObject"/> to evalutate</param>
         /// <returns>The resulting object</returns>
-        public static object Eval(Binding binding, DependencyObject dependencyObject = null)
+        public static object Eval(Binding binding, DependencyObject dependencyObject)
         {
             dependencyObject ??= new DependencyObject();
 
@@ -106,5 +109,15 @@ public static object Eval(Binding binding, DependencyObject dependencyObject = n
                 return dependencyObject.GetValue(DummyTextProperty);
             }
         }
+
+        /// <summary>
+        /// Evaluates a defined <see cref="Binding"/> on the given <see cref="DependencyObject"/>
+        /// </summary>
+        /// <param name="binding">The <see cref="Binding"/> to evaluate</param>
+        /// <returns>The resulting object</returns>
+        public static object Eval(Binding binding)
+        {
+            return Eval(binding, null);
+        }
     }
 }

src/MahApps.Metro/Controls/Helper/TextBoxHelper.cs

@@ -954,6 +954,8 @@ public static void ButtonClicked(object sender, RoutedEventArgs e)
                             case SelectionMode.Extended:
                                 multiSelectionComboBox.SelectedItems.Clear();
                                 break;
+                            default:
+                                throw new NotSupportedException("Unknown SelectionMode");
                         }
                     }
                 }

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

@@ -28,7 +28,7 @@ public interface ICompareObjectToString
     [MarkupExtensionReturnType(typeof(DefaultObjectToStringComparer))]
     public class DefaultObjectToStringComparer : MarkupExtension, ICompareObjectToString
     {
-        static DefaultObjectToStringComparer _Instance; 
+        private static DefaultObjectToStringComparer _Instance; 
 
         /// <inheritdoc/>
         public bool CheckIfStringMatchesObject(string input, object objectToCompare, StringComparison stringComparison, string stringFormat)
@@ -48,7 +48,7 @@ public bool CheckIfStringMatchesObject(string input, object objectToCompare, Str
             {
                 objectText = objectToCompare.ToString();
             }
-            else if (stringFormat.Contains('{') && stringFormat.Contains('{'))
+            else if (stringFormat.Contains('{') && stringFormat.Contains('}'))
             {
                 objectText = string.Format(stringFormat, objectToCompare);
             }

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

@@ -33,6 +33,8 @@ static MultiSelectionComboBox()
         {
             DefaultStyleKeyProperty.OverrideMetadata(typeof(MultiSelectionComboBox), new FrameworkPropertyMetadata(typeof(MultiSelectionComboBox)));
             TextProperty.OverrideMetadata(typeof(MultiSelectionComboBox), new FrameworkPropertyMetadata(String.Empty, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Journal, new PropertyChangedCallback(OnTextChanged)));
+            CommandManager.RegisterClassCommandBinding(typeof(MultiSelectionComboBox), new CommandBinding(ClearContentCommand, ExecutedClearContentCommand, CanExecuteClearContentCommand));
+            CommandManager.RegisterClassCommandBinding(typeof(MultiSelectionComboBox), new CommandBinding(RemoveItemCommand, RemoveItemCommand_Executed, RemoveItemCommand_CanExecute));
         }
 
         #endregion
@@ -138,40 +140,47 @@ 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 +255,25 @@ 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>
@@ -351,8 +365,14 @@ public IParseStringToObject StringToObjectParser
         /// </summary>
         public void ResetEditableText()
         {
-            SetCurrentValue(HasCustomTextProperty, BooleanBoxes.FalseBox);
+            var oldSelectionStart = PART_EditableTextBox.SelectionStart;
+            var oldSelectionLength = PART_EditableTextBox.SelectionLength;
+
+            HasCustomText = false;
             UpdateEditableText();
+
+            PART_EditableTextBox.SelectionStart = oldSelectionStart;
+            PART_EditableTextBox.SelectionLength = oldSelectionLength;
         }
 
         /// <summary>Identifies the <see cref="DisabledPopupOverlayContent"/> dependency property.</summary>
@@ -519,7 +539,9 @@ public int SelectItemsFromTextInputDelay
         private void UpdateEditableText()
         {
             if (PART_EditableTextBox is null || SelectedItems is null)
+            {
                 return;
+            }
 
             var selectedItemsText = GetSelectedItemsText();
 
@@ -578,19 +600,18 @@ 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)
         {
-            switch (orderBy)
+            if (orderBy == OrderSelectedItemsBy.SelectedOrder)
             {
-                case OrderSelectedItemsBy.SelectedOrder:
-                    SetCurrentValue(DisplaySelectedItemsProperty, SelectedItems);
-                    break;
-                case OrderSelectedItemsBy.ItemsSourceOrder:
-                    SetCurrentValue(DisplaySelectedItemsProperty, ((IEnumerable<object>)PART_PopupListBox.SelectedItems).OrderBy(o => Items.IndexOf(o)));
-                    break;
+                DisplaySelectedItems = SelectedItems;
+            }
+            else if (orderBy == OrderSelectedItemsBy.ItemsSourceOrder)
+            {
+                DisplaySelectedItems= ((IEnumerable<object>)SelectedItems).OrderBy(o => Items.IndexOf(o));
             }
         }
 
@@ -638,7 +659,7 @@ private void UpdateSelectedItemsFromTextTimer_Tick(object sender, EventArgs e)
                         SelectedItems.Clear();
                         break;
                     default:
-                        break;
+                        throw new NotSupportedException("Unknown SelectionMode");
                 }
                 return;
             }
@@ -671,7 +692,7 @@ private void UpdateSelectedItemsFromTextTimer_Tick(object sender, EventArgs e)
                 case SelectionMode.Multiple:
                 case SelectionMode.Extended:
 
