From 9f921f470f0d6b9c1c732aced97b9922d34f2fdf Mon Sep 17 00:00:00 2001
From: Sergei Papikian <50848091+I-SER-I@users.noreply.github.com>
Date: Wed, 3 Feb 2021 19:56:10 +0300
Subject: [PATCH] Added ConcurrentDictionary.Comparer property (#47787)

* fix #31350

* fix ref file

* fix getter

* added tests
---
 .../ref/System.Collections.Concurrent.cs       |  1 +
 .../Concurrent/ConcurrentDictionary.cs         |  2 ++
 .../ConcurrentDictionaryTests.cs               | 18 ++++++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/src/libraries/System.Collections.Concurrent/ref/System.Collections.Concurrent.cs b/src/libraries/System.Collections.Concurrent/ref/System.Collections.Concurrent.cs
index a7955b09120527..7fa61335bf028a 100644
--- a/src/libraries/System.Collections.Concurrent/ref/System.Collections.Concurrent.cs
+++ b/src/libraries/System.Collections.Concurrent/ref/System.Collections.Concurrent.cs
@@ -82,6 +82,7 @@ public ConcurrentDictionary(System.Collections.Generic.IEqualityComparer<TKey>?
         public ConcurrentDictionary(int concurrencyLevel, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> collection, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
         public ConcurrentDictionary(int concurrencyLevel, int capacity) { }
         public ConcurrentDictionary(int concurrencyLevel, int capacity, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
+        public System.Collections.Generic.IEqualityComparer<TKey> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
         public bool IsEmpty { get { throw null; } }
         public TValue this[TKey key] { get { throw null; } set { } }
diff --git a/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs b/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
index fb49255cfe655c..fc19e9c97b1870 100644
--- a/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
+++ b/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
@@ -1051,6 +1051,8 @@ public TValue this[TKey key]
         private static void ThrowKeyNotFoundException(TKey key) =>
             throw new KeyNotFoundException(SR.Format(SR.Arg_KeyNotFoundWithKey, key.ToString()));
 
+        public IEqualityComparer<TKey> Comparer => _comparer ?? _defaultComparer;
+
         /// <summary>
         /// Gets the number of key/value pairs contained in the <see
         /// cref="ConcurrentDictionary{TKey,TValue}"/>.
diff --git a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionaryTests.cs b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionaryTests.cs
index e466a37f920d7a..abc0598004f3be 100644
--- a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionaryTests.cs
+++ b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionaryTests.cs
@@ -666,6 +666,24 @@ void AssertDefaultComparerBehavior(ConcurrentDictionary<EqualityApiSpy, int> dic
             }
         }
 
+        [Fact]
+        public static void TestComparerGetter()
+        {
+            AssertComparerBehavior<string>(null);
+            AssertComparerBehavior<string>(EqualityComparer<string>.Default);
+            AssertComparerBehavior<string>(StringComparer.InvariantCulture);
+            AssertComparerBehavior<string>(StringComparer.OrdinalIgnoreCase);
+            AssertComparerBehavior<int>(EqualityComparer<int>.Default);
+            AssertComparerBehavior<bool>(EqualityComparer<bool>.Default);
+
+            void AssertComparerBehavior<T>(IEqualityComparer<T> comparer)
+            {
+                var dc = new ConcurrentDictionary<T, object>(comparer);
+                object expected = comparer ?? EqualityComparer<T>.Default;
+                Assert.Same(expected, dc.Comparer);
+            }
+        }
+
         private sealed class EqualityApiSpy : IEquatable<EqualityApiSpy>
         {
             public bool ObjectApiUsed { get; private set; }