From dc6ad37d410931edb6b46203fb73d6503ed29dc8 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Thu, 23 Feb 2023 18:00:59 -0500
Subject: [PATCH] Return singleton enumerators from IEnumerable.GetEnumerator
 for empty collections (#82499)

* Return singleton enumerators from IEnumerable.GetEnumerator for empty collections

Change the `IEnumerable<T>.GetEnumerator()` implementations on our core collection types to special-case Count==0 in order to return a single enumerator instead of allocating one a new each time.  This saves an allocation when enumerating these collections via the interface in exchange for an extra length check as part of GetEnumerator.

* Address PR feedback

- Create helper function for empty enumerator
- Add tests for singletons

* Fix a few tests
---
 .../Collections/Generic/EnumerableHelpers.cs  |   4 +
 .../Collections/IDictionary.Generic.Tests.cs  |   4 +-
 .../IDictionary.NonGeneric.Tests.cs           |   4 +-
 .../Collections/IEnumerable.Generic.Tests.cs  |  58 +++++++--
 .../IEnumerable.NonGeneric.Tests.cs           | 119 +++++++++++++++++-
 .../System/Collections/IList.Generic.Tests.cs |   4 +-
 .../ConcurrentDictionary.Generic.Tests.cs     |   1 +
 .../ConcurrentDictionary.NonGeneric.Tests.cs  |   2 +
 .../ImmutableArray.Generic.Tests.cs           |   1 +
 .../ImmutableArray.NonGeneric.Tests.cs        |   1 +
 .../System/Collections/Generic/LinkedList.cs  |  17 +--
 .../Collections/Generic/PriorityQueue.cs      |   6 +-
 .../Collections/Generic/SortedDictionary.cs   |  56 +++------
 .../System/Collections/Generic/SortedList.cs  |  46 +++----
 .../System/Collections/Generic/SortedSet.cs   |   2 +-
 .../src/System/Collections/Generic/Stack.cs   |  17 +--
 .../Dictionary.Generic.Tests.Keys.cs          |   3 +
 .../Dictionary.Generic.Tests.Values.cs        |   5 +
 .../Dictionary/Dictionary.Generic.Tests.cs    |   4 +
 .../Generic/Dictionary/Dictionary.Tests.cs    |   3 +
 ...t.Generic.Tests.AsNonGenericIEnumerable.cs |   1 +
 .../Generic/HashSet/HashSet.Generic.Tests.cs  |   2 +
 ...t.Generic.Tests.AsNonGenericICollection.cs |   3 +
 .../LinkedList/LinkedList.Generic.Tests.cs    |   3 +
 .../tests/Generic/List/List.Generic.Tests.cs  |   3 +
 .../Generic/Queue/Queue.Generic.Tests.cs      |   3 +-
 .../tests/Generic/Queue/Queue.Tests.cs        |   2 +
 .../SortedDictionary.Generic.Tests.Keys.cs    |   3 +
 .../SortedDictionary.Generic.Tests.Values.cs  |   3 +
 .../SortedDictionary.Generic.Tests.cs         |   4 +-
 .../SortedDictionary.Tests.cs                 |   2 +
 .../SortedList.Generic.Tests.Keys.cs          |   4 +
 .../SortedList.Generic.Tests.Values.cs        |   4 +
 .../SortedList/SortedList.Generic.Tests.cs    |   3 +
 .../Generic/SortedList/SortedList.Tests.cs    |   3 +-
 .../Generic/Stack/Stack.Generic.Tests.cs      |   3 +
 .../tests/Generic/Stack/Stack.Tests.cs        |   2 +
 .../src/System/ArraySegment.cs                |  10 +-
 .../System/Collections/Generic/Dictionary.cs  |  17 ++-
 .../src/System/Collections/Generic/HashSet.cs |   6 +-
 .../src/System/Collections/Generic/List.cs    |  13 +-
 .../src/System/Collections/Generic/Queue.cs   |  17 +--
 .../ObjectModel/ReadOnlyCollection.cs         |   5 +-
 .../CompilerServices/ConditionalWeakTable.cs  |   2 +-
 .../tests/System/ArraySegmentTests.cs         |   1 +
 45 files changed, 319 insertions(+), 157 deletions(-)

