From: buyaa-n <bunamnan@microsoft.com>
Date: Fri, 20 Sep 2019 00:19:48 +0000 (-0700)
Subject: Annotate System.Collections.Specialized for nullable (dotnet/corefx#41092)
X-Git-Tag: submit/tizen/20210909.063632~11031^2~433
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=22824d27c3129cf6773507a3ff7849832f300b5c;p=platform%2Fupstream%2Fdotnet%2Fruntime.git

Annotate System.Collections.Specialized for nullable (dotnet/corefx#41092)

Annotate System.Collections.NonGeneric for nullable

Commit migrated from https://github.com/dotnet/corefx/commit/0d6adbc80b1d8262771a4db71931c4383646ff1f
---

diff --git a/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.cs b/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.cs
index c485a17..43f8fc8 100644
--- a/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.cs
+++ b/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.cs
@@ -19,7 +19,7 @@ namespace System.Collections.Specialized
         public static int CreateMask(int previous) { throw null; }
         public static System.Collections.Specialized.BitVector32.Section CreateSection(short maxValue) { throw null; }
         public static System.Collections.Specialized.BitVector32.Section CreateSection(short maxValue, System.Collections.Specialized.BitVector32.Section previous) { throw null; }
-        public override bool Equals(object o) { throw null; }
+        public override bool Equals(object? o) { throw null; }
         public override int GetHashCode() { throw null; }
         public override string ToString() { throw null; }
         public static string ToString(System.Collections.Specialized.BitVector32 value) { throw null; }
@@ -29,7 +29,7 @@ namespace System.Collections.Specialized
             public short Mask { get { throw null; } }
             public short Offset { get { throw null; } }
             public bool Equals(System.Collections.Specialized.BitVector32.Section obj) { throw null; }
-            public override bool Equals(object o) { throw null; }
+            public override bool Equals(object? o) { throw null; }
             public override int GetHashCode() { throw null; }
             public static bool operator ==(System.Collections.Specialized.BitVector32.Section a, System.Collections.Specialized.BitVector32.Section b) { throw null; }
             public static bool operator !=(System.Collections.Specialized.BitVector32.Section a, System.Collections.Specialized.BitVector32.Section b) { throw null; }
@@ -47,11 +47,11 @@ namespace System.Collections.Specialized
         public bool IsFixedSize { get { throw null; } }
         public bool IsReadOnly { get { throw null; } }
         public bool IsSynchronized { get { throw null; } }
-        public object this[object key] { get { throw null; } set { } }
+        public object? this[object key] { get { throw null; } set { } }
         public System.Collections.ICollection Keys { get { throw null; } }
         public object SyncRoot { get { throw null; } }
         public System.Collections.ICollection Values { get { throw null; } }
-        public void Add(object key, object value) { }
+        public void Add(object key, object? value) { }
         public void Clear() { }
         public bool Contains(object key) { throw null; }
         public void CopyTo(System.Array array, int index) { }
@@ -61,24 +61,24 @@ namespace System.Collections.Specialized
     }
     public partial interface IOrderedDictionary : System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable
     {
-        object this[int index] { get; set; }
+        object? this[int index] { get; set; }
         new System.Collections.IDictionaryEnumerator GetEnumerator();
-        void Insert(int index, object key, object value);
+        void Insert(int index, object key, object? value);
         void RemoveAt(int index);
     }
     public partial class ListDictionary : System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable
     {
         public ListDictionary() { }
-        public ListDictionary(System.Collections.IComparer comparer) { }
+        public ListDictionary(System.Collections.IComparer? comparer) { }
         public int Count { get { throw null; } }
         public bool IsFixedSize { get { throw null; } }
         public bool IsReadOnly { get { throw null; } }
         public bool IsSynchronized { get { throw null; } }
-        public object this[object key] { get { throw null; } set { } }
+        public object? this[object key] { get { throw null; } set { } }
         public System.Collections.ICollection Keys { get { throw null; } }
         public object SyncRoot { get { throw null; } }
         public System.Collections.ICollection Values { get { throw null; } }
-        public void Add(object key, object value) { }
+        public void Add(object key, object? value) { }
         public void Clear() { }
         public bool Contains(object key) { throw null; }
         public void CopyTo(System.Array array, int index) { }
@@ -89,44 +89,44 @@ namespace System.Collections.Specialized
     public abstract partial class NameObjectCollectionBase : System.Collections.ICollection, System.Collections.IEnumerable, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
     {
         protected NameObjectCollectionBase() { }
-        protected NameObjectCollectionBase(System.Collections.IEqualityComparer equalityComparer) { }
+        protected NameObjectCollectionBase(System.Collections.IEqualityComparer? equalityComparer) { }
         [System.ObsoleteAttribute("Please use NameObjectCollectionBase(IEqualityComparer) instead.")]
-        protected NameObjectCollectionBase(System.Collections.IHashCodeProvider hashProvider, System.Collections.IComparer comparer) { }
+        protected NameObjectCollectionBase(System.Collections.IHashCodeProvider? hashProvider, System.Collections.IComparer? comparer) { }
         protected NameObjectCollectionBase(int capacity) { }
-        protected NameObjectCollectionBase(int capacity, System.Collections.IEqualityComparer equalityComparer) { }
+        protected NameObjectCollectionBase(int capacity, System.Collections.IEqualityComparer? equalityComparer) { }
         [System.ObsoleteAttribute("Please use NameObjectCollectionBase(Int32, IEqualityComparer) instead.")]
-        protected NameObjectCollectionBase(int capacity, System.Collections.IHashCodeProvider hashProvider, System.Collections.IComparer comparer) { }
+        protected NameObjectCollectionBase(int capacity, System.Collections.IHashCodeProvider? hashProvider, System.Collections.IComparer? comparer) { }
         protected NameObjectCollectionBase(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public virtual int Count { get { throw null; } }
         protected bool IsReadOnly { get { throw null; } set { } }
         public virtual System.Collections.Specialized.NameObjectCollectionBase.KeysCollection Keys { get { throw null; } }
         bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
-        protected void BaseAdd(string name, object value) { }
+        protected void BaseAdd(string? name, object? value) { }
         protected void BaseClear() { }
-        protected object BaseGet(int index) { throw null; }
-        protected object BaseGet(string name) { throw null; }
-        protected string[] BaseGetAllKeys() { throw null; }
-        protected object[] BaseGetAllValues() { throw null; }
-        protected object[] BaseGetAllValues(System.Type type) { throw null; }
-        protected string BaseGetKey(int index) { throw null; }
+        protected object? BaseGet(int index) { throw null; }
+        protected object? BaseGet(string? name) { throw null; }
+        protected string?[] BaseGetAllKeys() { throw null; }
+        protected object?[] BaseGetAllValues() { throw null; }
+        protected object?[] BaseGetAllValues(System.Type type) { throw null; }
+        protected string? BaseGetKey(int index) { throw null; }
         protected bool BaseHasKeys() { throw null; }
-        protected void BaseRemove(string name) { }
+        protected void BaseRemove(string? name) { }
         protected void BaseRemoveAt(int index) { }
-        protected void BaseSet(int index, object value) { }
-        protected void BaseSet(string name, object value) { }
+        protected void BaseSet(int index, object? value) { }
+        protected void BaseSet(string? name, object? value) { }
         public virtual System.Collections.IEnumerator 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) { }
         void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
         public partial class KeysCollection : System.Collections.ICollection, System.Collections.IEnumerable
         {
             internal KeysCollection() { }
             public int Count { get { throw null; } }
-            public string this[int index] { get { throw null; } }
+            public string? this[int index] { get { throw null; } }
             bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
             object System.Collections.ICollection.SyncRoot { get { throw null; } }
-            public virtual string Get(int index) { throw null; }
+            public virtual string? Get(int index) { throw null; }
             public System.Collections.IEnumerator GetEnumerator() { throw null; }
             void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
         }
@@ -134,58 +134,58 @@ namespace System.Collections.Specialized
     public partial class NameValueCollection : System.Collections.Specialized.NameObjectCollectionBase
     {
         public NameValueCollection() { }
-        public NameValueCollection(System.Collections.IEqualityComparer equalityComparer) { }
+        public NameValueCollection(System.Collections.IEqualityComparer? equalityComparer) { }
         [System.ObsoleteAttribute("Please use NameValueCollection(IEqualityComparer) instead.")]
-        public NameValueCollection(System.Collections.IHashCodeProvider hashProvider, System.Collections.IComparer comparer) { }
+        public NameValueCollection(System.Collections.IHashCodeProvider? hashProvider, System.Collections.IComparer? comparer) { }
         public NameValueCollection(System.Collections.Specialized.NameValueCollection col) { }
         public NameValueCollection(int capacity) { }
-        public NameValueCollection(int capacity, System.Collections.IEqualityComparer equalityComparer) { }
+        public NameValueCollection(int capacity, System.Collections.IEqualityComparer? equalityComparer) { }
