C#: Implement IReadOnlyDictionary<K,V> in MapField<K,V>

csharp/src/Google.Protobuf.Test/Collections/MapFieldTest.cs

@@ -540,6 +540,22 @@ namespace Google.Protobuf.Collections
             Assert.Throws<ArgumentException>(() => map.ToString());
         }
 
+#if !NET35
+        [Test]
+        public void IDictionaryKeys_Equals_IReadOnlyDictionaryKeys()
+        {
+            var map = new MapField<string, string> { { "foo", "bar" }, { "x", "y" } };
+            CollectionAssert.AreEquivalent(((IDictionary<string, string>)map).Keys, ((IReadOnlyDictionary<string, string>)map).Keys);
+        }
+
+        [Test]
+        public void IDictionaryValues_Equals_IReadOnlyDictionaryValues()
+        {
+            var map = new MapField<string, string> { { "foo", "bar" }, { "x", "y" } };
+            CollectionAssert.AreEquivalent(((IDictionary<string, string>)map).Values, ((IReadOnlyDictionary<string, string>)map).Values);
+        }
+#endif
+
         private static KeyValuePair<TKey, TValue> NewKeyValuePair<TKey, TValue>(TKey key, TValue value)
         {
             return new KeyValuePair<TKey, TValue>(key, value);

csharp/src/Google.Protobuf/Collections/MapField.cs

@@ -67,6 +67,9 @@ namespace Google.Protobuf.Collections
     /// </para>
     /// </remarks>
     public sealed class MapField<TKey, TValue> : IDeepCloneable<MapField<TKey, TValue>>, IDictionary<TKey, TValue>, IEquatable<MapField<TKey, TValue>>, IDictionary
+#if !NET35
+        , IReadOnlyDictionary<TKey, TValue>
+#endif
     {
         // TODO: Don't create the map/list until we have an entry. (Assume many maps will be empty.)
         private readonly Dictionary<TKey, LinkedListNode<KeyValuePair<TKey, TValue>>> map =
@@ -548,6 +551,14 @@ namespace Google.Protobuf.Collections
         }
         #endregion
 
+        #region IReadOnlyDictionary explicit interface implementation
+#if !NET35
+        IEnumerable<TKey> IReadOnlyDictionary<TKey, TValue>.Keys => Keys;
+
+        IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values => Values;
+#endif
+        #endregion
+
         private class DictionaryEnumerator : IDictionaryEnumerator
         {
             private readonly IEnumerator<KeyValuePair<TKey, TValue>> enumerator;