diff --git a/src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs b/src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs
index b8646e3e878a5d..f73a2d3193f32f 100644
--- a/src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs
+++ b/src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs
@@ -8,6 +8,10 @@ namespace System.Collections.Generic
     /// </summary>
     internal static partial class EnumerableHelpers
     {
+        /// <summary>Gets an enumerator singleton for an empty collection.</summary>
+        internal static IEnumerator<T> GetEmptyEnumerator<T>() =>
+            ((IEnumerable<T>)Array.Empty<T>()).GetEnumerator();
+
         /// <summary>Converts an enumerable to an array using the same logic as List{T}.</summary>
         /// <param name="source">The enumerable to convert.</param>
         /// <param name="length">The number of items stored in the resulting array, 0-indexed.</param>
diff --git a/src/libraries/Common/tests/System/Collections/IDictionary.Generic.Tests.cs b/src/libraries/Common/tests/System/Collections/IDictionary.Generic.Tests.cs
index 44e28ea3ecd8c0..22f3912b65e7dd 100644
--- a/src/libraries/Common/tests/System/Collections/IDictionary.Generic.Tests.cs
+++ b/src/libraries/Common/tests/System/Collections/IDictionary.Generic.Tests.cs
@@ -421,7 +421,7 @@ public void IDictionary_Generic_Keys_Enumeration_ParentDictionaryModifiedInvalid
                 ICollection<TKey> keys = dictionary.Keys;
                 IEnumerator<TKey> keysEnum = keys.GetEnumerator();
                 dictionary.Add(GetNewKey(dictionary), CreateTValue(3432));
-                if (IDictionary_Generic_Keys_Values_Enumeration_ThrowsInvalidOperation_WhenParentModified)
+                if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : IDictionary_Generic_Keys_Values_Enumeration_ThrowsInvalidOperation_WhenParentModified)
                 {
                     Assert.Throws<InvalidOperationException>(() => keysEnum.MoveNext());
                     Assert.Throws<InvalidOperationException>(() => keysEnum.Reset());
@@ -528,7 +528,7 @@ public void IDictionary_Generic_Values_Enumeration_ParentDictionaryModifiedInval
                 ICollection<TValue> values = dictionary.Values;
                 IEnumerator<TValue> valuesEnum = values.GetEnumerator();
                 dictionary.Add(GetNewKey(dictionary), CreateTValue(3432));
-                if (IDictionary_Generic_Keys_Values_Enumeration_ThrowsInvalidOperation_WhenParentModified)
+                if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : IDictionary_Generic_Keys_Values_Enumeration_ThrowsInvalidOperation_WhenParentModified)
                 {
                     Assert.Throws<InvalidOperationException>(() => valuesEnum.MoveNext());
                     Assert.Throws<InvalidOperationException>(() => valuesEnum.Reset());
diff --git a/src/libraries/Common/tests/System/Collections/IDictionary.NonGeneric.Tests.cs b/src/libraries/Common/tests/System/Collections/IDictionary.NonGeneric.Tests.cs
index 944d067300fb65..adca2780d96d5a 100644
--- a/src/libraries/Common/tests/System/Collections/IDictionary.NonGeneric.Tests.cs
+++ b/src/libraries/Common/tests/System/Collections/IDictionary.NonGeneric.Tests.cs
@@ -392,7 +392,7 @@ public void IDictionary_NonGeneric_Keys_Enumeration_ParentDictionaryModifiedInva
                 ICollection keys = dictionary.Keys;
                 IEnumerator keysEnum = keys.GetEnumerator();
                 dictionary.Add(GetNewKey(dictionary), CreateTValue(3432));
-                if (IDictionary_NonGeneric_Keys_Values_ParentDictionaryModifiedInvalidates)
+                if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : IDictionary_NonGeneric_Keys_Values_ParentDictionaryModifiedInvalidates)
                 {
                     Assert.Throws<InvalidOperationException>(() => keysEnum.MoveNext());
                     Assert.Throws<InvalidOperationException>(() => keysEnum.Reset());
@@ -487,7 +487,7 @@ public virtual void IDictionary_NonGeneric_Values_Enumeration_ParentDictionaryMo
                 ICollection values = dictionary.Values;
                 IEnumerator valuesEnum = values.GetEnumerator();
                 dictionary.Add(GetNewKey(dictionary), CreateTValue(3432));
-                if (IDictionary_NonGeneric_Keys_Values_ParentDictionaryModifiedInvalidates)
+                if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : IDictionary_NonGeneric_Keys_Values_ParentDictionaryModifiedInvalidates)
                 {
                     Assert.Throws<InvalidOperationException>(() => valuesEnum.MoveNext());
                     Assert.Throws<InvalidOperationException>(() => valuesEnum.Reset());
diff --git a/src/libraries/Common/tests/System/Collections/IEnumerable.Generic.Tests.cs b/src/libraries/Common/tests/System/Collections/IEnumerable.Generic.Tests.cs
index c23f9ca2542478..a71f72e6cc6fa9 100644
--- a/src/libraries/Common/tests/System/Collections/IEnumerable.Generic.Tests.cs
+++ b/src/libraries/Common/tests/System/Collections/IEnumerable.Generic.Tests.cs
@@ -85,6 +85,20 @@ public abstract partial class IEnumerable_Generic_Tests<T> : TestBase<T>
         /// </summary>
         protected virtual bool Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException => true;
 
+        /// <summary>
+        /// When calling MoveNext or Reset after modification of an empty enumeration, the resulting behavior is
+        /// undefined. Tests are included to cover two behavioral scenarios:
+        ///   - Throwing an InvalidOperationException
+        ///   - Execute MoveNext or Reset.
+        ///
+        /// If this property is set to true, the tests ensure that the exception is thrown. The default value is
+        /// <see cref="Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException"/>.
+        /// </summary>
+        protected virtual bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException;
+
+        /// <summary>Whether the enumerator returned from GetEnumerator is a singleton instance when the collection is empty.</summary>
+        protected virtual bool Enumerator_Empty_UsesSingletonInstance => false;
+
         /// <summary>
         /// Specifies whether this IEnumerable follows some sort of ordering pattern.
         /// </summary>
@@ -302,6 +316,30 @@ private void VerifyEnumerator(
 
         #region GetEnumerator()
 
