From 760185cbdac974d51db4e7282533ff2b260a2c25 Mon Sep 17 00:00:00 2001
From: Santiago Fernandez Madero <safern@microsoft.com>
Date: Thu, 13 Jun 2019 00:33:18 -0500
Subject: [PATCH] Add nullable annotations to System.Collections ref assembly
 (dotnet/corefx#38128)

* Add nullable annotations to System.Collections ref assembly

* Run GenAPI again with correct attributes format and output

* Enable nullable to project

* Apply nullable feedback, include attributes and object constraint to notnull


Commit migrated from https://github.com/dotnet/corefx/commit/cc8137ede09cdf16811767a6fb262fb498d85e67
---
 .../System.Collections/ref/System.Collections.cs   | 175 +++++++++++----------
 .../ref/System.Collections.csproj                  |   1 +
 2 files changed, 93 insertions(+), 83 deletions(-)

diff --git a/src/libraries/System.Collections/ref/System.Collections.cs b/src/libraries/System.Collections/ref/System.Collections.cs
index ab33d35..690e2e3 100644
--- a/src/libraries/System.Collections/ref/System.Collections.cs
+++ b/src/libraries/System.Collections/ref/System.Collections.cs
@@ -44,29 +44,31 @@ namespace System.Collections.Generic
 {
     public static partial class CollectionExtensions
     {
-        public static TValue GetValueOrDefault<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key) { throw null; }
-        public static TValue GetValueOrDefault<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue) { throw null; }
-        public static bool Remove<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, out TValue value) { throw null; }
-        public static bool TryAdd<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, TValue value) { throw null; }
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
+        public static TValue GetValueOrDefault<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key) where TKey : notnull { throw null; }
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
+        public static TValue GetValueOrDefault<TKey, TValue>(this System.Collections.Generic.IReadOnlyDictionary<TKey, TValue> dictionary, TKey key, [System.Diagnostics.CodeAnalysis.AllowNullAttribute] TValue defaultValue) where TKey : notnull { throw null; }
+        public static bool Remove<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) where TKey : notnull { throw null; }
+        public static bool TryAdd<TKey, TValue>(this System.Collections.Generic.IDictionary<TKey, TValue> dictionary, TKey key, TValue value) where TKey : notnull { throw null; }
     }
     public abstract partial class Comparer<T> : System.Collections.Generic.IComparer<T>, System.Collections.IComparer
     {
         protected Comparer() { }
         public static System.Collections.Generic.Comparer<T> Default { get { throw null; } }
-        public abstract int Compare(T x, T y);
+        public abstract int Compare([System.Diagnostics.CodeAnalysis.AllowNullAttribute] T x, [System.Diagnostics.CodeAnalysis.AllowNullAttribute] T y);
         public static System.Collections.Generic.Comparer<T> Create(System.Comparison<T> comparison) { throw null; }
-        int System.Collections.IComparer.Compare(object x, object y) { throw null; }
+        int System.Collections.IComparer.Compare(object? x, object? y) { throw null; }
     }
-    public partial class Dictionary<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
+    public partial class Dictionary<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable where TKey : notnull
     {
         public Dictionary() { }
         public Dictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary) { }
-        public Dictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IEqualityComparer<TKey> comparer) { }
+        public Dictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
         public Dictionary(System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> collection) { }
-        public Dictionary(System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> collection, System.Collections.Generic.IEqualityComparer<TKey> comparer) { }
-        public Dictionary(System.Collections.Generic.IEqualityComparer<TKey> comparer) { }
+        public Dictionary(System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>> collection, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
+        public Dictionary(System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
         public Dictionary(int capacity) { }
-        public Dictionary(int capacity, System.Collections.Generic.IEqualityComparer<TKey> comparer) { }
+        public Dictionary(int capacity, System.Collections.Generic.IEqualityComparer<TKey>? comparer) { }
         protected Dictionary(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public System.Collections.Generic.IEqualityComparer<TKey> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
@@ -81,7 +83,8 @@ namespace System.Collections.Generic
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
         bool System.Collections.IDictionary.IsFixedSize { get { throw null; } }
         bool System.Collections.IDictionary.IsReadOnly { get { throw null; } }