         [System.ObsoleteAttribute("Please use NameValueCollection(Int32, IEqualityComparer) instead.")]
-        public NameValueCollection(int capacity, System.Collections.IHashCodeProvider hashProvider, System.Collections.IComparer comparer) { }
+        public NameValueCollection(int capacity, System.Collections.IHashCodeProvider? hashProvider, System.Collections.IComparer? comparer) { }
         public NameValueCollection(int capacity, System.Collections.Specialized.NameValueCollection col) { }
         protected NameValueCollection(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public virtual string[] AllKeys { get { throw null; } }
-        public string this[int index] { get { throw null; } }
-        public string this[string name] { get { throw null; } set { } }
+        public virtual string?[] AllKeys { get { throw null; } }
+        public string? this[int index] { get { throw null; } }
+        public string? this[string? name] { get { throw null; } set { } }
         public void Add(System.Collections.Specialized.NameValueCollection c) { }
-        public virtual void Add(string name, string value) { }
+        public virtual void Add(string? name, string? value) { }
         public virtual void Clear() { }
         public void CopyTo(System.Array dest, int index) { }
-        public virtual string Get(int index) { throw null; }
-        public virtual string Get(string name) { throw null; }
-        public virtual string GetKey(int index) { throw null; }
-        public virtual string[] GetValues(int index) { throw null; }
-        public virtual string[] GetValues(string name) { throw null; }
+        public virtual string? Get(int index) { throw null; }
+        public virtual string? Get(string? name) { throw null; }
+        public virtual string? GetKey(int index) { throw null; }
+        public virtual string[]? GetValues(int index) { throw null; }
+        public virtual string[]? GetValues(string? name) { throw null; }
         public bool HasKeys() { throw null; }
         protected void InvalidateCachedArrays() { }
-        public virtual void Remove(string name) { }
-        public virtual void Set(string name, string value) { }
+        public virtual void Remove(string? name) { }
+        public virtual void Set(string? name, string? value) { }
     }
     public partial class OrderedDictionary : System.Collections.ICollection, System.Collections.IDictionary, System.Collections.IEnumerable, System.Collections.Specialized.IOrderedDictionary, System.Runtime.Serialization.IDeserializationCallback, System.Runtime.Serialization.ISerializable
     {
         public OrderedDictionary() { }
-        public OrderedDictionary(System.Collections.IEqualityComparer comparer) { }
+        public OrderedDictionary(System.Collections.IEqualityComparer? comparer) { }
         public OrderedDictionary(int capacity) { }
-        public OrderedDictionary(int capacity, System.Collections.IEqualityComparer comparer) { }
+        public OrderedDictionary(int capacity, System.Collections.IEqualityComparer? comparer) { }
         protected OrderedDictionary(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
         public int Count { get { throw null; } }
         public bool IsReadOnly { get { throw null; } }
-        public object this[int index] { get { throw null; } set { } }
-        public object this[object key] { get { throw null; } set { } }
+        public object? this[int index] { get { throw null; } set { } }
+        public object? this[object key] { get { throw null; } set { } }
         public System.Collections.ICollection Keys { get { throw null; } }
         bool System.Collections.ICollection.IsSynchronized { get { throw null; } }
         object System.Collections.ICollection.SyncRoot { get { throw null; } }
         bool System.Collections.IDictionary.IsFixedSize { get { throw null; } }
         public System.Collections.ICollection Values { get { throw null; } }
-        public void Add(object key, object value) { }
+        public void Add(object key, object? value) { }
         public System.Collections.Specialized.OrderedDictionary AsReadOnly() { throw null; }
         public void Clear() { }
         public bool Contains(object key) { throw null; }
         public void CopyTo(System.Array array, int index) { }
         public virtual System.Collections.IDictionaryEnumerator GetEnumerator() { throw null; }
         public virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context) { }
-        public void Insert(int index, object key, object value) { }
-        protected virtual void OnDeserialization(object sender) { }
+        public void Insert(int index, object key, object? value) { }
+        protected virtual void OnDeserialization(object? sender) { }
         public void Remove(object key) { }
         public void RemoveAt(int index) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
@@ -197,42 +197,42 @@ namespace System.Collections.Specialized
         public int Count { get { throw null; } }
         public bool IsReadOnly { get { throw null; } }
         public bool IsSynchronized { get { throw null; } }
-        public string this[int index] { get { throw null; } set { } }
+        public string? this[int index] { get { throw null; } set { } }
         public object 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 { } }
-        public int Add(string value) { throw null; }
+        object? System.Collections.IList.this[int index] { get { throw null; } set { } }
+        public int Add(string? value) { throw null; }
         public void AddRange(string[] value) { }
         public void Clear() { }
-        public bool Contains(string value) { throw null; }
+        public bool Contains(string? value) { throw null; }
         public void CopyTo(string[] array, int index) { }
         public System.Collections.Specialized.StringEnumerator GetEnumerator() { throw null; }
-        public int IndexOf(string value) { throw null; }
-        public void Insert(int index, string value) { }
-        public void Remove(string value) { }
+        public int IndexOf(string? value) { throw null; }
+        public void Insert(int index, string? value) { }
+        public void Remove(string? value) { }
         public void RemoveAt(int index) { }
         void System.Collections.ICollection.CopyTo(System.Array array, int index) { }
         System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw null; }
-        int System.Collections.IList.Add(object value) { throw null; }
-        bool System.Collections.IList.Contains(object value) { throw null; }
-        int System.Collections.IList.IndexOf(object value) { throw null; }
-        void System.Collections.IList.Insert(int index, object value) { }
-        void System.Collections.IList.Remove(object value) { }
+        int System.Collections.IList.Add(object? value) { throw null; }
+        bool System.Collections.IList.Contains(object? value) { throw null; }
+        int System.Collections.IList.IndexOf(object? value) { throw null; }
+        void System.Collections.IList.Insert(int index, object? value) { }
+        void System.Collections.IList.Remove(object? value) { }
     }
     public partial class StringDictionary : System.Collections.IEnumerable
     {
         public StringDictionary() { }
         public virtual int Count { get { throw null; } }
         public virtual bool IsSynchronized { get { throw null; } }
-        public virtual string this[string key] { get { throw null; } set { } }
+        public virtual string? this[string key] { get { throw null; } set { } }
         public virtual System.Collections.ICollection Keys { get { throw null; } }
         public virtual object SyncRoot { get { throw null; } }
         public virtual System.Collections.ICollection Values { get { throw null; } }
-        public virtual void Add(string key, string value) { }
+        public virtual void Add(string key, string? value) { }
         public virtual void Clear() { }
         public virtual bool ContainsKey(string key) { throw null; }
-        public virtual bool ContainsValue(string value) { throw null; }
+        public virtual bool ContainsValue(string? value) { throw null; }
         public virtual void CopyTo(System.Array array, int index) { }
         public virtual System.Collections.IEnumerator GetEnumerator() { throw null; }
         public virtual void Remove(string key) { }
@@ -240,7 +240,7 @@ namespace System.Collections.Specialized
     public partial class StringEnumerator
     {
         internal StringEnumerator() { }
-        public string Current { get { throw null; } }
+        public string? Current { get { throw null; } }
         public bool MoveNext() { throw null; }
         public void Reset() { }
     }
diff --git a/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.csproj b/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.csproj
index f453b4e..be44def 100644
--- a/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.csproj
+++ b/src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.csproj
@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Configurations>netcoreapp-Debug;netcoreapp-Release;uap-Debug;uap-Release</Configurations>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Collections.Specialized.cs" />
diff --git a/src/libraries/System.Collections.Specialized/src/System.Collections.Specialized.csproj b/src/libraries/System.Collections.Specialized/src/System.Collections.Specialized.csproj
index 803be56..f12c952 100644
--- a/src/libraries/System.Collections.Specialized/src/System.Collections.Specialized.csproj
+++ b/src/libraries/System.Collections.Specialized/src/System.Collections.Specialized.csproj
@@ -1,8 +1,9 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <RootNamespace>System.Collections.Specialized</RootNamespace>
     <AssemblyName>System.Collections.Specialized</AssemblyName>
     <Configurations>netcoreapp-Debug;netcoreapp-Release;uap-Windows_NT-Debug;uap-Windows_NT-Release</Configurations>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System\Collections\Specialized\BitVector32.cs" />
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/BitVector32.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/BitVector32.cs
index 33f23d5..a71558f 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/BitVector32.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/BitVector32.cs
@@ -185,7 +185,7 @@ namespace System.Collections.Specialized
             return new Section(CreateMaskFromHighValue(maxValue), offset);
         }
 