+        [Fact]
+        public void IEnumerable_NonGeneric_GetEnumerator_EmptyCollection_UsesSingleton()
+        {
+            IEnumerable enumerable = GenericIEnumerableFactory(0);
+
+            IEnumerator enumerator1 = enumerable.GetEnumerator();
+            try
+            {
+                IEnumerator enumerator2 = enumerable.GetEnumerator();
+                try
+                {
+                    Assert.Equal(Enumerator_Empty_UsesSingletonInstance, ReferenceEquals(enumerator1, enumerator2));
+                }
+                finally
+                {
+                    if (enumerator2 is IDisposable d2) d2.Dispose();
+                }
+            }
+            finally
+            {
+                if (enumerator1 is IDisposable d1) d1.Dispose();
+            }
+        }
+
         [Theory]
         [MemberData(nameof(ValidCollectionSizes))]
         public void IEnumerable_Generic_GetEnumerator_NoExceptionsWhileGetting(int count)
@@ -381,7 +419,7 @@ public void IEnumerable_Generic_Enumerator_MoveNext_ModifiedBeforeEnumeration_Th
                 {
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
                         }
@@ -427,7 +465,7 @@ public void IEnumerable_Generic_Enumerator_MoveNext_ModifiedDuringEnumeration_Th
                         enumerator.MoveNext();
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
                         }
@@ -471,7 +509,7 @@ public void IEnumerable_Generic_Enumerator_MoveNext_ModifiedAfterEnumeration_Thr
                     while (enumerator.MoveNext()) ;
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
                         }
@@ -602,8 +640,7 @@ public void IEnumerable_Generic_Enumerator_Current_BeforeFirstMoveNext_Undefined
             IEnumerable<T> enumerable = GenericIEnumerableFactory(count);
             using (IEnumerator<T> enumerator = enumerable.GetEnumerator())
             {
-                if (Enumerator_Current_UndefinedOperation_Throws ||
-                    (count == 0 && Enumerator_Empty_Current_UndefinedOperation_Throws))
+                if (count == 0 ? Enumerator_Empty_Current_UndefinedOperation_Throws : Enumerator_Current_UndefinedOperation_Throws)
                     Assert.Throws<InvalidOperationException>(() => enumerator.Current);
                 else
                     current = enumerator.Current;
@@ -619,8 +656,7 @@ public void IEnumerable_Generic_Enumerator_Current_AfterEndOfEnumerable_Undefine
             using (IEnumerator<T> enumerator = enumerable.GetEnumerator())
             {
                 while (enumerator.MoveNext()) ;
-                if (Enumerator_Current_UndefinedOperation_Throws ||
-                    (count == 0 && Enumerator_Empty_Current_UndefinedOperation_Throws))
+                if (count == 0 ? Enumerator_Empty_Current_UndefinedOperation_Throws : Enumerator_Current_UndefinedOperation_Throws)
                     Assert.Throws<InvalidOperationException>(() => enumerator.Current);
                 else
                     current = enumerator.Current;
@@ -639,7 +675,7 @@ public void IEnumerable_Generic_Enumerator_Current_ModifiedDuringEnumeration_Und
                 {
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_Current_UndefinedOperation_Throws)
+                        if (count == 0 ? Enumerator_Empty_Current_UndefinedOperation_Throws : Enumerator_Current_UndefinedOperation_Throws)
                             Assert.Throws<InvalidOperationException>(() => enumerator.Current);
                         else
                             current = enumerator.Current;
@@ -694,7 +730,7 @@ public void IEnumerable_Generic_Enumerator_Reset_ModifiedBeforeEnumeration_Throw
                 {
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
                         }
@@ -737,7 +773,7 @@ public void IEnumerable_Generic_Enumerator_Reset_ModifiedDuringEnumeration_Throw
                         enumerator.MoveNext();
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
                         }
@@ -781,7 +817,7 @@ public void IEnumerable_Generic_Enumerator_Reset_ModifiedAfterEnumeration_Throws
                     while (enumerator.MoveNext()) ;
                     if (ModifyEnumerable(enumerable))
                     {
-                        if (Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                        if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
                         }
diff --git a/src/libraries/Common/tests/System/Collections/IEnumerable.NonGeneric.Tests.cs b/src/libraries/Common/tests/System/Collections/IEnumerable.NonGeneric.Tests.cs
index 5760cbef9069c1..a441cf4d787330 100644
--- a/src/libraries/Common/tests/System/Collections/IEnumerable.NonGeneric.Tests.cs
+++ b/src/libraries/Common/tests/System/Collections/IEnumerable.NonGeneric.Tests.cs
@@ -60,6 +60,31 @@ public abstract partial class IEnumerable_NonGeneric_Tests : TestBase
         /// </summary>
         protected virtual bool Enumerator_Current_UndefinedOperation_Throws => false;
 
+        /// <summary>
+        /// When calling MoveNext or Reset after modification of the enumeration, the resulting behavior is
+        /// undefined. Tests are included to cover two behavioral scenarios:
+        ///   - Throwing an InvalidOperationException
+        ///   - Execute MoveNext or Reset.
+        ///
+        /// If this property is set to true, the tests ensure that the exception is thrown. The default value is
+        /// true.
+        /// </summary>
+        protected virtual bool Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException => true;
+
+        /// <summary>
+        /// When calling MoveNext or Reset after modification of an empty enumeration, the resulting behavior is
+        /// undefined. Tests are included to cover two behavioral scenarios:
+        ///   - Throwing an InvalidOperationException
+        ///   - Execute MoveNext or Reset.
+        ///
+        /// If this property is set to true, the tests ensure that the exception is thrown. The default value is
+        /// <see cref="Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException"/>.
+        /// </summary>
+        protected virtual bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException;
+
+        /// <summary>Whether the enumerator returned from GetEnumerator is a singleton instance when the collection is empty.</summary>
+        protected virtual bool Enumerator_Empty_UsesSingletonInstance => false;
+
         /// <summary>
         /// Whether the collection can be serialized.
         /// </summary>
@@ -83,6 +108,30 @@ protected enum EnumerableOrder
 
         #region GetEnumerator()
 
+        [Fact]
+        public void IEnumerable_NonGeneric_GetEnumerator_EmptyCollection_UsesSingleton()
+        {
+            IEnumerable enumerable = NonGenericIEnumerableFactory(0);
+
+            IEnumerator enumerator1 = enumerable.GetEnumerator();
+            try
+            {
+                IEnumerator enumerator2 = enumerable.GetEnumerator();
+                try
+                {
+                    Assert.Equal(Enumerator_Empty_UsesSingletonInstance, ReferenceEquals(enumerator1, enumerator2));
+                }
+                finally
+                {
+                    if (enumerator2 is IDisposable d2) d2.Dispose();
+                }
+            }
+            finally
+            {
+                if (enumerator1 is IDisposable d1) d1.Dispose();
+            }
+        }
+
         [Theory]
         [MemberData(nameof(ValidCollectionSizes))]
         public void IEnumerable_NonGeneric_GetEnumerator_NoExceptionsWhileGetting(int count)
@@ -141,7 +190,16 @@ public void IEnumerable_NonGeneric_Enumerator_MoveNext_ModifiedBeforeEnumeration
                 IEnumerable enumerable = NonGenericIEnumerableFactory(count);
                 IEnumerator enumerator = enumerable.GetEnumerator();
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                    }
+                    else
+                    {
+                        _ = enumerator.MoveNext();
+                    }
+                }
             });
         }
 
@@ -153,10 +211,21 @@ public void IEnumerable_NonGeneric_Enumerator_MoveNext_ModifiedDuringEnumeration
             {
                 IEnumerable enumerable = NonGenericIEnumerableFactory(count);
                 IEnumerator enumerator = enumerable.GetEnumerator();
+
                 for (int i = 0; i < count / 2; i++)
                     enumerator.MoveNext();
+
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                    }
+                    else
+                    {
+                        enumerator.MoveNext();
+                    }
+                }
             });
         }
 
@@ -170,7 +239,16 @@ public void IEnumerable_NonGeneric_Enumerator_MoveNext_ModifiedAfterEnumeration_
                 IEnumerator enumerator = enumerable.GetEnumerator();
                 while (enumerator.MoveNext()) ;
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.MoveNext());
+                    }
+                    else
+                    {
+                        _ = enumerator.MoveNext();
+                    }
+                }
             });
         }
 
@@ -290,7 +368,16 @@ public void IEnumerable_NonGeneric_Enumerator_Reset_ModifiedBeforeEnumeration_Th
                 IEnumerable enumerable = NonGenericIEnumerableFactory(count);
                 IEnumerator enumerator = enumerable.GetEnumerator();
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                    }
+                    else
+                    {
+                        enumerator.Reset();
+                    }
+                }
             });
         }
 