-        object System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
+        [System.Diagnostics.CodeAnalysis.DisallowNullAttribute]
+        object? System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
         System.Collections.ICollection System.Collections.IDictionary.Keys { get { throw null; } }
         System.Collections.ICollection System.Collections.IDictionary.Values { get { throw null; } }
         public System.Collections.Generic.Dictionary<TKey, TValue>.ValueCollection Values { get { throw null; } }
@@ -92,16 +95,16 @@ namespace System.Collections.Generic
         public int EnsureCapacity(int capacity) { throw null; }
         public System.Collections.Generic.Dictionary<TKey, TValue>.Enumerator GetEnumerator() { throw null; }
         public virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public virtual void OnDeserialization(object sender) { }
+        public virtual void OnDeserialization(object? sender) { }
         public bool Remove(TKey key) { throw null; }
-        public bool Remove(TKey key, out TValue value) { throw null; }
+        public bool Remove(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Add(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { }
         bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Contains(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { throw null; }
         void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.CopyTo(System.Collections.Generic.KeyValuePair<TKey, TValue>[] array, int index) { }
         bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Remove(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { throw null; }
         System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>> System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>.GetEnumerator() { throw null; }
         void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-        void System.Collections.IDictionary.Add(object key, object value) { }
+        void System.Collections.IDictionary.Add(object key, object? value) { }
         bool System.Collections.IDictionary.Contains(object key) { throw null; }
         System.Collections.IDictionaryEnumerator System.Collections.IDictionary.GetEnumerator() { throw null; }
         void System.Collections.IDictionary.Remove(object key) { }
@@ -109,7 +112,7 @@ namespace System.Collections.Generic
         public void TrimExcess() { }
         public void TrimExcess(int capacity) { }
         public bool TryAdd(TKey key, TValue value) { throw null; }
-        public bool TryGetValue(TKey key, out TValue value) { throw null; }
+        public bool TryGetValue(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.IDictionaryEnumerator, System.Collections.IEnumerator, System.IDisposable
         {
             private object _dummy;
@@ -117,8 +120,8 @@ namespace System.Collections.Generic
             public System.Collections.Generic.KeyValuePair<TKey, TValue> Current { get { throw null; } }
             System.Collections.DictionaryEntry System.Collections.IDictionaryEnumerator.Entry { get { throw null; } }
             object System.Collections.IDictionaryEnumerator.Key { get { throw null; } }
-            object System.Collections.IDictionaryEnumerator.Value { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IDictionaryEnumerator.Value { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
@@ -145,7 +148,7 @@ namespace System.Collections.Generic
                 private object _dummy;
                 private int _dummyPrimitive;
                 public TKey Current { get { throw null; } }
-                object System.Collections.IEnumerator.Current { get { throw null; } }
+                object? System.Collections.IEnumerator.Current { get { throw null; } }
                 public void Dispose() { }
                 public bool MoveNext() { throw null; }
                 void System.Collections.IEnumerator.Reset() { }
@@ -173,7 +176,7 @@ namespace System.Collections.Generic
                 private object _dummy;
                 private int _dummyPrimitive;
                 public TValue Current { get { throw null; } }
-                object System.Collections.IEnumerator.Current { get { throw null; } }
+                object? System.Collections.IEnumerator.Current { get { throw null; } }
                 public void Dispose() { }
                 public bool MoveNext() { throw null; }
                 void System.Collections.IEnumerator.Reset() { }
@@ -184,19 +187,21 @@ namespace System.Collections.Generic
     {
         protected EqualityComparer() { }
         public static System.Collections.Generic.EqualityComparer<T> Default { get { throw null; } }
-        public abstract bool Equals(T x, T y);
-        public abstract int GetHashCode(T obj);
-        bool System.Collections.IEqualityComparer.Equals(object x, object y) { throw null; }
-        int System.Collections.IEqualityComparer.GetHashCode(object obj) { throw null; }
+        public abstract bool Equals([System.Diagnostics.CodeAnalysis.AllowNullAttribute] T x, [System.Diagnostics.CodeAnalysis.AllowNullAttribute] T y);
