diff --git a/src/coreclr/src/System.Private.CoreLib/src/System/Environment.CoreCLR.cs b/src/coreclr/src/System.Private.CoreLib/src/System/Environment.CoreCLR.cs
index 59b2bd9d79040b..f141b1e9cfeb73 100644
--- a/src/coreclr/src/System.Private.CoreLib/src/System/Environment.CoreCLR.cs
+++ b/src/coreclr/src/System.Private.CoreLib/src/System/Environment.CoreCLR.cs
@@ -16,6 +16,7 @@ public static partial class Environment
 
         // Terminates this process with the given exit code.
         [DllImport(RuntimeHelpers.QCall, CharSet = CharSet.Unicode)]
+        [DoesNotReturn]
         private static extern void _Exit(int exitCode);
 
         [DoesNotReturn]
diff --git a/src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs b/src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs
index a21c8d894b12d5..7a9b6c0f1d50b8 100644
--- a/src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs
+++ b/src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.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.
 
+#nullable enable
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
 namespace System.Collections.Generic
@@ -15,7 +17,7 @@ private ReferenceEqualityComparer()
         {
         }
 
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             return ReferenceEquals(x, y);
         }
diff --git a/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs b/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
index ff155065feee20..4ef5ecdf2927a3 100644
--- a/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
+++ b/src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs
@@ -918,11 +918,13 @@ public TValue this[TKey key]
         // as these are uncommonly needed and when inlined are observed to prevent the inlining
         // of important methods like TryGetValue and ContainsKey.
 
+        [DoesNotReturn]
         private static void ThrowKeyNotFoundException(object key)
         {
             throw new KeyNotFoundException(SR.Format(SR.Arg_KeyNotFoundWithKey, key.ToString()));
         }
 
+        [DoesNotReturn]
         private static void ThrowKeyNullException()
         {
             throw new ArgumentNullException("key");
diff --git a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/IImmutableListQueries.cs b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/IImmutableListQueries.cs
index 36bcbe5e0e0021..0a6e1f904d34ad 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/IImmutableListQueries.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/IImmutableListQueries.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Collections.Immutable
 {
@@ -120,6 +121,7 @@ internal interface IImmutableListQueries<T> : IReadOnlyList<T>
         /// The first element that matches the conditions defined by the specified predicate,
         /// if found; otherwise, the default value for type <typeparamref name="T"/>.
         /// </returns>
+        [return: MaybeNull]
         T Find(Predicate<T> match);
 
         /// <summary>
@@ -193,6 +195,7 @@ internal interface IImmutableListQueries<T> : IReadOnlyList<T>
         /// The last element that matches the conditions defined by the specified predicate,
         /// if found; otherwise, the default value for type <typeparamref name="T"/>.
         /// </returns>
+        [return: MaybeNull]
         T FindLast(Predicate<T> match);
 