@@ -302,10 +389,21 @@ public void IEnumerable_NonGeneric_Enumerator_Reset_ModifiedDuringEnumeration_Th
             {
                 IEnumerable enumerable = NonGenericIEnumerableFactory(count);
                 IEnumerator enumerator = enumerable.GetEnumerator();
+
                 for (int i = 0; i < count / 2; i++)
                     enumerator.MoveNext();
+
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                    }
+                    else
+                    {
+                        enumerator.Reset();
+                    }
+                }
             });
         }
 
@@ -319,7 +417,16 @@ public void IEnumerable_NonGeneric_Enumerator_Reset_ModifiedAfterEnumeration_Thr
                 IEnumerator enumerator = enumerable.GetEnumerator();
                 while (enumerator.MoveNext()) ;
                 if (ModifyEnumerable(enumerable))
-                    Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                {
+                    if (count == 0 ? Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException : Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException)
+                    {
+                        Assert.Throws<InvalidOperationException>(() => enumerator.Reset());
+                    }
+                    else
+                    {
+                        enumerator.Reset();
+                    }
+                }
             });
         }
 
diff --git a/src/libraries/Common/tests/System/Collections/IList.Generic.Tests.cs b/src/libraries/Common/tests/System/Collections/IList.Generic.Tests.cs
index 820ea6407ab755..44f67e0ca24f83 100644
--- a/src/libraries/Common/tests/System/Collections/IList.Generic.Tests.cs
+++ b/src/libraries/Common/tests/System/Collections/IList.Generic.Tests.cs
@@ -614,7 +614,7 @@ public void IList_Generic_CurrentAtEnd_AfterAdd(int count)
                     while (enumerator.MoveNext()) ; // Go to end of enumerator
 
                     T current = default(T);
-                    if (Enumerator_Current_UndefinedOperation_Throws)
+                    if (count == 0 ? Enumerator_Empty_Current_UndefinedOperation_Throws : Enumerator_Current_UndefinedOperation_Throws)
                     {
                         Assert.Throws<InvalidOperationException>(() => enumerator.Current); // enumerator.Current should fail
                     }
@@ -630,7 +630,7 @@ public void IList_Generic_CurrentAtEnd_AfterAdd(int count)
                     {
                         collection.Add(CreateT(seed++));
 
-                        if (Enumerator_Current_UndefinedOperation_Throws)
+                        if (count == 0 ? Enumerator_Empty_Current_UndefinedOperation_Throws : Enumerator_Current_UndefinedOperation_Throws)
                         {
                             Assert.Throws<InvalidOperationException>(() => enumerator.Current); // enumerator.Current should fail
                         }
diff --git a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.Generic.Tests.cs b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.Generic.Tests.cs
index 1ec173dbc640db..f488444133a303 100644
--- a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.Generic.Tests.cs
+++ b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.Generic.Tests.cs
@@ -189,6 +189,7 @@ public abstract class ConcurrentDictionary_Generic_Tests<TKey, TValue> : IDictio
 
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
 
+        protected override bool Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override bool IDictionary_Generic_Keys_Values_Enumeration_ThrowsInvalidOperation_WhenParentModified => false;
 
         protected override bool IDictionary_Generic_Keys_Values_ModifyingTheDictionaryUpdatesTheCollection => false;
diff --git a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.NonGeneric.Tests.cs b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.NonGeneric.Tests.cs
index 9df2101c84a4e9..b7e63f51f402f9 100644
--- a/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.NonGeneric.Tests.cs
+++ b/src/libraries/System.Collections.Concurrent/tests/ConcurrentDictionary/ConcurrentDictionary.NonGeneric.Tests.cs
@@ -38,6 +38,8 @@ protected override object CreateTKey(int seed)
 
         protected override bool Enumerator_Current_UndefinedOperation_Throws => false;
 
+        protected override bool Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+
         protected override bool IDictionary_NonGeneric_Keys_Values_ModifyingTheDictionaryUpdatesTheCollection => false;
 
         protected override bool ICollection_NonGeneric_SupportsSyncRoot => false;
diff --git a/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.Generic.Tests.cs b/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.Generic.Tests.cs
index f1373db05c6423..a21bcf815d4f67 100644
--- a/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.Generic.Tests.cs
+++ b/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.Generic.Tests.cs
@@ -20,6 +20,7 @@ public abstract class ImmutableArray_Generic_Tests<T> : IList_Generic_Tests<T>
         protected override bool IsReadOnly => true;
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
         protected override Type IList_Generic_Item_InvalidIndex_ThrowType => typeof(IndexOutOfRangeException);
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
 
         protected override IList<T> GenericIListFactory() => GenericIListFactory(0);
diff --git a/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.NonGeneric.Tests.cs b/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.NonGeneric.Tests.cs
index 0d9410cedca93e..fabefa12d4cd47 100644
--- a/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.NonGeneric.Tests.cs
+++ b/src/libraries/System.Collections.Immutable/tests/ImmutableArray/ImmutableArray.NonGeneric.Tests.cs
@@ -15,6 +15,7 @@ public class ImmutableArray_NonGeneric_Tests : IList_NonGeneric_Tests
 
         protected override Type IList_NonGeneric_Item_InvalidIndex_ThrowType => typeof(IndexOutOfRangeException);
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
 
         protected override Type ICollection_NonGeneric_CopyTo_TwoDimensionArray_ThrowType => typeof(RankException);
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/LinkedList.cs b/src/libraries/System.Collections/src/System/Collections/Generic/LinkedList.cs
index 0e99312eba2895..a50dce3b158946 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/LinkedList.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/LinkedList.cs
@@ -283,15 +283,11 @@ public void CopyTo(T[] array, int index)
             return null;
         }
 
-        public Enumerator GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        public Enumerator GetEnumerator() => new Enumerator(this);
 
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            return GetEnumerator();
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
+            Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<T>() :
+            GetEnumerator();
 
         public bool Remove(T value)
         {
@@ -497,10 +493,7 @@ void ICollection.CopyTo(Array array, int index)
             }
         }
 