+        public abstract int GetHashCode([System.Diagnostics.CodeAnalysis.DisallowNullAttribute] T obj);
+        bool System.Collections.IEqualityComparer.Equals(object? x, object? y) { throw null; }
+#pragma warning disable CS8617 // TODO-NULLABLE: Covariant parameter types (https://github.com/dotnet/roslyn/issues/30958)
+        int System.Collections.IEqualityComparer.GetHashCode(object? obj) { throw null; }
+#pragma warning restore CS8617
     }
     public partial class HashSet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.ISet<T>, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
     {
         public HashSet() { }
         public HashSet(System.Collections.Generic.IEnumerable<T> collection) { }
-        public HashSet(System.Collections.Generic.IEnumerable<T> collection, System.Collections.Generic.IEqualityComparer<T> comparer) { }
-        public HashSet(System.Collections.Generic.IEqualityComparer<T> comparer) { }
+        public HashSet(System.Collections.Generic.IEnumerable<T> collection, System.Collections.Generic.IEqualityComparer<T>? comparer) { }
+        public HashSet(System.Collections.Generic.IEqualityComparer<T>? comparer) { }
         public HashSet(int capacity) { }
-        public HashSet(int capacity, System.Collections.Generic.IEqualityComparer<T> comparer) { }
+        public HashSet(int capacity, System.Collections.Generic.IEqualityComparer<T>? comparer) { }
         protected HashSet(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public System.Collections.Generic.IEqualityComparer<T> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
@@ -217,7 +222,7 @@ namespace System.Collections.Generic
         public bool IsProperSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool IsSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool IsSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        public virtual void OnDeserialization(object sender) { }
+        public virtual void OnDeserialization(object? sender) { }
         public bool Overlaps(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool Remove(T item) { throw null; }
         public int RemoveWhere(System.Predicate<T> match) { throw null; }
@@ -227,7 +232,7 @@ namespace System.Collections.Generic
         System.Collections.Generic.IEnumerator<T> System.Collections.Generic.IEnumerable<T>.GetEnumerator() { throw null; }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         public void TrimExcess() { }
-        public bool TryGetValue(T equalValue, out T actualValue) { throw null; }
+        public bool TryGetValue(T equalValue, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T actualValue) { throw null; }
         public void UnionWith(System.Collections.Generic.IEnumerable<T> other) { }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<T>, System.Collections.IEnumerator, System.IDisposable
         {
@@ -235,7 +240,7 @@ namespace System.Collections.Generic
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
@@ -244,9 +249,9 @@ namespace System.Collections.Generic
     public sealed partial class LinkedListNode<T>
     {
         public LinkedListNode(T value) { }
-        public System.Collections.Generic.LinkedList<T> List { get { throw null; } }
-        public System.Collections.Generic.LinkedListNode<T> Next { get { throw null; } }
-        public System.Collections.Generic.LinkedListNode<T> Previous { get { throw null; } }
+        public System.Collections.Generic.LinkedList<T>? List { get { throw null; } }
+        public System.Collections.Generic.LinkedListNode<T>? Next { get { throw null; } }
+        public System.Collections.Generic.LinkedListNode<T>? Previous { get { throw null; } }
         public T Value { get { throw null; } set { } }
     }
     public partial class LinkedList<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.ICollection, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
@@ -255,8 +260,8 @@ namespace System.Collections.Generic
         public LinkedList(System.Collections.Generic.IEnumerable<T> collection) { }
         protected LinkedList(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public int Count { get { throw null; } }
-        public System.Collections.Generic.LinkedListNode<T> First { get { throw null; } }
-        public System.Collections.Generic.LinkedListNode<T> Last { get { throw null; } }
+        public System.Collections.Generic.LinkedListNode<T>? First { get { throw null; } }
+        public System.Collections.Generic.LinkedListNode<T>? Last { get { throw null; } }
         bool System.Collections.Generic.ICollection<T>.IsReadOnly { get { throw null; } }
         bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
@@ -271,11 +276,11 @@ namespace System.Collections.Generic
         public void Clear() { }
         public bool Contains(T value) { throw null; }
         public void CopyTo(T[] array, int index) { }
-        public System.Collections.Generic.LinkedListNode<T> Find(T value) { throw null; }
-        public System.Collections.Generic.LinkedListNode<T> FindLast(T value) { throw null; }
+        public System.Collections.Generic.LinkedListNode<T>? Find(T value) { throw null; }
