dotnet-core/release-notes/2.1/api-diff/2.0-vs-2.1_System.Collections.Immutable.md

System.Collections.Immutable

 namespace System.Collections.Immutable {
     public struct ImmutableArray<T> : ICollection, ICollection<T>, IEnumerable, IEnumerable<T>, IEquatable<ImmutableArray<T>>, IImmutableList<T>, IList, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T>, IStructuralComparable, IStructuralEquatable {
+        public ref readonly T ItemRef(int index);
         public sealed class Builder : ICollection<T>, IEnumerable, IEnumerable<T>, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T> {
+            public ref readonly T ItemRef(int index);
         }
     }
     public sealed class ImmutableList<T> : ICollection, ICollection<T>, IEnumerable, IEnumerable<T>, IImmutableList<T>, IList, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T> {
+        public ref readonly T ItemRef(int index);
         public sealed class Builder : ICollection, ICollection<T>, IEnumerable, IEnumerable<T>, IList, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T> {
+            public ref readonly T ItemRef(int index);
         }
     }
     public sealed class ImmutableQueue<T> : IEnumerable, IEnumerable<T>, IImmutableQueue<T> {
+        public ref readonly T PeekRef();
     }
     public sealed class ImmutableSortedDictionary<TKey, TValue> : ICollection, ICollection<KeyValuePair<TKey, TValue>>, IDictionary, IDictionary<TKey, TValue>, IEnumerable, IEnumerable<KeyValuePair<TKey, TValue>>, IImmutableDictionary<TKey, TValue>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IReadOnlyDictionary<TKey, TValue> {
+        public ref readonly TValue ValueRef(TKey key);
         public sealed class Builder : ICollection, ICollection<KeyValuePair<TKey, TValue>>, IDictionary, IDictionary<TKey, TValue>, IEnumerable, IEnumerable<KeyValuePair<TKey, TValue>>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IReadOnlyDictionary<TKey, TValue> {
+            public ref readonly TValue ValueRef(TKey key);
         }
     }
     public sealed class ImmutableSortedSet<T> : ICollection, ICollection<T>, IEnumerable, IEnumerable<T>, IImmutableSet<T>, IList, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T>, ISet<T> {
+        public ref readonly T ItemRef(int index);
         public sealed class Builder : ICollection, ICollection<T>, IEnumerable, IEnumerable<T>, IReadOnlyCollection<T>, ISet<T> {
+            public ref readonly T ItemRef(int index);
         }
     }
     public sealed class ImmutableStack<T> : IEnumerable, IEnumerable<T>, IImmutableStack<T> {
+        public ref readonly T PeekRef();
     }
 }