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return GetEnumerator();
-        }
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         public struct Enumerator : IEnumerator<T>, IEnumerator, ISerializable, IDeserializationCallback
         {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs b/src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs
index 67f2ae3c176d13..e8d8221641cd2a 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/PriorityQueue.cs
@@ -961,9 +961,11 @@ void IEnumerator.Reset()
             /// <returns>An <see cref="Enumerator"/> for the <see cref="UnorderedItems"/>.</returns>
             public Enumerator GetEnumerator() => new Enumerator(_queue);
 
-            IEnumerator<(TElement Element, TPriority Priority)> IEnumerable<(TElement Element, TPriority Priority)>.GetEnumerator() => GetEnumerator();
+            IEnumerator<(TElement Element, TPriority Priority)> IEnumerable<(TElement Element, TPriority Priority)>.GetEnumerator() =>
+                _queue.Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<(TElement Element, TPriority Priority)>() :
+                GetEnumerator();
 
-            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+            IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<(TElement Element, TPriority Priority)>)this).GetEnumerator();
         }
     }
 }
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedDictionary.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedDictionary.cs
index 08335221c4011a..64d745a778fcef 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedDictionary.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedDictionary.cs
@@ -247,15 +247,11 @@ public void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
             _set.CopyTo(array, index);
         }
 
-        public Enumerator GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        public Enumerator GetEnumerator() => new Enumerator(this, Enumerator.KeyValuePair);
 
-        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator() =>
+            Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<KeyValuePair<TKey, TValue>>() :
+            GetEnumerator();
 
         public bool Remove(TKey key)
         {
@@ -389,10 +385,7 @@ private static bool IsCompatibleKey(object key)
             return (key is TKey);
         }
 
-        IDictionaryEnumerator IDictionary.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.DictEntry);
-        }
+        IDictionaryEnumerator IDictionary.GetEnumerator() => new Enumerator(this, Enumerator.DictEntry);
 
         void IDictionary.Remove(object key)
         {
@@ -412,10 +405,7 @@ object ICollection.SyncRoot
             get { return ((ICollection)_set).SyncRoot; }
         }
 
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<KeyValuePair<TKey, TValue>>)this).GetEnumerator();
 
         public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>, IDictionaryEnumerator
         {
@@ -541,20 +531,13 @@ public KeyCollection(SortedDictionary<TKey, TValue> dictionary)
                 _dictionary = dictionary;
             }
 
-            public Enumerator GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            public Enumerator GetEnumerator() => new Enumerator(_dictionary);
 
-            IEnumerator<TKey> IEnumerable<TKey>.GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            IEnumerator<TKey> IEnumerable<TKey>.GetEnumerator() =>
+                Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<TKey>() :
+                GetEnumerator();
 
-            IEnumerator IEnumerable.GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<TKey>)this).GetEnumerator();
 
             public void CopyTo(TKey[] array, int index)
             {
@@ -709,20 +692,13 @@ public ValueCollection(SortedDictionary<TKey, TValue> dictionary)
                 _dictionary = dictionary;
             }
 
-            public Enumerator GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            public Enumerator GetEnumerator() => new Enumerator(_dictionary);
 
-            IEnumerator<TValue> IEnumerable<TValue>.GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            IEnumerator<TValue> IEnumerable<TValue>.GetEnumerator() =>
+                Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<TValue>() :
+                GetEnumerator();
 
-            IEnumerator IEnumerable.GetEnumerator()
-            {
-                return new Enumerator(_dictionary);
-            }
+            IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<TValue>)this).GetEnumerator();
 
             public void CopyTo(TValue[] array, int index)
             {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedList.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedList.cs
index 3204094bf71ecd..167e1be161f027 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedList.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedList.cs
@@ -542,25 +542,15 @@ public void SetValueAtIndex(int index, TValue value)
             version++;
         }
 
-        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() => new Enumerator(this, Enumerator.KeyValuePair);
 
-        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator() =>
+            Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<KeyValuePair<TKey, TValue>>() :
+            GetEnumerator();
 
-        IDictionaryEnumerator IDictionary.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.DictEntry);
-        }
+        IDictionaryEnumerator IDictionary.GetEnumerator() => new Enumerator(this, Enumerator.DictEntry);
 
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return new Enumerator(this, Enumerator.KeyValuePair);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<KeyValuePair<TKey, TValue>>)this).GetEnumerator();
 
         /// <summary>
         /// Gets the key corresponding to the specified index.
@@ -1091,15 +1081,11 @@ public TKey this[int index]
                 }
             }
 
-            public IEnumerator<TKey> GetEnumerator()
-            {
-                return new SortedListKeyEnumerator(_dict);
-            }
+            public IEnumerator<TKey> GetEnumerator() =>
+                Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<TKey>() :
+                new SortedListKeyEnumerator(_dict);
 
-            IEnumerator IEnumerable.GetEnumerator()
-            {
-                return new SortedListKeyEnumerator(_dict);
-            }
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
             public int IndexOf(TKey key)
             {
@@ -1209,15 +1195,11 @@ public TValue this[int index]
                 }
             }
 
-            public IEnumerator<TValue> GetEnumerator()
-            {
-                return new SortedListValueEnumerator(_dict);
-            }
+            public IEnumerator<TValue> GetEnumerator() =>
+                Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<TValue>() :
+                new SortedListValueEnumerator(_dict);
 
-            IEnumerator IEnumerable.GetEnumerator()
-            {
-                return new SortedListValueEnumerator(_dict);
-            }
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
             public int IndexOf(TValue value)
             {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
index 57f3ed24e596ef..3282996e5bc5d3 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
@@ -591,7 +591,7 @@ void ICollection.CopyTo(Array array, int index)
 
         IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
 
-        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         #endregion
 
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/Stack.cs b/src/libraries/System.Collections/src/System/Collections/Generic/Stack.cs
index ae5cc7bf0f93a7..2bc209c6a01e0b 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/Stack.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/Stack.cs
@@ -152,21 +152,14 @@ void ICollection.CopyTo(Array array, int arrayIndex)
         }
 
         // Returns an IEnumerator for this Stack.
-        public Enumerator GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        public Enumerator GetEnumerator() => new Enumerator(this);
 
         /// <internalonly/>
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
+            Count == 0 ? EnumerableHelpers.GetEmptyEnumerator<T>() :
+            GetEnumerator();
 