         /// <summary>
diff --git a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs
index 8fc536af27da4a..e740e1afc442aa 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs
@@ -221,7 +221,7 @@ public ImmutableHashSet<T> Remove(T item)
         [Pure]
         public bool TryGetValue(T equalValue, out T actualValue)
         {
-            int hashCode = _equalityComparer.GetHashCode(equalValue);
+            int hashCode = equalValue != null ? _equalityComparer.GetHashCode(equalValue) : 0;
             HashBucket bucket;
             if (_root.TryGetValue(hashCode, out bucket))
             {
@@ -639,7 +639,7 @@ private static bool IsSupersetOf(IEnumerable<T> other, MutationInput origin)
         private static MutationResult Add(T item, MutationInput origin)
         {
             OperationResult result;
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket = origin.Root.GetValueOrDefault(hashCode);
             var newBucket = bucket.Add(item, origin.EqualityComparer, out result);
             if (result == OperationResult.NoChangeRequired)
@@ -658,7 +658,7 @@ private static MutationResult Add(T item, MutationInput origin)
         private static MutationResult Remove(T item, MutationInput origin)
         {
             var result = OperationResult.NoChangeRequired;
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket;
             var newRoot = origin.Root;
             if (origin.Root.TryGetValue(hashCode, out bucket))
@@ -680,7 +680,7 @@ private static MutationResult Remove(T item, MutationInput origin)
         /// </summary>
         private static bool Contains(T item, MutationInput origin)
         {
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket;
             if (origin.Root.TryGetValue(hashCode, out bucket))
             {
@@ -701,7 +701,7 @@ private static MutationResult Union(IEnumerable<T> other, MutationInput origin)
             var newRoot = origin.Root;
             foreach (var item in other.GetEnumerableDisposable<T, Enumerator>())
             {
-                int hashCode = origin.EqualityComparer.GetHashCode(item);
+                int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
                 HashBucket bucket = newRoot.GetValueOrDefault(hashCode);
                 OperationResult result;
                 var newBucket = bucket.Add(item, origin.EqualityComparer, out result);
@@ -812,7 +812,7 @@ private static MutationResult Except(IEnumerable<T> other, IEqualityComparer<T>
             var newRoot = root;
             foreach (var item in other.GetEnumerableDisposable<T, Enumerator>())
             {
-                int hashCode = equalityComparer.GetHashCode(item);
+                int hashCode = item != null ? equalityComparer.GetHashCode(item) : 0;
                 HashBucket bucket;
                 if (newRoot.TryGetValue(hashCode, out bucket))
                 {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/HashSetEqualityComparer.cs b/src/libraries/System.Collections/src/System/Collections/Generic/HashSetEqualityComparer.cs
index 5cfae13f3f31ec..8415e534569903 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/HashSetEqualityComparer.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/HashSetEqualityComparer.cs
@@ -30,7 +30,10 @@ public int GetHashCode(HashSet<T>? obj)
             {
                 foreach (T t in obj)
                 {
-                    hashCode = hashCode ^ (_comparer.GetHashCode(t) & 0x7FFFFFFF);
+                    if (t != null)
+                    {
+                        hashCode ^= (_comparer.GetHashCode(t) & 0x7FFFFFFF);
+                    }
                 }
             } // else returns hashcode of 0 for null hashsets
             return hashCode;
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
index bcbc46e8b7042b..4611785eb9ed80 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
@@ -341,7 +341,7 @@ internal override int TotalCount()
             // This passes functionality down to the underlying tree, clipping edges if necessary
             // There's nothing gained by having a nested subset. May as well draw it from the base
             // Cannot increase the bounds of the subset, can only decrease it
-            public override SortedSet<T> GetViewBetween(T lowerValue, T upperValue)
+            public override SortedSet<T> GetViewBetween([AllowNull] T lowerValue, [AllowNull] T upperValue)
             {
                 if (_lBoundActive && Comparer.Compare(_min, lowerValue) > 0)
                 {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
index d82c4052aa86ec..c896834c997103 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs
@@ -806,7 +806,7 @@ public static IEqualityComparer<SortedSet<T>> CreateSetComparer(IEqualityCompare
         /// <param name="set2">The second set.</param>
         /// <param name="comparer">The fallback comparer to use if the sets do not have equal comparers.</param>
         /// <returns><c>true</c> if the sets have equal contents; otherwise, <c>false</c>.</returns>
-        internal static bool SortedSetEquals(SortedSet<T> set1, SortedSet<T> set2, IComparer<T> comparer)
+        internal static bool SortedSetEquals(SortedSet<T>? set1, SortedSet<T>? set2, IComparer<T> comparer)
         {
             if (set1 == null)
             {
diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSetEqualityComparer.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSetEqualityComparer.cs
index e0e59ff8704c95..dc262258981d24 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSetEqualityComparer.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSetEqualityComparer.cs
@@ -27,7 +27,7 @@ private SortedSetEqualityComparer(IComparer<T>? comparer, IEqualityComparer<T>?
         }
 
         // Use _comparer to keep equals properties intact; don't want to choose one of the comparers.
-        public bool Equals(SortedSet<T> x, SortedSet<T> y) => SortedSet<T>.SortedSetEquals(x, y, _comparer);
+        public bool Equals(SortedSet<T>? x, SortedSet<T>? y) => SortedSet<T>.SortedSetEquals(x, y, _comparer);
 
         // IMPORTANT: this part uses the fact that GetHashCode() is consistent with the notion of equality in the set.
         public int GetHashCode(SortedSet<T> obj)
@@ -37,7 +37,10 @@ public int GetHashCode(SortedSet<T> obj)
             {
                 foreach (T t in obj)
                 {
-                    hashCode = hashCode ^ (_memberEqualityComparer.GetHashCode(t) & 0x7FFFFFFF);
+                    if (t != null)
+                    {
+                        hashCode ^= (_memberEqualityComparer.GetHashCode(t) & 0x7FFFFFFF);
+                    }
                 }
             }
             // Returns 0 for null sets.
diff --git a/src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs b/src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs
index 71a388e0e25e3c..6a1af13a3d4ae2 100644
--- a/src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs
+++ b/src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs
@@ -6,6 +6,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics;
 using System.Linq;
 
 namespace Microsoft.Internal.Collections
@@ -144,11 +145,13 @@ private static List<T> FastAppendToListAllowNulls<T>(this List<T>? source, T val
         }
 
         public static List<T>? FastAppendToListAllowNulls<T>(
-                        this List<T>? source, T value,
+                        this List<T>? source, T? value,
                         IEnumerable<T>? second)
+            where T : class
         {
             if (second == null)
             {
+                Debug.Assert(value != null);
                 source = source.FastAppendToListAllowNulls(value);
             }
             else
diff --git a/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Primitives/ComposablePartDefinition.cs b/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Primitives/ComposablePartDefinition.cs
index 039e4642888196..31d7d84f6ced12 100644
--- a/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Primitives/ComposablePartDefinition.cs
+++ b/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Primitives/ComposablePartDefinition.cs
@@ -103,7 +103,7 @@ protected ComposablePartDefinition()
         /// </remarks>
         public abstract ComposablePart CreatePart();
 