+        public System.Collections.Generic.LinkedListNode<T>? FindLast(T value) { throw null; }
         public System.Collections.Generic.LinkedList<T>.Enumerator GetEnumerator() { throw null; }
         public virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public virtual void OnDeserialization(object sender) { }
+        public virtual void OnDeserialization(object? sender) { }
         public void Remove(System.Collections.Generic.LinkedListNode<T> node) { }
         public bool Remove(T value) { throw null; }
         public void RemoveFirst() { }
@@ -290,11 +295,11 @@ namespace System.Collections.Generic
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
-            void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object sender) { }
+            void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object? sender) { }
             void System.Runtime.Serialization.ISerializable.GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         }
     }
@@ -311,13 +316,13 @@ namespace System.Collections.Generic
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
         bool System.Collections.IList.IsFixedSize { get { throw null; } }
         bool System.Collections.IList.IsReadOnly { get { throw null; } }
-        object System.Collections.IList.this[int index] { get { throw null; } set { } }
+        object? System.Collections.IList.this[int index] { get { throw null; } set { } }
         public void Add(T item) { }
         public void AddRange(System.Collections.Generic.IEnumerable<T> collection) { }
         public System.Collections.ObjectModel.ReadOnlyCollection<T> AsReadOnly() { throw null; }
-        public int BinarySearch(int index, int count, T item, System.Collections.Generic.IComparer<T> comparer) { throw null; }
+        public int BinarySearch(int index, int count, T item, System.Collections.Generic.IComparer<T>? comparer) { throw null; }
         public int BinarySearch(T item) { throw null; }
-        public int BinarySearch(T item, System.Collections.Generic.IComparer<T> comparer) { throw null; }
+        public int BinarySearch(T item, System.Collections.Generic.IComparer<T>? comparer) { throw null; }
         public void Clear() { }
         public bool Contains(T item) { throw null; }
         public System.Collections.Generic.List<TOutput> ConvertAll<TOutput>(System.Converter<T, TOutput> converter) { throw null; }
@@ -325,11 +330,13 @@ namespace System.Collections.Generic
         public void CopyTo(T[] array) { }
         public void CopyTo(T[] array, int arrayIndex) { }
         public bool Exists(System.Predicate<T> match) { throw null; }
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public T Find(System.Predicate<T> match) { throw null; }
         public System.Collections.Generic.List<T> FindAll(System.Predicate<T> match) { throw null; }
         public int FindIndex(int startIndex, int count, System.Predicate<T> match) { throw null; }
         public int FindIndex(int startIndex, System.Predicate<T> match) { throw null; }
         public int FindIndex(System.Predicate<T> match) { throw null; }
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public T FindLast(System.Predicate<T> match) { throw null; }
         public int FindLastIndex(int startIndex, int count, System.Predicate<T> match) { throw null; }
         public int FindLastIndex(int startIndex, System.Predicate<T> match) { throw null; }
@@ -352,17 +359,17 @@ namespace System.Collections.Generic
         public void Reverse() { }
         public void Reverse(int index, int count) { }
         public void Sort() { }
-        public void Sort(System.Collections.Generic.IComparer<T> comparer) { }
+        public void Sort(System.Collections.Generic.IComparer<T>? comparer) { }
         public void Sort(System.Comparison<T> comparison) { }
-        public void Sort(int index, int count, System.Collections.Generic.IComparer<T> comparer) { }
+        public void Sort(int index, int count, System.Collections.Generic.IComparer<T>? comparer) { }
         System.Collections.Generic.IEnumerator<T> System.Collections.Generic.IEnumerable<T>.GetEnumerator() { throw null; }
         void System.Collections.ICollection.CopyTo(System.Array array, int arrayIndex) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        int System.Collections.IList.Add(object item) { throw null; }
-        bool System.Collections.IList.Contains(object item) { throw null; }
-        int System.Collections.IList.IndexOf(object item) { throw null; }
-        void System.Collections.IList.Insert(int index, object item) { }
-        void System.Collections.IList.Remove(object item) { }
+        int System.Collections.IList.Add(object? item) { throw null; }
+        bool System.Collections.IList.Contains(object? item) { throw null; }
+        int System.Collections.IList.IndexOf(object? item) { throw null; }
+        void System.Collections.IList.Insert(int index, object? item) { }
+        void System.Collections.IList.Remove(object? item) { }
         public T[] ToArray() { throw null; }