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         public void TrimExcess()
         {
diff --git a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Keys.cs b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Keys.cs
index f4386897680552..a71dad9a34d3c2 100644
--- a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Keys.cs
+++ b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Keys.cs
@@ -9,6 +9,8 @@ namespace System.Collections.Tests
 {
     public class Dictionary_Generic_Tests_Keys : ICollection_Generic_Tests<string>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
         protected override bool DefaultValueAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
@@ -61,6 +63,7 @@ public class Dictionary_Generic_Tests_Keys_AsICollection : ICollection_NonGeneri
         protected override bool NullAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override Type ICollection_NonGeneric_CopyTo_ArrayOfEnumType_ThrowType => typeof(ArgumentException);
         protected override bool SupportsSerialization => false;
diff --git a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Values.cs b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Values.cs
index 936705f8215b3f..efb7ef3b29ccb0 100644
--- a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Values.cs
+++ b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.Values.cs
@@ -13,6 +13,9 @@ public class Dictionary_Generic_Tests_Values : ICollection_Generic_Tests<string>
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
 
         protected override ICollection<string> GenericICollectionFactory()
         {
@@ -62,10 +65,12 @@ public class Dictionary_Generic_Tests_Values_AsICollection : ICollection_NonGene
         protected override bool NullAllowed => true;
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override Type ICollection_NonGeneric_CopyTo_ArrayOfEnumType_ThrowType => typeof(ArgumentException);
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
         protected override bool SupportsSerialization => false;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override Type ICollection_NonGeneric_CopyTo_IndexLargerThanArrayCount_ThrowType => typeof(ArgumentOutOfRangeException);
 
diff --git a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.cs
index b42b1733f0742f..fa12ef55d6c540 100644
--- a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Generic.Tests.cs
@@ -14,6 +14,10 @@ namespace System.Collections.Tests
     /// </summary>
     public abstract class Dictionary_Generic_Tests<TKey, TValue> : IDictionary_Generic_Tests<TKey, TValue>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+
         protected override ModifyOperation ModifyEnumeratorThrows => ModifyOperation.Add | ModifyOperation.Insert;
 
         protected override ModifyOperation ModifyEnumeratorAllowed => ModifyOperation.Overwrite | ModifyOperation.Remove | ModifyOperation.Clear;
diff --git a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Tests.cs b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Tests.cs
index 31271dc03c217d..1a0845a3529865 100644
--- a/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Tests.cs
@@ -12,6 +12,9 @@ namespace System.Collections.Tests
 {
     public partial class Dictionary_IDictionary_NonGeneric_Tests : IDictionary_NonGeneric_Tests
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+
         protected override IDictionary NonGenericIDictionaryFactory()
         {
             return new Dictionary<string, string>();
diff --git a/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.AsNonGenericIEnumerable.cs b/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.AsNonGenericIEnumerable.cs
index ff8901ae2afed5..70671721d338af 100644
--- a/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.AsNonGenericIEnumerable.cs
+++ b/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.AsNonGenericIEnumerable.cs
@@ -16,6 +16,7 @@ protected override IEnumerable NonGenericIEnumerableFactory(int count)
             return set;
         }
 
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
 
         protected override ModifyOperation ModifyEnumeratorThrows => PlatformDetection.IsNetFramework ? base.ModifyEnumeratorThrows : (base.ModifyEnumeratorAllowed & ~ModifyOperation.Remove);
diff --git a/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.cs
index 65261bfd26e422..c6aee132375a75 100644
--- a/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/HashSet/HashSet.Generic.Tests.cs
@@ -15,6 +15,8 @@ namespace System.Collections.Tests
     public abstract class HashSet_Generic_Tests<T> : ISet_Generic_Tests<T>
     {
         #region ISet<T> Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
 
         protected override bool ResetImplemented => true;
 
diff --git a/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.AsNonGenericICollection.cs b/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.AsNonGenericICollection.cs
index 1b4cf93cabc287..a3854d76ebda12 100644
--- a/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.AsNonGenericICollection.cs
+++ b/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.AsNonGenericICollection.cs
@@ -8,6 +8,9 @@ namespace System.Collections.Tests
 {
     public class LinkedList_ICollection_NonGeneric_Tests : ICollection_NonGeneric_Tests
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+
         protected override Type ICollection_NonGeneric_CopyTo_ArrayOfEnumType_ThrowType => typeof(ArgumentException);
 
         protected override void AddToCollection(ICollection collection, int numberOfItemsToAdd)
diff --git a/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.cs
index 9d656a84bd98c0..7f0998cac8789e 100644
--- a/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/LinkedList/LinkedList.Generic.Tests.cs
@@ -12,6 +12,9 @@ namespace System.Collections.Tests
     public abstract partial class LinkedList_Generic_Tests<T> : ICollection_Generic_Tests<T>
     {
         #region ICollection<T> Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override ICollection<T> GenericICollectionFactory()
         {
diff --git a/src/libraries/System.Collections/tests/Generic/List/List.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/List/List.Generic.Tests.cs
index 079acc7ade7d42..e10c12c48013c1 100644
--- a/src/libraries/System.Collections/tests/Generic/List/List.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/List/List.Generic.Tests.cs
@@ -12,6 +12,9 @@ namespace System.Collections.Tests
     public abstract partial class List_Generic_Tests<T> : IList_Generic_Tests<T>
     {
         #region IList<T> Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override IList<T> GenericIListFactory()
         {
diff --git a/src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs
index d38d98655b737c..3fd069023ca253 100644
--- a/src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Queue/Queue.Generic.Tests.cs
@@ -50,8 +50,9 @@ protected override IEnumerable<T> GenericIEnumerableFactory(int count)
         protected override bool Contains(IEnumerable<T> enumerable, T value) => ((Queue<T>)enumerable).Contains(value);
         protected override void CopyTo(IEnumerable<T> enumerable, T[] array, int index) => ((Queue<T>)enumerable).CopyTo(array, index);
         protected override bool Remove(IEnumerable<T> enumerable) => ((Queue<T>)enumerable).TryDequeue(out _);
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
-
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override Type IGenericSharedAPI_CopyTo_IndexLargerThanArrayCount_ThrowType => typeof(ArgumentOutOfRangeException);
 