-        public override bool Equals(object o)
+        public override bool Equals(object? o)
         {
             if (!(o is BitVector32))
             {
@@ -254,7 +254,7 @@ namespace System.Collections.Specialized
                 }
             }
 
-            public override bool Equals(object o)
+            public override bool Equals(object? o)
             {
                 if (o is Section)
                     return Equals((Section)o);
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/HybridDictionary.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/HybridDictionary.cs
index abe2856..13aa099 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/HybridDictionary.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/HybridDictionary.cs
@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics;
+
 namespace System.Collections.Specialized
 {
     /// <devdoc>
@@ -25,8 +27,8 @@ namespace System.Collections.Specialized
         private const int FixedSizeCutoverPoint = 6;
 
         // Instance variables. This keeps the HybridDictionary very light-weight when empty
-        private ListDictionary list; // Do not rename (binary serialization)
-        private Hashtable hashtable; // Do not rename (binary serialization)
+        private ListDictionary? list; // Do not rename (binary serialization)
+        private Hashtable? hashtable; // Do not rename (binary serialization)
         private readonly bool caseInsensitive; // Do not rename (binary serialization)
 
         public HybridDictionary()
@@ -58,7 +60,7 @@ namespace System.Collections.Specialized
             }
         }
 
-        public object this[object key]
+        public object? this[object key]
         {
             get
             {
@@ -66,7 +68,7 @@ namespace System.Collections.Specialized
                 // Although we never made the same guarantee for HybridDictionary,
                 // it is still nice to do the same thing here since we have recommended
                 // HybridDictionary as replacement for Hashtable.
-                ListDictionary cachedList = list;
+                ListDictionary? cachedList = list;
                 if (hashtable != null)
                 {
                     return hashtable[key];
@@ -97,6 +99,7 @@ namespace System.Collections.Specialized
                     if (list.Count >= CutoverPoint - 1)
                     {
                         ChangeOver();
+                        Debug.Assert(hashtable != null);
                         hashtable[key] = value;
                     }
                     else
@@ -126,6 +129,7 @@ namespace System.Collections.Specialized
 
         private void ChangeOver()
         {
+            Debug.Assert(list != null);
             IDictionaryEnumerator en = list.GetEnumerator();
             Hashtable newTable;
             if (caseInsensitive)
@@ -152,7 +156,7 @@ namespace System.Collections.Specialized
         {
             get
             {
-                ListDictionary cachedList = list;
+                ListDictionary? cachedList = list;
                 if (hashtable != null)
                 {
                     return hashtable.Count;
@@ -230,7 +234,7 @@ namespace System.Collections.Specialized
             }
         }
 
-        public void Add(object key, object value)
+        public void Add(object key, object? value)
         {
             if (hashtable != null)
             {
@@ -246,6 +250,7 @@ namespace System.Collections.Specialized
                 else if (list.Count + 1 >= CutoverPoint)
                 {
                     ChangeOver();
+                    Debug.Assert(hashtable != null);
                     hashtable.Add(key, value);
                 }
                 else
@@ -274,7 +279,7 @@ namespace System.Collections.Specialized
 
         public bool Contains(object key)
         {
-            ListDictionary cachedList = list;
+            ListDictionary? cachedList = list;
             if (hashtable != null)
             {
                 return hashtable.Contains(key);
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/IOrderedDictionary.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/IOrderedDictionary.cs
index bee80fa..16dbf91 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/IOrderedDictionary.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/IOrderedDictionary.cs
@@ -19,7 +19,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// Returns the object at the given index
         /// </devdoc>
-        object this[int index] { get; set; }
+        object? this[int index] { get; set; }
 
         // Returns an IDictionaryEnumerator for this dictionary.
         new IDictionaryEnumerator GetEnumerator();
@@ -28,7 +28,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// Inserts the given object, with the given key, at the given index
         /// </devdoc>
-        void Insert(int index, object key, object value);
+        void Insert(int index, object key, object? value);
 
         /// <devdoc>
         /// Removes the object and key at the given index
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/ListDictionary.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/ListDictionary.cs
index 74d3b13..ac2a1e9 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/ListDictionary.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/ListDictionary.cs
@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics;
+
 namespace System.Collections.Specialized
 {
     /// <devdoc>
@@ -15,21 +17,21 @@ namespace System.Collections.Specialized
     [System.Runtime.CompilerServices.TypeForwardedFrom("System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class ListDictionary : IDictionary
     {
-        private DictionaryNode head; // Do not rename (binary serialization)
+        private DictionaryNode? head; // Do not rename (binary serialization)
         private int version; // Do not rename (binary serialization)
         private int count; // Do not rename (binary serialization)
-        private readonly IComparer comparer; // Do not rename (binary serialization)
+        private readonly IComparer? comparer; // Do not rename (binary serialization)
 
         public ListDictionary()
         {
         }
 
-        public ListDictionary(IComparer comparer)
+        public ListDictionary(IComparer? comparer)
         {
             this.comparer = comparer;
         }
 
-        public object this[object key]
+        public object? this[object key]
         {
             get
             {
@@ -37,7 +39,7 @@ namespace System.Collections.Specialized
                 {
                     throw new ArgumentNullException(nameof(key));
                 }
-                DictionaryNode node = head;
+                DictionaryNode? node = head;
                 if (comparer == null)
                 {
                     while (node != null)
@@ -71,8 +73,8 @@ namespace System.Collections.Specialized
                     throw new ArgumentNullException(nameof(key));
                 }
                 version++;
-                DictionaryNode last = null;
-                DictionaryNode node;
+                DictionaryNode? last = null;
+                DictionaryNode? node;
                 for (node = head; node != null; node = node.next)
                 {
                     object oldKey = node.key;
@@ -154,16 +156,16 @@ namespace System.Collections.Specialized
             }
         }
 