         public void TrimExcess() { }
         public bool TrueForAll(System.Predicate<T> match) { throw null; }
@@ -372,7 +379,7 @@ namespace System.Collections.Generic
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
@@ -398,26 +405,26 @@ namespace System.Collections.Generic
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         public T[] ToArray() { throw null; }
         public void TrimExcess() { }
-        public bool TryDequeue(out T result) { throw null; }
-        public bool TryPeek(out T result) { throw null; }
+        public bool TryDequeue([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T result) { throw null; }
+        public bool TryPeek([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T result) { throw null; }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<T>, System.Collections.IEnumerator, System.IDisposable
         {
             private T _currentElement;
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
         }
     }
-    public partial class SortedDictionary<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable
+    public partial class SortedDictionary<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable where TKey : notnull
     {
         public SortedDictionary() { }
-        public SortedDictionary(System.Collections.Generic.IComparer<TKey> comparer) { }
+        public SortedDictionary(System.Collections.Generic.IComparer<TKey>? comparer) { }
         public SortedDictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary) { }
-        public SortedDictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IComparer<TKey> comparer) { }
+        public SortedDictionary(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IComparer<TKey>? comparer) { }
         public System.Collections.Generic.IComparer<TKey> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
         public TValue this[TKey key] { get { throw null; } set { } }
@@ -431,7 +438,7 @@ namespace System.Collections.Generic
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
         bool System.Collections.IDictionary.IsFixedSize { get { throw null; } }
         bool System.Collections.IDictionary.IsReadOnly { get { throw null; } }
-        object System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
+        object? System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
         System.Collections.ICollection System.Collections.IDictionary.Keys { get { throw null; } }
         System.Collections.ICollection System.Collections.IDictionary.Values { get { throw null; } }
         public System.Collections.Generic.SortedDictionary<TKey, TValue>.ValueCollection Values { get { throw null; } }
@@ -447,12 +454,12 @@ namespace System.Collections.Generic
         bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Remove(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { throw null; }
         System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>> System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>.GetEnumerator() { throw null; }
         void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
-        void System.Collections.IDictionary.Add(object key, object value) { }
+        void System.Collections.IDictionary.Add(object key, object? value) { }
         bool System.Collections.IDictionary.Contains(object key) { throw null; }
         System.Collections.IDictionaryEnumerator System.Collections.IDictionary.GetEnumerator() { throw null; }
         void System.Collections.IDictionary.Remove(object key) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        public bool TryGetValue(TKey key, out TValue value) { throw null; }
+        public bool TryGetValue(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.IDictionaryEnumerator, System.Collections.IEnumerator, System.IDisposable
         {
             private object _dummy;
@@ -460,8 +467,8 @@ namespace System.Collections.Generic
             public System.Collections.Generic.KeyValuePair<TKey, TValue> Current { get { throw null; } }
             System.Collections.DictionaryEntry System.Collections.IDictionaryEnumerator.Entry { get { throw null; } }
             object System.Collections.IDictionaryEnumerator.Key { get { throw null; } }
-            object System.Collections.IDictionaryEnumerator.Value { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IDictionaryEnumerator.Value { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
@@ -486,7 +493,7 @@ namespace System.Collections.Generic
             {
                 private object _dummy;
                 public TKey Current { get { throw null; } }
-                object System.Collections.IEnumerator.Current { get { throw null; } }
+                object? System.Collections.IEnumerator.Current { get { throw null; } }
                 public void Dispose() { }
                 public bool MoveNext() { throw null; }
                 void System.Collections.IEnumerator.Reset() { }
@@ -512,21 +519,21 @@ namespace System.Collections.Generic
             {
                 private object _dummy;
                 public TValue Current { get { throw null; } }