         #endregion
diff --git a/src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs b/src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs
index 8f8afc317fcd1d..62b855d3f5dc74 100644
--- a/src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Queue/Queue.Tests.cs
@@ -10,6 +10,8 @@ namespace System.Collections.Tests
     public class Queue_ICollection_NonGeneric_Tests : ICollection_NonGeneric_Tests
     {
         #region ICollection Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override Type ICollection_NonGeneric_CopyTo_ArrayOfEnumType_ThrowType => typeof(ArgumentException);
 
diff --git a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Keys.cs b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Keys.cs
index 55d7e446de8706..db626df07701af 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Keys.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Keys.cs
@@ -9,6 +9,8 @@ namespace System.Collections.Tests
 {
     public class SortedDictionary_Generic_Tests_Keys : ICollection_Generic_Tests<string>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
         protected override bool DefaultValueAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
@@ -59,6 +61,7 @@ public class SortedDictionary_Generic_Tests_Keys_AsICollection : ICollection_Non
         protected override bool NullAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
         protected override ICollection NonGenericICollectionFactory()
diff --git a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Values.cs b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Values.cs
index 3030fdaaa08205..fd4669f7707dd4 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Values.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.Values.cs
@@ -9,6 +9,8 @@ namespace System.Collections.Tests
 {
     public class SortedDictionary_Generic_Tests_Values : ICollection_Generic_Tests<string>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
         protected override bool DefaultValueAllowed => true;
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
@@ -61,6 +63,7 @@ public class SortedDictionary_Generic_Tests_Values_AsICollection : ICollection_N
         protected override bool NullAllowed => true;
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override IEnumerable<ModifyEnumerable> GetModifyEnumerables(ModifyOperation operations) => new List<ModifyEnumerable>();
         protected override bool SupportsSerialization => false;
diff --git a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.cs
index cac00bc6723b4d..830181188c5fa3 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Generic.Tests.cs
@@ -13,7 +13,9 @@ namespace System.Collections.Tests
     public abstract class SortedDictionary_Generic_Tests<TKey, TValue> : IDictionary_Generic_Tests<TKey, TValue>
     {
         #region IDictionary<TKey, TValue> Helper Methods
-
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override bool DefaultValueWhenNotAllowed_Throws { get { return false; } }
 
         protected override IDictionary<TKey, TValue> GenericIDictionaryFactory()
diff --git a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Tests.cs b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Tests.cs
index f83ed756cc7f19..7b393933029491 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedDictionary/SortedDictionary.Tests.cs
@@ -9,6 +9,8 @@ namespace System.Collections.Tests
     public class SortedDictionary_IDictionary_NonGeneric_Tests : IDictionary_NonGeneric_Tests
     {
         #region IDictionary Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override IDictionary NonGenericIDictionaryFactory()
         {
diff --git a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Keys.cs b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Keys.cs
index 58e69384033d0d..4c5971cd359159 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Keys.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Keys.cs
@@ -9,6 +9,9 @@ namespace System.Collections.Tests
 {
     public class SortedList_Generic_Tests_Keys : IList_Generic_Tests<string>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override bool DefaultValueAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
@@ -57,6 +60,7 @@ public class SortedList_Generic_Tests_Keys_AsICollection : ICollection_NonGeneri
         protected override bool NullAllowed => false;
         protected override bool DuplicateValuesAllowed => false;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override bool SupportsSerialization => false;
 
diff --git a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Values.cs b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Values.cs
index 14eeaa0166b208..32ec7e651f3cbb 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Values.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.Values.cs
@@ -9,6 +9,9 @@ namespace System.Collections.Tests
 {
     public class SortedList_Generic_Tests_Values : IList_Generic_Tests<string>
     {
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override bool DefaultValueAllowed => true;
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
@@ -57,6 +60,7 @@ public class SortedList_Generic_Tests_Values_AsICollection : ICollection_NonGene
         protected override bool NullAllowed => true;
         protected override bool DuplicateValuesAllowed => true;
         protected override bool IsReadOnly => true;
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override bool SupportsSerialization => false;
 
diff --git a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.cs
index 7fc503b3b96c97..2a7966dab14e9e 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Generic.Tests.cs
@@ -13,6 +13,9 @@ namespace System.Collections.Tests
     public abstract class SortedList_Generic_Tests<TKey, TValue> : IDictionary_Generic_Tests<TKey, TValue>
     {
         #region IDictionary<TKey, TValue> Helper Methods
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override IDictionary<TKey, TValue> GenericIDictionaryFactory()
         {
diff --git a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Tests.cs b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Tests.cs
index 85bec3a725bacc..d0c7435ff5bf6a 100644
--- a/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/SortedList/SortedList.Tests.cs
@@ -9,7 +9,8 @@ namespace System.Collections.Tests
     public class SortedList_IDictionary_NonGeneric_Tests : IDictionary_NonGeneric_Tests
     {
         #region IDictionary Helper Methods
-
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override IDictionary NonGenericIDictionaryFactory()
         {
             return new SortedList<string, string>();
diff --git a/src/libraries/System.Collections/tests/Generic/Stack/Stack.Generic.Tests.cs b/src/libraries/System.Collections/tests/Generic/Stack/Stack.Generic.Tests.cs
index 1edc037d7cd210..4c5c5a49d243e1 100644
--- a/src/libraries/System.Collections/tests/Generic/Stack/Stack.Generic.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Stack/Stack.Generic.Tests.cs
@@ -32,6 +32,9 @@ protected Stack<T> GenericStackFactory(int count)
 
         protected override Type IGenericSharedAPI_CopyTo_IndexLargerThanArrayCount_ThrowType => typeof(ArgumentOutOfRangeException);
 
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
+
         #endregion
 
         protected override IEnumerable<T> GenericIEnumerableFactory()
diff --git a/src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs b/src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs
index 76a6e74e4f69bd..05a4cb340eae6d 100644
--- a/src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs
+++ b/src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs
@@ -23,7 +23,9 @@ protected override ICollection NonGenericICollectionFactory()
             return new Stack<string>();
         }
 
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
+        protected override bool Enumerator_Empty_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
 
         protected override Type ICollection_NonGeneric_CopyTo_IndexLargerThanArrayCount_ThrowType => typeof(ArgumentOutOfRangeException);
 
diff --git a/src/libraries/System.Private.CoreLib/src/System/ArraySegment.cs b/src/libraries/System.Private.CoreLib/src/System/ArraySegment.cs
index a5431a9209295c..45fe8fdee166aa 100644
--- a/src/libraries/System.Private.CoreLib/src/System/ArraySegment.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/ArraySegment.cs
@@ -261,12 +261,18 @@ bool ICollection<T>.Remove(T item)
 
         #region IEnumerable<T>
 
-        IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
+        IEnumerator<T> IEnumerable<T>.GetEnumerator()
+        {
+            ThrowInvalidOperationIfDefault();
+            return
+                Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
+                new Enumerator(this);
+        }
         #endregion
 
