Validate source collection, and add tests.

src/Avalonia.Controls/ItemsSourceView.cs

@@ -42,19 +42,16 @@ namespace Avalonia.Controls
         public ItemsSourceView(IEnumerable source)
         {
             source = source ?? throw new ArgumentNullException(nameof(source));
-
-            if (source is IList list)
-            {
-                _inner = list;
-            }
-            else if (source is IEnumerable<object> objectEnumerable)
+            _inner = source switch
             {
-                _inner = new List<object>(objectEnumerable);
-            }
-            else
-            {
-                _inner = new List<object>(source.Cast<object>());
-            }
+                ItemsSourceView => throw new ArgumentException("Cannot wrap an existing ItemsSourceView.", nameof(source)),
+                IList list => list,
+                INotifyCollectionChanged => throw new ArgumentException(
+                    "Collection implements INotifyCollectionChanged by not IList.",
+                    nameof(source)),
+                IEnumerable<object> iObj => new List<object>(iObj),
+                _ => new List<object>(source.Cast<object>())
+            };
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/ItemsSourceViewTests.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.Text;
+using Avalonia.Collections;
+using Avalonia.Diagnostics;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class ItemsSourceViewTests
+    {
+        [Fact]
+        public void Only_Subscribes_To_Source_CollectionChanged_When_CollectionChanged_Subscribed()
+        {
+            var source = new AvaloniaList<string>();
+            var target = new ItemsSourceView<string>(source);
+            var debug = (INotifyCollectionChangedDebug)source;
+
+            Assert.Null(debug.GetCollectionChangedSubscribers());
+
+            void Handler(object sender, NotifyCollectionChangedEventArgs e) { }
+            target.CollectionChanged += Handler;
+
+            Assert.NotNull(debug.GetCollectionChangedSubscribers());
+            Assert.Equal(1, debug.GetCollectionChangedSubscribers().Length);
+
+            target.CollectionChanged -= Handler;
+
+            Assert.Null(debug.GetCollectionChangedSubscribers());
+        }
+
+        [Fact]
+        public void Cannot_Wrap_An_ItemsSourceView_In_Another()
+        {
+            var source = new ItemsSourceView<string>(new string[0]);
+            Assert.Throws<ArgumentException>(() => new ItemsSourceView<string>(source));
+        }
+
+        [Fact]
+        public void Cannot_Create_ItemsSourceView_With_Collection_That_Implements_INCC_But_Not_List()
+        {
+            var source = new InvalidCollection();
+            Assert.Throws<ArgumentException>(() => new ItemsSourceView<string>(source));
+        }
+
+        private class InvalidCollection : INotifyCollectionChanged, IEnumerable<string>
+        {
+            public event NotifyCollectionChangedEventHandler CollectionChanged;
+
+            public IEnumerator<string> GetEnumerator()
+            {
+                yield break;
+            }
+
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                yield break;
+            }
+        }
+    }
+}