diff --git a/src/mscorlib/model.xml b/src/mscorlib/model.xml
index b4cd2bdbbfd0..e186fc3cacde 100644
--- a/src/mscorlib/model.xml
+++ b/src/mscorlib/model.xml
@@ -341,6 +341,7 @@
         <Member Name="get_Array" />
         <Member Name="get_Count" />
         <Member Name="get_Offset" />
+        <Member Name="GetEnumerator" />
         <Member Name="GetHashCode" />
         <Member Name="op_Equality(System.ArraySegment&lt;T&gt;,System.ArraySegment&lt;T&gt;)" />
         <Member Name="op_Inequality(System.ArraySegment&lt;T&gt;,System.ArraySegment&lt;T&gt;)" />
@@ -348,6 +349,13 @@
         <Member MemberType="Property" Name="Count" />
         <Member MemberType="Property" Name="Offset" />
     </Type>
+    <Type Name="System.ArraySegment&lt;T&gt;+Enumerator">
+      <Member Name="Dispose" />
+      <Member Name="get_Current" />
+      <Member Name="MoveNext" />
+      <Member MemberType="Property" Name="Current" />
+      <Member Name="System.Collections.IEnumerator.Reset" />
+    </Type>
     <Type Name="System.ArrayTypeMismatchException">
       <Member Name="#ctor" />
       <Member Name="#ctor(System.String)" />
diff --git a/src/mscorlib/src/System/ArraySegment.cs b/src/mscorlib/src/System/ArraySegment.cs
index 1e76d174ced0..676452abae81 100644
--- a/src/mscorlib/src/System/ArraySegment.cs
+++ b/src/mscorlib/src/System/ArraySegment.cs
@@ -28,10 +28,10 @@ namespace System
     [Serializable]
     public struct ArraySegment<T> : IList<T>, IReadOnlyList<T>
     {
-        private T[] _array;
-        private int _offset;
-        private int _count;
-        
+        private readonly T[] _array;
+        private readonly int _offset;
+        private readonly int _count;
+
         public ArraySegment(T[] array)
         {
             if (array == null)
@@ -67,7 +67,7 @@ public T[] Array
                 Contract.Assert(    (null == _array && 0 == _offset && 0 == _count)
                                  || (null != _array && _offset >= 0 && _count >= 0 && _offset + _count <= _array.Length),
                                 "ArraySegment is invalid");
-                
+
                 return _array;
             }
         }
@@ -109,7 +109,16 @@ public int Count
                 return _count;
             }
         }
-        
+
+        public Enumerator GetEnumerator()
+        {
+            if (_array == null)
+                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
+            Contract.EndContractBlock();
+
+            return new Enumerator(this);
+        }
+
         public override int GetHashCode()
         {
             if (_array == null)
@@ -184,7 +193,7 @@ int IList<T>.IndexOf(T item)
 
             int index = System.Array.IndexOf<T>(_array, item, _offset, _count);
 
-            Contract.Assert(index == -1 || 
+            Contract.Assert(index == -1 ||
                             (index >= _offset && index < _offset + _count));
 
             return index >= 0 ? index - _offset : -1;
@@ -269,46 +278,34 @@ bool ICollection<T>.Remove(T item)
         #endregion
 
         #region IEnumerable<T>
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            if (_array == null)
-                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
 
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
         #endregion
 
         #region IEnumerable
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            if (_array == null)
-                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
 
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
         #endregion
 
         [Serializable]
-        private sealed class ArraySegmentEnumerator : IEnumerator<T>
+        public struct Enumerator : IEnumerator<T>
         {
-            private T[] _array;
-            private int _start;
-            private int _end;
+            private readonly T[] _array;
+            private readonly int _start;
+            private readonly int _end; // cache Offset + Count, since it's a little slow
             private int _current;
 
-            internal ArraySegmentEnumerator(ArraySegment<T> arraySegment)
+            internal Enumerator(ArraySegment<T> arraySegment)
             {
                 Contract.Requires(arraySegment.Array != null);
                 Contract.Requires(arraySegment.Offset >= 0);
                 Contract.Requires(arraySegment.Count >= 0);
                 Contract.Requires(arraySegment.Offset + arraySegment.Count <= arraySegment.Array.Length);
 
-                _array = arraySegment._array;
-                _start = arraySegment._offset;
-                _end = _start + arraySegment._count;
-                _current = _start - 1;
+                _array = arraySegment.Array;
+                _start = arraySegment.Offset;
+                _end = arraySegment.Offset + arraySegment.Count;
+                _current = arraySegment.Offset - 1;
             }
 
             public bool MoveNext()
@@ -325,19 +322,15 @@ public T Current
             {
                 get
                 {
-                    if (_current < _start) ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumNotStarted();
-                    if (_current >= _end) ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumEnded();
+                    if (_current < _start)
+                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumNotStarted();
+                    if (_current >= _end)
+                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumEnded();
                     return _array[_current];
                 }
             }
 
-            object IEnumerator.Current
-            {
-                get
-                {
-                    return Current;
-                }
-            }
+            object IEnumerator.Current => Current;
 
             void IEnumerator.Reset()
             {