         #region IEnumerable
 
-        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
         #endregion
 
         private void ThrowInvalidOperationIfDefault()
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs
index 3b840d04efafd7..f959827b1da605 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs
@@ -339,7 +339,8 @@ private void CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
         public Enumerator GetEnumerator() => new Enumerator(this, Enumerator.KeyValuePair);
 
         IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator() =>
-            new Enumerator(this, Enumerator.KeyValuePair);
+            Count == 0 ? SZGenericArrayEnumerator<KeyValuePair<TKey, TValue>>.Empty :
+            GetEnumerator();
 
         public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
         {
@@ -1100,7 +1101,7 @@ void ICollection.CopyTo(Array array, int index)
             }
         }
 
-        IEnumerator IEnumerable.GetEnumerator() => new Enumerator(this, Enumerator.KeyValuePair);
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<KeyValuePair<TKey, TValue>>)this).GetEnumerator();
 
         /// <summary>
         /// Ensures that the dictionary can hold up to 'capacity' entries without any further expansion of its backing storage
@@ -1513,9 +1514,11 @@ bool ICollection<TKey>.Remove(TKey item)
                 return false;
             }
 
-            IEnumerator<TKey> IEnumerable<TKey>.GetEnumerator() => new Enumerator(_dictionary);
+            IEnumerator<TKey> IEnumerable<TKey>.GetEnumerator() =>
+                Count == 0 ? SZGenericArrayEnumerator<TKey>.Empty :
+                GetEnumerator();
 
-            IEnumerator IEnumerable.GetEnumerator() => new Enumerator(_dictionary);
+            IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<TKey>)this).GetEnumerator();
 
             void ICollection.CopyTo(Array array, int index)
             {
@@ -1705,9 +1708,11 @@ void ICollection<TValue>.Clear() =>
 
             bool ICollection<TValue>.Contains(TValue item) => _dictionary.ContainsValue(item);
 
-            IEnumerator<TValue> IEnumerable<TValue>.GetEnumerator() => new Enumerator(_dictionary);
+            IEnumerator<TValue> IEnumerable<TValue>.GetEnumerator() =>
+                Count == 0 ? SZGenericArrayEnumerator<TValue>.Empty :
+                GetEnumerator();
 
-            IEnumerator IEnumerable.GetEnumerator() => new Enumerator(_dictionary);
+            IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<TValue>)this).GetEnumerator();
 
             void ICollection.CopyTo(Array array, int index)
             {
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs
index fe7912af6a710e..7537e2d4b80f90 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs
@@ -358,9 +358,11 @@ public bool Remove(T item)
 
         public Enumerator GetEnumerator() => new Enumerator(this);
 
-        IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
+            Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
+            GetEnumerator();
 
-        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         #endregion
 
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs
index 1cfe6a699b2421..b3c165595b7654 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs
@@ -625,14 +625,13 @@ public void ForEach(Action<T> action)
         // while an enumeration is in progress, the MoveNext and
         // GetObject methods of the enumerator will throw an exception.
         //
-        public Enumerator GetEnumerator()
-            => new Enumerator(this);
+        public Enumerator GetEnumerator() => new Enumerator(this);
 
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-            => new Enumerator(this);
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
+            Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
+            GetEnumerator();
 
-        IEnumerator IEnumerable.GetEnumerator()
-            => new Enumerator(this);
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         public List<T> GetRange(int index, int count)
         {
@@ -1145,6 +1144,8 @@ public bool TrueForAll(Predicate<T> match)
 
         public struct Enumerator : IEnumerator<T>, IEnumerator
         {
+            internal static IEnumerator<T>? s_emptyEnumerator;
+
             private readonly List<T> _list;
             private int _index;
             private readonly int _version;
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Queue.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Queue.cs
index 98a2f202556a35..ba27a5ed9d397c 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Queue.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Queue.cs
@@ -183,21 +183,14 @@ public void Enqueue(T item)
 
         // GetEnumerator returns an IEnumerator over this Queue.  This
         // Enumerator will support removing.
-        public Enumerator GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        public Enumerator GetEnumerator() => new Enumerator(this);
 
         /// <internalonly/>
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() =>
+            Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
+            GetEnumerator();
 
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return new Enumerator(this);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator();
 
         // Removes the object at the head of the queue and returns it. If the queue
         // is empty, this method throws an
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs
index 57be3812a5142c..874db5fad5f47e 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/ObjectModel/ReadOnlyCollection.cs
@@ -43,9 +43,8 @@ public void CopyTo(T[] array, int index)
         }
 
         public IEnumerator<T> GetEnumerator() =>
-            list.Count == 0 ?
-                SZGenericArrayEnumerator<T>.Empty :
-                list.GetEnumerator();
+            list.Count == 0 ? SZGenericArrayEnumerator<T>.Empty :
+            list.GetEnumerator();
 
         public int IndexOf(T value)
         {
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs
index 0c9806eadf4cf7..7a8f8da784c088 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs
@@ -258,7 +258,7 @@ IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.
             {
                 Container c = _container;
                 return c is null || c.FirstFreeEntry == 0 ?
-                    ((IEnumerable<KeyValuePair<TKey, TValue>>)Array.Empty<KeyValuePair<TKey, TValue>>()).GetEnumerator() :
+                    SZGenericArrayEnumerator<KeyValuePair<TKey, TValue>>.Empty :
                     new Enumerator(this);
             }
         }
diff --git a/src/libraries/System.Runtime/tests/System/ArraySegmentTests.cs b/src/libraries/System.Runtime/tests/System/ArraySegmentTests.cs
index bbe9a8646e2fc3..171b9bf5d24eb5 100644
--- a/src/libraries/System.Runtime/tests/System/ArraySegmentTests.cs
+++ b/src/libraries/System.Runtime/tests/System/ArraySegmentTests.cs
@@ -24,6 +24,7 @@ protected override IList<T> GenericIListFactory(int count)
             return Factory(count * 2, count / 2, count);
         }
 
+        protected override bool Enumerator_Empty_UsesSingletonInstance => true;
         protected override bool Enumerator_Current_UndefinedOperation_Throws => true;
         protected override bool Enumerator_ModifiedDuringEnumeration_ThrowsInvalidOperationException => false;
         protected override bool IsReadOnly_ValidityValue => true;