Fixed bug in ReadOnlyDictionary's IDictionary.this[object] implementation. This method didn't adhere to IDictionary's contract to return null on a missing key. (#36926)

Author: dotnetjunkie

src/libraries/System.ObjectModel/src/System/Collections/ObjectModel/ReadOnlyDictionary.cs

@@ -161,7 +161,14 @@ void IDictionary.Remove(object key)
                     return null;
                 }
 
-                return this[(TKey)key];
+                if (m_dictionary.TryGetValue((TKey)key, out TValue value))
+                {
+                    return value;
+                }
+                else
+                {
+                    return null;
+                }
             }
             set => throw new NotSupportedException(SR.NotSupported_ReadOnlyCollection);
         }

src/libraries/System.ObjectModel/tests/ReadOnlyDictionary/ReadOnlyDictionaryTests.cs

@@ -142,6 +142,20 @@ public static void GetValuesTests()
             helper.Values_get_Tests();
         }
 
+        /// <summary>
+        /// Tests that the explicit IDictionary.Item[] implementation returns null on a non-existing key.
+        /// </summary>
+        [Fact]
+        public static void IDictionaryItemTests()
+        {
+            KeyValuePair<int, string>[] expectedArr = new KeyValuePair<int, string>[] {
+                new KeyValuePair<int, string>(1, "one")
+            };
+            DummyDictionary<int, string> dummyExpectedDict = new DummyDictionary<int, string>(expectedArr);
+            IDictionary dictionary = new ReadOnlyDictionary<int, string>(dummyExpectedDict);
+            Assert.Null(dictionary[2]);
+        }
+
         /// <summary>
         /// Tests that items can be retrieved by key from the Dictionary.
         /// </summary>