-        public void Add(object key, object value)
+        public void Add(object key, object? value)
         {
             if (key == null)
             {
                 throw new ArgumentNullException(nameof(key));
             }
             version++;
-            DictionaryNode last = null;
-            DictionaryNode node;
-            for (node = head; node != null; node = node.next)
+            DictionaryNode? last = null;
+
+            for (DictionaryNode? node = head; node != null; node = node.next)
             {
                 object oldKey = node.key;
                 if ((comparer == null) ? oldKey.Equals(key) : comparer.Compare(oldKey, key) == 0)
@@ -200,7 +202,7 @@ namespace System.Collections.Specialized
             {
                 throw new ArgumentNullException(nameof(key));
             }
-            for (DictionaryNode node = head; node != null; node = node.next)
+            for (DictionaryNode? node = head; node != null; node = node.next)
             {
                 object oldKey = node.key;
                 if ((comparer == null) ? oldKey.Equals(key) : comparer.Compare(oldKey, key) == 0)
@@ -221,7 +223,7 @@ namespace System.Collections.Specialized
             if (array.Length - index < count)
                 throw new ArgumentException(SR.Arg_InsufficientSpace);
 
-            for (DictionaryNode node = head; node != null; node = node.next)
+            for (DictionaryNode? node = head; node != null; node = node.next)
             {
                 array.SetValue(new DictionaryEntry(node.key, node.value), index);
                 index++;
@@ -245,8 +247,8 @@ namespace System.Collections.Specialized
                 throw new ArgumentNullException(nameof(key));
             }
             version++;
-            DictionaryNode last = null;
-            DictionaryNode node;
+            DictionaryNode? last = null;
+            DictionaryNode? node;
             for (node = head; node != null; node = node.next)
             {
                 object oldKey = node.key;
@@ -266,6 +268,7 @@ namespace System.Collections.Specialized
             }
             else
             {
+                Debug.Assert(last != null);
                 last.next = node.next;
             }
             count--;
@@ -274,7 +277,7 @@ namespace System.Collections.Specialized
         private class NodeEnumerator : IDictionaryEnumerator
         {
             private readonly ListDictionary _list;
-            private DictionaryNode _current;
+            private DictionaryNode? _current;
             private readonly int _version;
             private bool _start;
 
@@ -319,7 +322,7 @@ namespace System.Collections.Specialized
                 }
             }
 
-            public object Value
+            public object? Value
             {
                 get
                 {
@@ -378,7 +381,7 @@ namespace System.Collections.Specialized
                 if (index < 0)
                     throw new ArgumentOutOfRangeException(nameof(index), index, SR.ArgumentOutOfRange_NeedNonNegNum_Index);
 
-                for (DictionaryNode node = _list.head; node != null; node = node.next)
+                for (DictionaryNode? node = _list.head; node != null; node = node.next)
                 {
                     array.SetValue(_isKeys ? node.key : node.value, index);
                     index++;
@@ -390,7 +393,7 @@ namespace System.Collections.Specialized
                 get
                 {
                     int count = 0;
-                    for (DictionaryNode node = _list.head; node != null; node = node.next)
+                    for (DictionaryNode? node = _list.head; node != null; node = node.next)
                     {
                         count++;
                     }
@@ -423,7 +426,7 @@ namespace System.Collections.Specialized
             private class NodeKeyValueEnumerator : IEnumerator
             {
                 private readonly ListDictionary _list;
-                private DictionaryNode _current;
+                private DictionaryNode? _current;
                 private readonly int _version;
                 private readonly bool _isKeys;
                 private bool _start;
@@ -437,7 +440,7 @@ namespace System.Collections.Specialized
                     _current = null;
                 }
 
-                public object Current
+                public object? Current
                 {
                     get
                     {
@@ -483,9 +486,9 @@ namespace System.Collections.Specialized
         [System.Runtime.CompilerServices.TypeForwardedFrom("System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
         public class DictionaryNode
         {
-            public object key; // Do not rename (binary serialization)
-            public object value; // Do not rename (binary serialization)
-            public DictionaryNode next; // Do not rename (binary serialization)
+            public object key = null!; // Do not rename (binary serialization)
+            public object? value; // Do not rename (binary serialization)
+            public DictionaryNode? next; // Do not rename (binary serialization)
         }
     }
 }
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameObjectCollectionBase.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameObjectCollectionBase.cs
index f809126..19de422 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameObjectCollectionBase.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameObjectCollectionBase.cs
@@ -24,11 +24,11 @@ namespace System.Collections.Specialized
     public abstract class NameObjectCollectionBase : ICollection, ISerializable, IDeserializationCallback
     {
         private bool _readOnly = false;
-        private ArrayList _entriesArray;
+        private ArrayList _entriesArray = null!; // initialized in Reset method, called from constructor
         private IEqualityComparer _keyComparer;
-        private volatile Hashtable _entriesTable;
-        private volatile NameObjectEntry _nullKeyEntry;
-        private KeysCollection _keys;
+        private volatile Hashtable _entriesTable = null!; // initialized in Reset method, called from constructor
+        private volatile NameObjectEntry? _nullKeyEntry;
+        private KeysCollection? _keys;
         private int _version;
 
         private static readonly StringComparer s_defaultComparer = CultureInfo.InvariantCulture.CompareInfo.GetStringComparer(CompareOptions.IgnoreCase);
@@ -41,26 +41,26 @@ namespace System.Collections.Specialized
         {
         }
 
-        protected NameObjectCollectionBase(IEqualityComparer equalityComparer)
+        protected NameObjectCollectionBase(IEqualityComparer? equalityComparer)
         {
             _keyComparer = (equalityComparer == null) ? s_defaultComparer : equalityComparer;
             Reset();
         }
 
-        protected NameObjectCollectionBase(int capacity, IEqualityComparer equalityComparer) : this(equalityComparer)
+        protected NameObjectCollectionBase(int capacity, IEqualityComparer? equalityComparer) : this(equalityComparer)
         {
             Reset(capacity);
         }
 
         [Obsolete("Please use NameObjectCollectionBase(IEqualityComparer) instead.")]
-        protected NameObjectCollectionBase(IHashCodeProvider hashProvider, IComparer comparer)
+        protected NameObjectCollectionBase(IHashCodeProvider? hashProvider, IComparer? comparer)
         {
             _keyComparer = new CompatibleComparer(hashProvider, comparer);
             Reset();
         }
 
         [Obsolete("Please use NameObjectCollectionBase(Int32, IEqualityComparer) instead.")]
-        protected NameObjectCollectionBase(int capacity, IHashCodeProvider hashProvider, IComparer comparer)
+        protected NameObjectCollectionBase(int capacity, IHashCodeProvider? hashProvider, IComparer? comparer)
         {
             _keyComparer = new CompatibleComparer(hashProvider, comparer);
             Reset(capacity);
@@ -87,7 +87,7 @@ namespace System.Collections.Specialized
             throw new PlatformNotSupportedException();
         }
 
-        public virtual void OnDeserialization(object sender)
+        public virtual void OnDeserialization(object? sender)
         {
             throw new PlatformNotSupportedException();
         }
@@ -112,10 +112,10 @@ namespace System.Collections.Specialized
             _version++;
         }
 
-        private NameObjectEntry FindEntry(string key)
+        private NameObjectEntry? FindEntry(string? key)
         {
             if (key != null)
-                return (NameObjectEntry)_entriesTable[key];
+                return (NameObjectEntry?)_entriesTable[key];
             else
                 return _nullKeyEntry;
         }
@@ -159,7 +159,7 @@ namespace System.Collections.Specialized
         ///    <para>Adds an entry with the specified key and value into the
         ///    <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected void BaseAdd(string name, object value)
+        protected void BaseAdd(string? name, object? value)
         {
             if (_readOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
@@ -188,7 +188,7 @@ namespace System.Collections.Specialized
         ///    <para>Removes the entries with the specified key from the
         ///    <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected void BaseRemove(string name)
+        protected void BaseRemove(string? name)
         {
             if (_readOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
@@ -230,7 +230,7 @@ namespace System.Collections.Specialized
             if (_readOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
 
