refactor: AsReadOnly for ICollection

src/Common/Extensions/CollectionExtensions.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Common.Extensions
+{
+    public static class CollectionExtensions
+    {
+        // Taken from https://stackoverflow.com/a/34362585/157971
+        public static IReadOnlyCollection<T> AsReadOnly<T>(this ICollection<T> source)
+        {
+            if (source == null)
+            {
+                throw new ArgumentNullException(nameof(source));
+            }
+
+            return source as IReadOnlyCollection<T> ?? new ReadOnlyCollectionAdapter<T>(source);
+        }
+
+        private sealed class ReadOnlyCollectionAdapter<T> : IReadOnlyCollection<T>
+        {
+            private readonly ICollection<T> _source;
+            public ReadOnlyCollectionAdapter(ICollection<T> source) => _source = source;
+            public int Count => _source.Count;
+            public IEnumerator<T> GetEnumerator() => _source.GetEnumerator();
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        }
+    }
+}