-                object System.Collections.IEnumerator.Current { get { throw null; } }
+                object? System.Collections.IEnumerator.Current { get { throw null; } }
                 public void Dispose() { }
                 public bool MoveNext() { throw null; }
                 void System.Collections.IEnumerator.Reset() { }
             }
         }
     }
-    public partial class SortedList<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable
+    public partial class SortedList<TKey, TValue> : System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IDictionary<TKey, TValue>, System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyCollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>, System.Collections.Generic.IReadOnlyDictionary<TKey, TValue>, System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable where TKey : notnull
     {
         public SortedList() { }
-        public SortedList(System.Collections.Generic.IComparer<TKey> comparer) { }
+        public SortedList(System.Collections.Generic.IComparer<TKey>? comparer) { }
         public SortedList(System.Collections.Generic.IDictionary<TKey, TValue> dictionary) { }
-        public SortedList(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IComparer<TKey> comparer) { }
+        public SortedList(System.Collections.Generic.IDictionary<TKey, TValue> dictionary, System.Collections.Generic.IComparer<TKey>? comparer) { }
         public SortedList(int capacity) { }
-        public SortedList(int capacity, System.Collections.Generic.IComparer<TKey> comparer) { }
+        public SortedList(int capacity, System.Collections.Generic.IComparer<TKey>? comparer) { }
         public int Capacity { get { throw null; } set { } }
         public System.Collections.Generic.IComparer<TKey> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
@@ -541,7 +548,7 @@ namespace System.Collections.Generic
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
         bool System.Collections.IDictionary.IsFixedSize { get { throw null; } }
         bool System.Collections.IDictionary.IsReadOnly { get { throw null; } }
-        object System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
+        object? System.Collections.IDictionary.this[object key] { get { throw null; } set { } }
         System.Collections.ICollection System.Collections.IDictionary.Keys { get { throw null; } }
         System.Collections.ICollection System.Collections.IDictionary.Values { get { throw null; } }
         public System.Collections.Generic.IList<TValue> Values { get { throw null; } }
@@ -559,25 +566,27 @@ namespace System.Collections.Generic
         void System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.CopyTo(System.Collections.Generic.KeyValuePair<TKey, TValue>[] array, int arrayIndex) { }
         bool System.Collections.Generic.ICollection<System.Collections.Generic.KeyValuePair<TKey, TValue>>.Remove(System.Collections.Generic.KeyValuePair<TKey, TValue> keyValuePair) { throw null; }
         System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<TKey, TValue>> System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey, TValue>>.GetEnumerator() { throw null; }
-        void System.Collections.ICollection.CopyTo(System.Array array, int arrayIndex) { }
-        void System.Collections.IDictionary.Add(object key, object value) { }
+        void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
+        void System.Collections.IDictionary.Add(object key, object? value) { }
         bool System.Collections.IDictionary.Contains(object key) { throw null; }
         System.Collections.IDictionaryEnumerator System.Collections.IDictionary.GetEnumerator() { throw null; }
         void System.Collections.IDictionary.Remove(object key) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         public void TrimExcess() { }
-        public bool TryGetValue(TKey key, out TValue value) { throw null; }
+        public bool TryGetValue(TKey key, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out TValue value) { throw null; }
     }
     public partial class SortedSet<T> : System.Collections.Generic.ICollection<T>, System.Collections.Generic.IEnumerable<T>, System.Collections.Generic.IReadOnlyCollection<T>, System.Collections.Generic.ISet<T>, System.Collections.ICollection, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
     {
         public SortedSet() { }
-        public SortedSet(System.Collections.Generic.IComparer<T> comparer) { }
+        public SortedSet(System.Collections.Generic.IComparer<T>? comparer) { }
         public SortedSet(System.Collections.Generic.IEnumerable<T> collection) { }
-        public SortedSet(System.Collections.Generic.IEnumerable<T> collection, System.Collections.Generic.IComparer<T> comparer) { }
+        public SortedSet(System.Collections.Generic.IEnumerable<T> collection, System.Collections.Generic.IComparer<T>? comparer) { }
         protected SortedSet(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public System.Collections.Generic.IComparer<T> Comparer { get { throw null; } }
         public int Count { get { throw null; } }