-            string key = BaseGetKey(index);
+            string? key = BaseGetKey(index);
 
             if (key != null)
             {
@@ -268,9 +268,9 @@ namespace System.Collections.Specialized
         ///    <para>Gets the value of the first entry with the specified key from
         ///       the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected object BaseGet(string name)
+        protected object? BaseGet(string? name)
         {
-            NameObjectEntry e = FindEntry(name);
+            NameObjectEntry? e = FindEntry(name);
             return (e != null) ? e.Value : null;
         }
 
@@ -280,12 +280,12 @@ namespace System.Collections.Specialized
         /// into the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/>
         /// instance.</para>
         /// </devdoc>
-        protected void BaseSet(string name, object value)
+        protected void BaseSet(string? name, object? value)
         {
             if (_readOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
 
-            NameObjectEntry entry = FindEntry(name);
+            NameObjectEntry?  entry = FindEntry(name);
             if (entry != null)
             {
                 entry.Value = value;
@@ -305,9 +305,9 @@ namespace System.Collections.Specialized
         ///    <para>Gets the value of the entry at the specified index of
         ///       the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected object BaseGet(int index)
+        protected object? BaseGet(int index)
         {
-            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index];
+            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index]!;   // no null entry added to the array
             return entry.Value;
         }
 
@@ -316,9 +316,9 @@ namespace System.Collections.Specialized
         ///    <see cref='System.Collections.Specialized.NameObjectCollectionBase'/>
         ///    instance.</para>
         /// </devdoc>
-        protected string BaseGetKey(int index)
+        protected string? BaseGetKey(int index)
         {
-            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index];
+            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index]!;
             return entry.Key;
         }
 
@@ -326,12 +326,12 @@ namespace System.Collections.Specialized
         ///    <para>Sets the value of the entry at the specified index of
         ///       the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected void BaseSet(int index, object value)
+        protected void BaseSet(int index, object? value)
         {
             if (_readOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
 
-            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index];
+            NameObjectEntry entry = (NameObjectEntry)_entriesArray[index]!;
             entry.Value = value;
             _version++;
         }
@@ -400,10 +400,10 @@ namespace System.Collections.Specialized
         /// <para>Returns a <see cref='string' qualify='true'/> array containing all the keys in the
         /// <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected string[] BaseGetAllKeys()
+        protected string?[] BaseGetAllKeys()
         {
             int n = _entriesArray.Count;
-            string[] allKeys = new string[n];
+            string?[] allKeys = new string[n];
 
             for (int i = 0; i < n; i++)
                 allKeys[i] = BaseGetKey(i);
@@ -415,10 +415,10 @@ namespace System.Collections.Specialized
         /// <para>Returns an <see cref='object' qualify='true'/> array containing all the values in the
         /// <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected object[] BaseGetAllValues()
+        protected object?[] BaseGetAllValues()
         {
             int n = _entriesArray.Count;
-            object[] allValues = new object[n];
+            object?[] allValues = new object[n];
 
             for (int i = 0; i < n; i++)
                 allValues[i] = BaseGet(i);
@@ -430,14 +430,14 @@ namespace System.Collections.Specialized
         ///    <para>Returns an array of the specified type containing
         ///       all the values in the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        protected object[] BaseGetAllValues(Type type)
+        protected object?[] BaseGetAllValues(Type type)
         {
             int n = _entriesArray.Count;
             if (type == null)
             {
                 throw new ArgumentNullException(nameof(type));
             }
-            object[] allValues = (object[])Array.CreateInstance(type, n);
+            object?[] allValues = (object?[])Array.CreateInstance(type, n);
 
             for (int i = 0; i < n; i++)
             {
@@ -471,14 +471,14 @@ namespace System.Collections.Specialized
 
         internal class NameObjectEntry
         {
-            internal NameObjectEntry(string name, object value)
+            internal NameObjectEntry(string? name, object? value)
             {
                 Key = name;
                 Value = value;
             }
 
-            internal string Key;
-            internal object Value;
+            internal string? Key;
+            internal object? Value;
         }
 
         //
@@ -522,7 +522,7 @@ namespace System.Collections.Specialized
                 _pos = -1;
             }
 
-            public object Current
+            public object? Current
             {
                 get
                 {
@@ -559,7 +559,7 @@ namespace System.Collections.Specialized
             /// <devdoc>
             ///    <para> Gets the key at the specified index of the collection.</para>
             /// </devdoc>
-            public virtual string Get(int index)
+            public virtual string? Get(int index)
             {
                 return _coll.BaseGetKey(index);
             }
@@ -567,7 +567,7 @@ namespace System.Collections.Specialized
             /// <devdoc>
             ///    <para>Represents the entry at the specified index of the collection.</para>
             /// </devdoc>
-            public string this[int index]
+            public string? this[int index]
             {
                 get
                 {
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameValueCollection.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameValueCollection.cs
index 38c8813..c8793d3 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameValueCollection.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/NameValueCollection.cs
@@ -8,6 +8,7 @@
  *
  */
 
+using System.Diagnostics;
 using System.Runtime.Serialization;
 using System.Text;
 
@@ -19,8 +20,8 @@ namespace System.Collections.Specialized
     /// </devdoc>
     public class NameValueCollection : NameObjectCollectionBase
     {
-        private string[] _all; // Do not rename (binary serialization)
-        private string[] _allKeys; // Do not rename (binary serialization)
+        private string?[]? _all; // Do not rename (binary serialization)
+        private string?[]? _allKeys; // Do not rename (binary serialization)
 
         //
         // Constructors
@@ -43,11 +44,11 @@ namespace System.Collections.Specialized
         public NameValueCollection(NameValueCollection col)
             : base(col != null ? col.Comparer : null)
         {
-            Add(col);
+            Add(col!);
         }
 
         [Obsolete("Please use NameValueCollection(IEqualityComparer) instead.")]
-        public NameValueCollection(IHashCodeProvider hashProvider, IComparer comparer)
+        public NameValueCollection(IHashCodeProvider? hashProvider, IComparer? comparer)
             : base(hashProvider, comparer)
         {
         }
@@ -61,11 +62,11 @@ namespace System.Collections.Specialized
         {
         }
 
-        public NameValueCollection(IEqualityComparer equalityComparer) : base(equalityComparer)
+        public NameValueCollection(IEqualityComparer? equalityComparer) : base(equalityComparer)
         {
         }
 
-        public NameValueCollection(int capacity, IEqualityComparer equalityComparer)
+        public NameValueCollection(int capacity, IEqualityComparer? equalityComparer)
             : base(capacity, equalityComparer)
         {
         }
@@ -89,7 +90,7 @@ namespace System.Collections.Specialized
         }
 
         [Obsolete("Please use NameValueCollection(Int32, IEqualityComparer) instead.")]
-        public NameValueCollection(int capacity, IHashCodeProvider hashProvider, IComparer comparer)
+        public NameValueCollection(int capacity, IHashCodeProvider? hashProvider, IComparer? comparer)
             : base(capacity, hashProvider, comparer)
         {
         }
@@ -111,22 +112,24 @@ namespace System.Collections.Specialized
             _allKeys = null;
         }
 
-        private static string GetAsOneString(ArrayList list)
+        private static string? GetAsOneString(ArrayList? list)
         {
             int n = (list != null) ? list.Count : 0;
 
             if (n == 1)
             {
-                return (string)list[0];
+                Debug.Assert(list != null);
+                return (string?)list[0];
             }
             else if (n > 1)
             {
-                StringBuilder s = new StringBuilder((string)list[0]);
+                Debug.Assert(list != null);
+                StringBuilder s = new StringBuilder((string?)list[0]);
 
                 for (int i = 1; i < n; i++)
                 {
                     s.Append(',');
-                    s.Append((string)list[i]);
+                    s.Append((string?)list[i]);
                 }
 
                 return s.ToString();
@@ -137,14 +140,14 @@ namespace System.Collections.Specialized
             }
         }
 
