Fix PhotoTagger multiselect.

Bug introduced in 0d20a11, due to
failure to convert ObservableCollection to ReadOnlyObservableCollection.

Author: adam-azarchs

PhotoTagger.Wpf/ValueConverters.cs

@@ -1,6 +1,7 @@
 using PhotoTagger.Imaging;
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Globalization;
 using System.Linq;
 using System.Windows;
@@ -182,4 +183,24 @@ public object ConvertBack(object value, Type targetType,
             throw new NotSupportedException();
         }
     }
+
+    [ValueConversion(typeof(ObservableCollection<Photo>),
+                     typeof(ReadOnlyObservableCollection<Photo>))]
+    public class ObservableCollectionToReadOnlyConverter : IValueConverter {
+        public object Convert(object value, Type targetType,
+                              object parameter, CultureInfo culture) {
+            if (value is null) {
+                return null;
+            } else if (value is ObservableCollection<Photo> c) {
+                return new ReadOnlyObservableCollection<Photo>(c);
+            } else {
+                throw new NotSupportedException();
+            }
+        }
+
+        public object ConvertBack(object value, Type targetType,
+                                  object parameter, CultureInfo culture) {
+            throw new NotSupportedException();
+        }
+    }
 }

PhotoTagger/TaggerWindow.xaml

@@ -13,6 +13,7 @@
         <ptwpf:AnyToEnabledValueConverter x:Key="AnyToEnabledValueConverter"/>
         <ptwpf:ElementCountToVisibilityValueConverter x:Key="ElementCountToVisibilityValueConverter"/>
         <ptwpf:AnyChangedToEnabledValueConverter x:Key="AnyChangedToEnabledValueConverter"/>
+        <ptwpf:ObservableCollectionToReadOnlyConverter x:Key="ObservableCollectionToReadOnlyConverter"/>
         <Style TargetType="{x:Type Button}">
             <Setter Property="Padding" Value="1,1" />
             <Setter Property="Margin" Value="1,1" />
@@ -87,9 +88,17 @@
                     </Binding>
                 </local:SinglePhotoEdit.Visibility>
             </local:SinglePhotoEdit>
-            <local:MultiPhotoEdit PhotoSet="{Binding SelectedPhotos,
-                                             ElementName=window,
-                                             Mode=OneWay}">
+            <local:MultiPhotoEdit>
+                <local:MultiPhotoEdit.PhotoSet>
+                    <Binding>
+                        <Binding.ElementName>window</Binding.ElementName>
+                        <Binding.Path>SelectedPhotos</Binding.Path>
+                        <Binding.Mode>OneWay</Binding.Mode>
+                        <Binding.Converter>
+                            <StaticResource ResourceKey="ObservableCollectionToReadOnlyConverter"/>
+                        </Binding.Converter>
+                    </Binding>
+                </local:MultiPhotoEdit.PhotoSet>
                 <local:MultiPhotoEdit.Visibility>
                     <Binding Path="SelectedPhotos.Count"
                              ElementName="window"