Fix MultipleChoiceFilter: concurrent filter may lock out each other

Author: tom-englert

src/DataGridExtensions/DataGridFilterColumnControl.cs

@@ -176,7 +176,7 @@ public object? Filter
         /// You may need to include "NotifyOnTargetUpdated=true" in the binding of the DataGrid.ItemsSource to get up-to-date
         /// values when the source object changes.
         /// </remarks>
-        public IEnumerable<string> Values => InternalValues().Distinct().ToList().AsReadOnly();
+        public IReadOnlyCollection<string> Values => InternalValues().Distinct().ToList().AsReadOnly();
 
         /// <summary>
         /// Returns all distinct source values of this column as string.
@@ -186,7 +186,7 @@ public object? Filter
         /// You may need to include "NotifyOnTargetUpdated=true" in the binding of the DataGrid.ItemsSource to get up-to-date 
         /// values when the source object changes.
         /// </remarks>
-        public IEnumerable<string> SourceValues
+        public IReadOnlyCollection<string> SourceValues
         {
             get
             {
@@ -205,7 +205,7 @@ public IEnumerable<string> SourceValues
         /// You may need to include "NotifyOnTargetUpdated=true" in the binding of the DataGrid.ItemsSource to get up-to-date 
         /// values when the source object changes.
         /// </remarks>
-        public IEnumerable<string> SelectableValues
+        public IReadOnlyCollection<string> SelectableValues
         {
             get
             {

src/DataGridExtensions/MultipleChoiceFilter.cs

@@ -65,12 +65,12 @@ public bool HasTextFilter
         public static readonly DependencyProperty HasTextFilterProperty = DependencyProperty.Register(
             "HasTextFilter", typeof(bool), typeof(MultipleChoiceFilter), new PropertyMetadata(default(bool)));
 
-        private IEnumerable<string?>? SourceValues => (IEnumerable<string?>?)GetValue(SourceValuesProperty);
+        private IReadOnlyCollection<string?>? SourceValues => (IReadOnlyCollection<string?>?)GetValue(SourceValuesProperty);
 
         private static readonly DependencyProperty SourceValuesProperty =
-            DependencyProperty.Register("SourceValues", typeof(IList<string>), typeof(MultipleChoiceFilter), new FrameworkPropertyMetadata(null, (sender, e) => ((MultipleChoiceFilter)sender).SourceValues_Changed((IList<string>)e.NewValue)));
+            DependencyProperty.Register("SourceValues", typeof(IReadOnlyCollection<string>), typeof(MultipleChoiceFilter), new FrameworkPropertyMetadata(null, (sender, e) => ((MultipleChoiceFilter)sender).SourceValues_Changed((IReadOnlyCollection<string>)e.NewValue)));
 
-        private void SourceValues_Changed(IEnumerable<string?>? newValue)
+        private void SourceValues_Changed(IReadOnlyCollection<string?>? newValue)
         {
             OnSourceValuesChanged(newValue);
         }
@@ -197,12 +197,12 @@ protected virtual MultipleChoiceContentFilter CreateFilter(IEnumerable<string?>?
         /// Called when the source values have changed.
         /// </summary>
         /// <param name="newValue">The new value.</param>
-        protected virtual void OnSourceValuesChanged(IEnumerable<string?>? newValue)
+        protected virtual void OnSourceValuesChanged(IReadOnlyCollection<string?>? newValue)
         {
             var values = Values;
             var filterRegex = Filter?.Regex;
 
-            if (newValue == null)
+            if (newValue == null || !newValue.Any())
             {
                 values.Clear();
             }