-        private static string[] GetAsStringArray(ArrayList list)
+        private static string[]? GetAsStringArray(ArrayList? list)
         {
             int n = (list != null) ? list.Count : 0;
             if (n == 0)
                 return null;
 
             string[] array = new string[n];
-            list.CopyTo(0, array, 0, n);
+            list!.CopyTo(0, array, 0, n);
             return array;
         }
 
@@ -168,8 +171,8 @@ namespace System.Collections.Specialized
 
             for (int i = 0; i < n; i++)
             {
-                string key = c.GetKey(i);
-                string[] values = c.GetValues(i);
+                string? key = c.GetKey(i);
+                string[]? values = c.GetValues(i);
 
                 if (values != null)
                 {
@@ -221,7 +224,7 @@ namespace System.Collections.Specialized
             int n = Count;
             if (_all == null)
             {
-                string[] all = new string[n];
+                string?[] all = new string[n];
                 for (int i = 0; i < n; i++)
                 {
                     all[i] = Get(i);
@@ -262,14 +265,14 @@ namespace System.Collections.Specialized
         ///    <para>Adds an entry with the specified name and value into the
         ///    <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public virtual void Add(string name, string value)
+        public virtual void Add(string? name, string? value)
         {
             if (IsReadOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
 
             InvalidateCachedArrays();
 
-            ArrayList values = (ArrayList)BaseGet(name);
+            ArrayList? values = (ArrayList?)BaseGet(name);
 
             if (values == null)
             {
@@ -290,25 +293,25 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para> Gets the values associated with the specified key from the <see cref='System.Collections.Specialized.NameValueCollection'/> combined into one comma-separated list.</para>
         /// </devdoc>
-        public virtual string Get(string name)
+        public virtual string? Get(string? name)
         {
-            ArrayList values = (ArrayList)BaseGet(name);
+            ArrayList? values = (ArrayList?)BaseGet(name);
             return GetAsOneString(values);
         }
 
         /// <devdoc>
         /// <para>Gets the values associated with the specified key from the <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public virtual string[] GetValues(string name)
+        public virtual string[]? GetValues(string? name)
         {
-            ArrayList values = (ArrayList)BaseGet(name);
+            ArrayList? values = (ArrayList?)BaseGet(name);
             return GetAsStringArray(values);
         }
 
         /// <devdoc>
         /// <para>Adds a value to an entry in the <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public virtual void Set(string name, string value)
+        public virtual void Set(string? name, string? value)
         {
             if (IsReadOnly)
                 throw new NotSupportedException(SR.CollectionReadOnly);
@@ -323,7 +326,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Removes the entries with the specified key from the <see cref='System.Collections.Specialized.NameObjectCollectionBase'/> instance.</para>
         /// </devdoc>
-        public virtual void Remove(string name)
+        public virtual void Remove(string? name)
         {
             InvalidateCachedArrays();
             BaseRemove(name);
@@ -333,7 +336,7 @@ namespace System.Collections.Specialized
         ///    <para> Represents the entry with the specified key in the
         ///    <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public string this[string name]
+        public string? this[string? name]
         {
             get
             {
@@ -355,25 +358,25 @@ namespace System.Collections.Specialized
         ///       Gets the values at the specified index of the <see cref='System.Collections.Specialized.NameValueCollection'/> combined into one
         ///       comma-separated list.</para>
         /// </devdoc>
-        public virtual string Get(int index)
+        public virtual string? Get(int index)
         {
-            ArrayList values = (ArrayList)BaseGet(index);
+            ArrayList? values = (ArrayList?)BaseGet(index);
             return GetAsOneString(values);
         }
 
         /// <devdoc>
         ///    <para> Gets the values at the specified index of the <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public virtual string[] GetValues(int index)
+        public virtual string[]? GetValues(int index)
         {
-            ArrayList values = (ArrayList)BaseGet(index);
+            ArrayList? values = (ArrayList?)BaseGet(index);
             return GetAsStringArray(values);
         }
 
         /// <devdoc>
         /// <para>Gets the key at the specified index of the <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public virtual string GetKey(int index)
+        public virtual string? GetKey(int index)
         {
             return BaseGetKey(index);
         }
@@ -381,7 +384,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Represents the entry at the specified index of the <see cref='System.Collections.Specialized.NameValueCollection'/>.</para>
         /// </devdoc>
-        public string this[int index]
+        public string? this[int index]
         {
             get
             {
@@ -396,7 +399,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Gets all the keys in the <see cref='System.Collections.Specialized.NameValueCollection'/>. </para>
         /// </devdoc>
-        public virtual string[] AllKeys
+        public virtual string?[] AllKeys
         {
             get
             {
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/OrderedDictionary.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/OrderedDictionary.cs
index 30c9b96..870f528 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/OrderedDictionary.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/OrderedDictionary.cs
@@ -24,12 +24,12 @@ namespace System.Collections.Specialized
     [System.Runtime.CompilerServices.TypeForwardedFrom("System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
     public class OrderedDictionary : IOrderedDictionary, ISerializable, IDeserializationCallback
     {
-        private ArrayList _objectsArray;
-        private Hashtable _objectsTable;
+        private ArrayList? _objectsArray;
+        private Hashtable? _objectsTable;
         private int _initialCapacity;
-        private IEqualityComparer _comparer;
+        private IEqualityComparer? _comparer;
         private bool _readOnly;
-        private readonly SerializationInfo _siInfo; //A temporary variable which we need during deserialization.
+        private readonly SerializationInfo? _siInfo; //A temporary variable which we need during deserialization.
 
         private const string KeyComparerName = "KeyComparer"; // Do not rename (binary serialization)
         private const string ArrayListName = "ArrayList"; // Do not rename (binary serialization)
@@ -44,11 +44,11 @@ namespace System.Collections.Specialized
         {
         }
 
-        public OrderedDictionary(IEqualityComparer comparer) : this(0, comparer)
+        public OrderedDictionary(IEqualityComparer? comparer) : this(0, comparer)
         {
         }
 
-        public OrderedDictionary(int capacity, IEqualityComparer comparer)
+        public OrderedDictionary(int capacity, IEqualityComparer? comparer)
         {
             _initialCapacity = capacity;
             _comparer = comparer;
@@ -128,26 +128,14 @@ namespace System.Collections.Specialized
         {
             get
             {
-                EnsureObjectsArray();
-                return new OrderedDictionaryKeyValueCollection(_objectsArray, true);
+                ArrayList objectsArray = EnsureObjectsArray();
+                return new OrderedDictionaryKeyValueCollection(objectsArray, true);
             }
         }
 
-        private void EnsureObjectsArray()
-        {
-            if (_objectsArray == null)
-            {
-                _objectsArray = new ArrayList(_initialCapacity);
-            }
-        }
+        private ArrayList EnsureObjectsArray() => _objectsArray ??= new ArrayList(_initialCapacity);
 
-        private void EnsureObjectsTable()
-        {
-            if (_objectsTable == null)
-            {
-                _objectsTable = new Hashtable(_initialCapacity, _comparer);
-            }
-        }
+        private Hashtable EnsureObjectsTable() => _objectsTable ??= new Hashtable(_initialCapacity, _comparer);
 
         /// <devdoc>
         /// The SyncRoot object.  Not used because IsSynchronized is false
@@ -157,12 +145,12 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// Gets or sets the object at the specified index
         /// </devdoc>
-        public object this[int index]
+        public object? this[int index]
         {
             get
             {
-                EnsureObjectsArray();
-                return ((DictionaryEntry)_objectsArray[index]).Value;
+                ArrayList objectsArray = EnsureObjectsArray();
+                return ((DictionaryEntry)objectsArray[index]!).Value;
             }
             set
             {
@@ -174,18 +162,18 @@ namespace System.Collections.Specialized
                 {
                     throw new ArgumentOutOfRangeException(nameof(index));
                 }
-                EnsureObjectsArray();
-                EnsureObjectsTable();
-                object key = ((DictionaryEntry)_objectsArray[index]).Key;
-                _objectsArray[index] = new DictionaryEntry(key, value);
-                _objectsTable[key] = value;
+                ArrayList objectsArray = EnsureObjectsArray();
+                Hashtable objectsTable = EnsureObjectsTable();
+                object key = ((DictionaryEntry)objectsArray[index]!).Key;
+                objectsArray[index] = new DictionaryEntry(key, value);
+                objectsTable[key] = value;
             }
         }
 