+        [System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public T Max { get { throw null; } }
+        [System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public T Min { get { throw null; } }
         bool System.Collections.Generic.ICollection<T>.IsReadOnly { get { throw null; } }
         bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
@@ -589,7 +598,7 @@ namespace System.Collections.Generic
         public void CopyTo(T[] array, int index) { }
         public void CopyTo(T[] array, int index, int count) { }
         public static System.Collections.Generic.IEqualityComparer<System.Collections.Generic.SortedSet<T>> CreateSetComparer() { throw null; }
-        public static System.Collections.Generic.IEqualityComparer<System.Collections.Generic.SortedSet<T>> CreateSetComparer(System.Collections.Generic.IEqualityComparer<T> memberEqualityComparer) { throw null; }
+        public static System.Collections.Generic.IEqualityComparer<System.Collections.Generic.SortedSet<T>> CreateSetComparer(System.Collections.Generic.IEqualityComparer<T>? memberEqualityComparer) { throw null; }
         public void ExceptWith(System.Collections.Generic.IEnumerable<T> other) { }
         public System.Collections.Generic.SortedSet<T>.Enumerator GetEnumerator() { throw null; }
         protected virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
@@ -599,7 +608,7 @@ namespace System.Collections.Generic
         public bool IsProperSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool IsSubsetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool IsSupersetOf(System.Collections.Generic.IEnumerable<T> other) { throw null; }
-        protected virtual void OnDeserialization(object sender) { }
+        protected virtual void OnDeserialization(object? sender) { }
         public bool Overlaps(System.Collections.Generic.IEnumerable<T> other) { throw null; }
         public bool Remove(T item) { throw null; }
         public int RemoveWhere(System.Predicate<T> match) { throw null; }
@@ -610,20 +619,20 @@ namespace System.Collections.Generic
         System.Collections.Generic.IEnumerator<T> System.Collections.Generic.IEnumerable<T>.GetEnumerator() { throw null; }
         void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object sender) { }
+        void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object? sender) { }
         void System.Runtime.Serialization.ISerializable.GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public bool TryGetValue(T equalValue, out T actualValue) { throw null; }
+        public bool TryGetValue(T equalValue, [System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T actualValue) { throw null; }
         public void UnionWith(System.Collections.Generic.IEnumerable<T> other) { }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<T>, System.Collections.IEnumerator, System.IDisposable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
         {
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
-            void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object sender) { }
+            void System.Runtime.Serialization.IDeserializationCallback.OnDeserialization(object? sender) { }
             void System.Runtime.Serialization.ISerializable.GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         }
     }
@@ -647,15 +656,15 @@ namespace System.Collections.Generic
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
         public T[] ToArray() { throw null; }
         public void TrimExcess() { }
-        public bool TryPeek(out T result) { throw null; }
-        public bool TryPop(out T result) { throw null; }
+        public bool TryPeek([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T result) { throw null; }
+        public bool TryPop([System.Diagnostics.CodeAnalysis.MaybeNullWhenAttribute(false)] out T result) { throw null; }
         public partial struct Enumerator : System.Collections.Generic.IEnumerator<T>, System.Collections.IEnumerator, System.IDisposable
         {
             private T _currentElement;
             private object _dummy;
             private int _dummyPrimitive;
             public T Current { get { throw null; } }
-            object System.Collections.IEnumerator.Current { get { throw null; } }
+            object? System.Collections.IEnumerator.Current { get { throw null; } }
             public void Dispose() { }
             public bool MoveNext() { throw null; }
             void System.Collections.IEnumerator.Reset() { }
diff --git a/src/libraries/System.Collections/ref/System.Collections.csproj b/src/libraries/System.Collections/ref/System.Collections.csproj
index 6644040..a82f8a3 100644
--- a/src/libraries/System.Collections/ref/System.Collections.csproj
+++ b/src/libraries/System.Collections/ref/System.Collections.csproj
@@ -2,6 +2,7 @@
   <PropertyGroup>
     <ProjectGuid>{C746D448-E7C3-4850-9CA7-D3F1FA49742F}</ProjectGuid>
     <Configurations>netcoreapp-Debug;netcoreapp-Release;uap-Debug;uap-Release</Configurations>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Collections.cs" />
-- 
2.7.4