-                    var strings = Text.Split(new string[] { Separator }, StringSplitOptions.RemoveEmptyEntries);
+                    var strings = Text.Split(new [] { Separator }, StringSplitOptions.RemoveEmptyEntries);
 
                     SelectedItems.Clear();
 
@@ -698,7 +719,7 @@ private void UpdateSelectedItemsFromTextTimer_Tick(object sender, EventArgs e)
                     }
                     break;
                 default:
-                    break;
+                    throw new NotSupportedException("Unknown SelectionMode");
             }
 
 
@@ -752,7 +773,7 @@ private object TryAddObjectFromString(string input)
         // Clear Text Command
         public static RoutedUICommand ClearContentCommand { get; } = new RoutedUICommand("ClearContent", nameof(ClearContentCommand), typeof(MultiSelectionComboBox));
 
-        private void ExecutedClearContentCommand(object sender, ExecutedRoutedEventArgs e)
+        private static void ExecutedClearContentCommand(object sender, ExecutedRoutedEventArgs e)
         {
             if (sender is MultiSelectionComboBox multiSelectionCombo)
             {
@@ -771,12 +792,14 @@ private void ExecutedClearContentCommand(object sender, ExecutedRoutedEventArgs
                         case SelectionMode.Extended:
                             multiSelectionCombo.SelectedItems.Clear();
                             break;
+                        default:
+                            throw new NotSupportedException("Unknown SelectionMode");
                     }
                 }
             }
         }
 
-        private void CanExecuteClearContentCommand(object sender, CanExecuteRoutedEventArgs e)
+        private static void CanExecuteClearContentCommand(object sender, CanExecuteRoutedEventArgs e)
         {
             e.CanExecute = false;
             if (sender is MultiSelectionComboBox multiSelectionComboBox)
@@ -787,7 +810,7 @@ private void CanExecuteClearContentCommand(object sender, CanExecuteRoutedEventA
 
         public static RoutedUICommand RemoveItemCommand { get; } = new RoutedUICommand("Remove item", nameof(RemoveItemCommand), typeof(MultiSelectionComboBox));
 
-        private void RemoveItemCommand_Executed(object sender, ExecutedRoutedEventArgs e)
+        private static void RemoveItemCommand_Executed(object sender, ExecutedRoutedEventArgs e)
         {
             if (sender is MultiSelectionComboBox multiSelectionCombo && multiSelectionCombo.SelectedItems.Contains(e.Parameter))
             {
@@ -800,7 +823,7 @@ private void RemoveItemCommand_Executed(object sender, ExecutedRoutedEventArgs e
             }
         }
 
-        private void RemoveItemCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
+        private static void RemoveItemCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
         {
             e.CanExecute = false;
             if (sender is MultiSelectionComboBox)
@@ -857,15 +880,13 @@ public override void OnApplyTemplate()
             {
                 PART_PopupListBox.SelectionChanged -= PART_PopupListBox_SelectionChanged;
                 PART_PopupListBox.SelectionChanged += PART_PopupListBox_SelectionChanged;
+                SelectedItems = PART_PopupListBox.SelectedItems;
             }
             else
             {
                 throw new MahAppsException($"The template part \"{nameof(PART_PopupListBox)}\" could not be found.");
             }
 
-            CommandBindings.Add(new CommandBinding(ClearContentCommand, ExecutedClearContentCommand, CanExecuteClearContentCommand));
-            CommandBindings.Add(new CommandBinding(RemoveItemCommand, RemoveItemCommand_Executed, RemoveItemCommand_CanExecute));
-
             // Do update the text 
             UpdateEditableText();
             UpdateDisplaySelectedItems();
@@ -889,7 +910,10 @@ protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
             }
 
             // If we have the ItemsSource set, we need to exit here. 
-            if (ReadLocalValue(ItemsSourceProperty) != DependencyProperty.UnsetValue) return;
+            if ((PART_PopupListBox?.Items as IList)?.IsReadOnly ?? true)
+            {
+                return;
+            }
 
             switch (e.Action)
             {
@@ -908,11 +932,7 @@ protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    // TODO Add Handler
-                    break;
                 case NotifyCollectionChangedAction.Move:
-                    // TODO Add Handler
-                    break;
                 case NotifyCollectionChangedAction.Reset:
                     PART_PopupListBox.Items.Clear();
                     foreach (var item in Items)
@@ -921,7 +941,7 @@ protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
                     }
                     break;
                 default:
-                    break;
+                    throw new NotSupportedException("Unsupported NotifyCollectionChangedAction");
             }
         }
 
@@ -951,10 +971,16 @@ protected override void OnDropDownOpened(EventArgs e)
 
             PART_PopupListBox.Focus();
 
-            if (PART_PopupListBox.Items.Count == 0) return;
+            if (PART_PopupListBox.Items.Count == 0)
+            {
+                return;
+            }
 
             var index = PART_PopupListBox.SelectedIndex;
-            if (index < 0) index = 0;
+            if (index < 0)
+            {
+                index = 0;
+            }
 
             Action action = () =>
             {
@@ -1052,7 +1078,10 @@ private void MultiSelectionComboBox_Loaded(object sender, EventArgs e)
             Loaded -= MultiSelectionComboBox_Loaded;
 
             // If we have the ItemsSource set, we need to exit here. 
-            if (ReadLocalValue(ItemsSourceProperty) != DependencyProperty.UnsetValue) return;
+            if ((PART_PopupListBox?.Items as IList)?.IsReadOnly ?? true)
+            {
+                return;
+            }
 
             PART_PopupListBox.Items.Clear();
             foreach (var item in Items)