         /// <devdoc>
         /// Gets or sets the object with the specified key
         /// </devdoc>
-        public object this[object key]
+        public object? this[object key]
         {
             get
             {
@@ -201,12 +189,12 @@ namespace System.Collections.Specialized
                 {
                     throw new NotSupportedException(SR.OrderedDictionary_ReadOnly);
                 }
-                EnsureObjectsTable();
-                if (_objectsTable.Contains(key))
+                Hashtable objectsTable = EnsureObjectsTable();
+                if (objectsTable.Contains(key))
                 {
-                    _objectsTable[key] = value;
-                    EnsureObjectsArray();
-                    _objectsArray[IndexOfKey(key)] = new DictionaryEntry(key, value);
+                    objectsTable[key] = value;
+                    ArrayList objectsArray = EnsureObjectsArray();
+                    objectsArray[IndexOfKey(key)] = new DictionaryEntry(key, value);
                 }
                 else
                 {
@@ -222,24 +210,24 @@ namespace System.Collections.Specialized
         {
             get
             {
-                EnsureObjectsArray();
-                return new OrderedDictionaryKeyValueCollection(_objectsArray, false);
+                ArrayList objectsArray = EnsureObjectsArray();
+                return new OrderedDictionaryKeyValueCollection(objectsArray, false);
             }
         }
 
         /// <devdoc>
         /// Adds a new entry to the table with the lowest-available index.
         /// </devdoc>
-        public void Add(object key, object value)
+        public void Add(object key, object? value)
         {
             if (_readOnly)
             {
                 throw new NotSupportedException(SR.OrderedDictionary_ReadOnly);
             }
-            EnsureObjectsTable();
-            EnsureObjectsArray();
-            _objectsTable.Add(key, value);
-            _objectsArray.Add(new DictionaryEntry(key, value));
+            Hashtable objectsTable = EnsureObjectsTable();
+            ArrayList objectsArray = EnsureObjectsArray();
+            objectsTable.Add(key, value);
+            objectsArray.Add(new DictionaryEntry(key, value));
         }
 
         /// <devdoc>
@@ -290,8 +278,8 @@ namespace System.Collections.Specialized
         /// </devdoc>
         public void CopyTo(Array array, int index)
         {
-            EnsureObjectsTable();
-            _objectsTable.CopyTo(array, index);
+            Hashtable objectsTable = EnsureObjectsTable();
+            objectsTable.CopyTo(array, index);
         }
 
         private int IndexOfKey(object key)
@@ -302,7 +290,7 @@ namespace System.Collections.Specialized
             }
             for (int i = 0; i < _objectsArray.Count; i++)
             {
-                object o = ((DictionaryEntry)_objectsArray[i]).Key;
+                object o = ((DictionaryEntry)_objectsArray[i]!).Key;
                 if (_comparer != null)
                 {
                     if (_comparer.Equals(o, key))
@@ -324,7 +312,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// Inserts a new object at the given index with the given key.
         /// </devdoc>
-        public void Insert(int index, object key, object value)
+        public void Insert(int index, object key, object? value)
         {
             if (_readOnly)
             {
@@ -334,10 +322,10 @@ namespace System.Collections.Specialized
             {
                 throw new ArgumentOutOfRangeException(nameof(index));
             }
-            EnsureObjectsTable();
-            EnsureObjectsArray();
-            _objectsTable.Add(key, value);
-            _objectsArray.Insert(index, new DictionaryEntry(key, value));
+            Hashtable objectsTable = EnsureObjectsTable();
+            ArrayList objectsArray = EnsureObjectsArray();
+            objectsTable.Add(key, value);
+            objectsArray.Insert(index, new DictionaryEntry(key, value));
         }
 
         /// <devdoc>
@@ -353,11 +341,11 @@ namespace System.Collections.Specialized
             {
                 throw new ArgumentOutOfRangeException(nameof(index));
             }
-            // The 'index >= Count' check above ensures
-            // that the '_objectsArray' and '_objectsTable' objects are initialized.
-            object key = ((DictionaryEntry)_objectsArray[index]).Key;
-            _objectsArray.RemoveAt(index);
-            _objectsTable.Remove(key);
+            Hashtable objectsTable = EnsureObjectsTable();
+            ArrayList objectsArray = EnsureObjectsArray();
+            object key = ((DictionaryEntry)objectsArray[index]!).Key;
+            objectsArray.RemoveAt(index);
+            objectsTable.Remove(key);
         }
 
         /// <devdoc>
@@ -380,25 +368,25 @@ namespace System.Collections.Specialized
                 return;
             }
 
-            EnsureObjectsTable();
-            EnsureObjectsArray();
-            _objectsTable.Remove(key);
-            _objectsArray.RemoveAt(index);
+            Hashtable objectsTable = EnsureObjectsTable();
+            ArrayList objectsArray = EnsureObjectsArray();
+            objectsTable.Remove(key);
+            objectsArray.RemoveAt(index);
         }
 
 #region IDictionary implementation
         public virtual IDictionaryEnumerator GetEnumerator()
         {
-            EnsureObjectsArray();
-            return new OrderedDictionaryEnumerator(_objectsArray, OrderedDictionaryEnumerator.DictionaryEntry);
+            ArrayList objectsArray = EnsureObjectsArray();
+            return new OrderedDictionaryEnumerator(objectsArray, OrderedDictionaryEnumerator.DictionaryEntry);
         }
 #endregion
 
 #region IEnumerable implementation
         IEnumerator IEnumerable.GetEnumerator()
         {
-            EnsureObjectsArray();
-            return new OrderedDictionaryEnumerator(_objectsArray, OrderedDictionaryEnumerator.DictionaryEntry);
+            ArrayList objectsArray = EnsureObjectsArray();
+            return new OrderedDictionaryEnumerator(objectsArray, OrderedDictionaryEnumerator.DictionaryEntry);
         }
 #endregion
 
@@ -415,34 +403,34 @@ namespace System.Collections.Specialized
             info.AddValue(InitCapacityName, _initialCapacity);
 
             object[] serArray = new object[Count];
-            EnsureObjectsArray();
-            _objectsArray.CopyTo(serArray);
+            ArrayList objectsArray = EnsureObjectsArray();
+            objectsArray.CopyTo(serArray);
             info.AddValue(ArrayListName, serArray);
         }
 #endregion
 
 #region IDeserializationCallback implementation
-        void IDeserializationCallback.OnDeserialization(object sender)
+        void IDeserializationCallback.OnDeserialization(object? sender)
         {
             OnDeserialization(sender);
         }
 
-        protected virtual void OnDeserialization(object sender)
+        protected virtual void OnDeserialization(object? sender)
         {
             if (_siInfo == null)
             {
                 throw new SerializationException(SR.Serialization_InvalidOnDeser);
             }
-            _comparer = (IEqualityComparer)_siInfo.GetValue(KeyComparerName, typeof(IEqualityComparer));
+            _comparer = (IEqualityComparer?)_siInfo.GetValue(KeyComparerName, typeof(IEqualityComparer));
             _readOnly = _siInfo.GetBoolean(ReadOnlyName);
             _initialCapacity = _siInfo.GetInt32(InitCapacityName);
 
-            object[] serArray = (object[])_siInfo.GetValue(ArrayListName, typeof(object[]));
+            object[]? serArray = (object[]?)_siInfo.GetValue(ArrayListName, typeof(object[]));
 
             if (serArray != null)
             {
-                EnsureObjectsTable();
-                EnsureObjectsArray();
+                Hashtable objectsTable = EnsureObjectsTable();
+                ArrayList objectsArray = EnsureObjectsArray();
                 foreach (object o in serArray)
                 {
                     DictionaryEntry entry;
@@ -455,8 +443,8 @@ namespace System.Collections.Specialized
                     {
                         throw new SerializationException(SR.OrderedDictionary_SerializationMismatch);
                     }
-                    _objectsArray.Add(entry);
-                    _objectsTable.Add(entry.Key, entry.Value);
+                    objectsArray.Add(entry);
+                    objectsTable.Add(entry.Key, entry.Value);
                 }
             }
         }
@@ -483,10 +471,11 @@ namespace System.Collections.Specialized
             /// <devdoc>
             /// Retrieves the current DictionaryEntry.  This is the same as Entry, but not strongly-typed.
             /// </devdoc>
-            public object Current
+            public object? Current
             {
                 get
                 {
+                    Debug.Assert(_arrayEnumerator.Current != null);
                     if (_objectReturnType == Keys)
                     {
                         return ((DictionaryEntry)_arrayEnumerator.Current).Key;
@@ -506,6 +495,7 @@ namespace System.Collections.Specialized
             {
                 get
                 {
+                    Debug.Assert(_arrayEnumerator.Current != null);
                     return new DictionaryEntry(((DictionaryEntry)_arrayEnumerator.Current).Key, ((DictionaryEntry)_arrayEnumerator.Current).Value);
                 }
             }
@@ -517,6 +507,7 @@ namespace System.Collections.Specialized
             {
                 get
                 {
+                    Debug.Assert(_arrayEnumerator.Current != null);
                     return ((DictionaryEntry)_arrayEnumerator.Current).Key;
                 }
             }
@@ -524,10 +515,11 @@ namespace System.Collections.Specialized
             /// <devdoc>
             /// Retrieves the value of the current DictionaryEntry
             /// </devdoc>
-            public object Value
+            public object? Value
             {
                 get
                 {
+                    Debug.Assert(_arrayEnumerator.Current != null);
                     return ((DictionaryEntry)_arrayEnumerator.Current).Value;
                 }
             }
@@ -571,8 +563,9 @@ namespace System.Collections.Specialized
                     throw new ArgumentNullException(nameof(array));
                 if (index < 0)
                     throw new ArgumentOutOfRangeException(nameof(index), index, SR.ArgumentOutOfRange_NeedNonNegNum_Index);
-                foreach (object o in _objects)
+                foreach (object? o in _objects)
                 {
+                    Debug.Assert(o != null);
                     array.SetValue(_isKeys ? ((DictionaryEntry)o).Key : ((DictionaryEntry)o).Value, index);
                     index++;
                 }
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringCollection.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringCollection.cs
index 4e4e1eb..9fd899d 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringCollection.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringCollection.cs
@@ -17,11 +17,11 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Represents the entry at the specified index of the <see cref='System.Collections.Specialized.StringCollection'/>.</para>
         /// </devdoc>
-        public string this[int index]
+        public string? this[int index]
         {
             get
             {
-                return ((string)data[index]);
+                return ((string?)data[index]);
             }
             set
             {
@@ -62,7 +62,7 @@ namespace System.Collections.Specialized
         ///    <para>Adds a string with the specified value to the
         ///    <see cref='System.Collections.Specialized.StringCollection'/> .</para>
         /// </devdoc>
-        public int Add(string value)
+        public int Add(string? value)
         {
             return data.Add(value);
         }
@@ -93,7 +93,7 @@ namespace System.Collections.Specialized
         ///    <see cref='System.Collections.Specialized.StringCollection'/> contains a string with the specified
         ///       value.</para>
         /// </devdoc>
-        public bool Contains(string value)
+        public bool Contains(string? value)
         {
             return data.Contains(value);
         }
@@ -120,7 +120,7 @@ namespace System.Collections.Specialized
         ///    <para>Returns the index of the first occurrence of a string in
         ///       the <see cref='System.Collections.Specialized.StringCollection'/> .</para>
         /// </devdoc>
-        public int IndexOf(string value)
+        public int IndexOf(string? value)
         {
             return data.IndexOf(value);
         }
@@ -129,7 +129,7 @@ namespace System.Collections.Specialized
         /// <para>Inserts a string into the <see cref='System.Collections.Specialized.StringCollection'/> at the specified
         ///    index.</para>
         /// </devdoc>
-        public void Insert(int index, string value)
+        public void Insert(int index, string? value)
         {
             data.Insert(index, value);
         }
@@ -162,7 +162,7 @@ namespace System.Collections.Specialized
         ///    <para> Removes a specific string from the
         ///    <see cref='System.Collections.Specialized.StringCollection'/> .</para>
         /// </devdoc>
-        public void Remove(string value)
+        public void Remove(string? value)
         {
             data.Remove(value);
         }
@@ -186,7 +186,7 @@ namespace System.Collections.Specialized
             }
         }
 
-        object IList.this[int index]
+        object? IList.this[int index]
         {
             get
             {
@@ -194,34 +194,34 @@ namespace System.Collections.Specialized
             }
             set
             {
-                this[index] = (string)value;
+                this[index] = (string?)value;
             }
         }
 
-        int IList.Add(object value)
+        int IList.Add(object? value)
         {
-            return Add((string)value);
+            return Add((string?)value);
         }
 
-        bool IList.Contains(object value)
+        bool IList.Contains(object? value)
         {
-            return Contains((string)value);
+            return Contains((string?)value);
         }
 
 
-        int IList.IndexOf(object value)
+        int IList.IndexOf(object? value)
         {
-            return IndexOf((string)value);
+            return IndexOf((string?)value);
         }
 
-        void IList.Insert(int index, object value)
+        void IList.Insert(int index, object? value)
         {
-            Insert(index, (string)value);
+            Insert(index, (string?)value);
         }
 
-        void IList.Remove(object value)
+        void IList.Remove(object? value)
         {
-            Remove((string)value);
+            Remove((string?)value);
         }
 
         void ICollection.CopyTo(Array array, int index)
@@ -246,11 +246,11 @@ namespace System.Collections.Specialized
             _baseEnumerator = _temp.GetEnumerator();
         }
 
-        public string Current
+        public string? Current
         {
             get
             {
-                return (string)(_baseEnumerator.Current);
+                return (string?)(_baseEnumerator.Current);
             }
         }
 
diff --git a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringDictionary.cs b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringDictionary.cs
index bfec498..011f334 100644
--- a/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringDictionary.cs
+++ b/src/libraries/System.Collections.Specialized/src/System/Collections/Specialized/StringDictionary.cs
@@ -59,7 +59,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         ///    <para>Gets or sets the value associated with the specified key.</para>
         /// </devdoc>
-        public virtual string this[string key]
+        public virtual string? this[string key]
         {
             get
             {
@@ -68,7 +68,7 @@ namespace System.Collections.Specialized
                     throw new ArgumentNullException(nameof(key));
                 }
 
-                return (string)contents[key.ToLowerInvariant()];
+                return (string?)contents[key.ToLowerInvariant()];
             }
             set
             {
@@ -118,7 +118,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Adds an entry with the specified key and value into the StringDictionary.</para>
         /// </devdoc>
-        public virtual void Add(string key, string value)
+        public virtual void Add(string key, string? value)
         {
             if (key == null)
             {
@@ -152,7 +152,7 @@ namespace System.Collections.Specialized
         /// <devdoc>
         /// <para>Determines if the StringDictionary contains a specific value.</para>
         /// </devdoc>
-        public virtual bool ContainsValue(string value)
+        public virtual bool ContainsValue(string? value)
         {
             return contents.ContainsValue(value);
         }