-        internal virtual bool TryGetExports(ImportDefinition definition, [NotNullWhen(true)] out Tuple<ComposablePartDefinition, ExportDefinition>? singleMatch, out IEnumerable<Tuple<ComposablePartDefinition, ExportDefinition>>? multipleMatches)
+        internal virtual bool TryGetExports(ImportDefinition definition, out Tuple<ComposablePartDefinition, ExportDefinition>? singleMatch, out IEnumerable<Tuple<ComposablePartDefinition, ExportDefinition>>? multipleMatches)
         {
             singleMatch = null;
             multipleMatches = null;
diff --git a/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs b/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs
index ebc0705dd8cde5..0f02be5bb1d2e3 100644
--- a/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs
+++ b/src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs
@@ -23,13 +23,13 @@ internal class ImportType
         private readonly bool _isOpenGeneric = false;
 
         [ThreadStatic]
-        internal static Dictionary<Type, Func<Export, object>>? _castSingleValueCache;
+        internal static Dictionary<Type, Func<Export, object>?>? _castSingleValueCache;
 
-        private static Dictionary<Type, Func<Export, object>> CastSingleValueCache
+        private static Dictionary<Type, Func<Export, object>?> CastSingleValueCache
         {
             get
             {
-                return _castSingleValueCache = _castSingleValueCache ?? new Dictionary<Type, Func<Export, object>>();
+                return _castSingleValueCache = _castSingleValueCache ?? new Dictionary<Type, Func<Export, object>?>();
             }
         }
 
@@ -170,7 +170,7 @@ private static bool IsLazyGenericType(Type genericType)
             return (genericType == LazyOfTType) || (genericType == LazyOfTMType);
         }
 
-        private static bool TryGetCastFunction(Type genericType, bool isOpenGeneric, Type[] arguments, [NotNullWhen(true)] out Func<Export, object>? castFunction)
+        private static bool TryGetCastFunction(Type genericType, bool isOpenGeneric, Type[] arguments, out Func<Export, object>? castFunction)
         {
             castFunction = null;
 
diff --git a/src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs b/src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs
index cd655d9f0366cc..329e585226469f 100644
--- a/src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs
+++ b/src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs
@@ -86,7 +86,10 @@ public static int ListHashCode<T>(this ReadOnlyCollection<T> list)
             int h = 6551;
             foreach (T t in list)
             {
-                h ^= (h << 5) ^ cmp.GetHashCode(t);
+                if (t != null)
+                {
+                    h ^= (h << 5) ^ cmp.GetHashCode(t);
+                }
             }
             return h;
         }
diff --git a/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Interpreter/LightCompiler.cs b/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Interpreter/LightCompiler.cs
index 61520cc3599251..4959f96de8267c 100644
--- a/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Interpreter/LightCompiler.cs
+++ b/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Interpreter/LightCompiler.cs
@@ -108,7 +108,7 @@ internal bool HasHandler(InterpretedFrame frame, Exception exception, [NotNullWh
                 // Unreachable.
                 // Want to assert that this case isn't hit, but an assertion failure here will be eaten because
                 // we are in an exception filter. Therefore return true here and assert in the catch block.
-                handler = null;
+                handler = null!;
                 unwrappedException = exception;
                 return true;
             }
@@ -215,8 +215,9 @@ internal sealed class DebugInfo
         private class DebugInfoComparer : IComparer<DebugInfo>
         {
             //We allow comparison between int and DebugInfo here
-            int IComparer<DebugInfo>.Compare(DebugInfo d1, DebugInfo d2)
+            int IComparer<DebugInfo>.Compare(DebugInfo? d1, DebugInfo? d2)
             {
+                Debug.Assert(d1 != null && d2 != null);
                 if (d1.Index > d2.Index) return 1;
                 else if (d1.Index == d2.Index) return 0;
                 else return -1;
diff --git a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Partitioning/HashRepartitionStream.cs b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Partitioning/HashRepartitionStream.cs
index 0314d2bac60742..e90a0a9002585b 100644
--- a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Partitioning/HashRepartitionStream.cs
+++ b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Partitioning/HashRepartitionStream.cs
@@ -91,19 +91,16 @@ internal int GetHashCode(TInputOutput element)
         {
             return
                 (HashCodeMask &
-                    (_elementComparer == null ?
-                        (element == null ? NULL_ELEMENT_HASH_CODE : element.GetHashCode()) :
-                        _elementComparer.GetHashCode(element)))
-                        % _distributionMod;
+                    (element == null ? NULL_ELEMENT_HASH_CODE : (_elementComparer?.GetHashCode(element) ?? element.GetHashCode())))
+                % _distributionMod;
         }
 
         internal int GetHashCode(THashKey key)
         {
             return
                 (HashCodeMask &
-                    (_keyComparer == null ?
-                        (key == null ? NULL_ELEMENT_HASH_CODE : key.GetHashCode()) :
-                        _keyComparer.GetHashCode(key))) % _distributionMod;
+                    (key == null ? NULL_ELEMENT_HASH_CODE : (_keyComparer?.GetHashCode(key) ?? key.GetHashCode())))
+                % _distributionMod;
         }
     }
 }
diff --git a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Unary/ForAllOperator.cs b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Unary/ForAllOperator.cs
index 116170fd1d9de1..c50b7fc481c033 100644
--- a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Unary/ForAllOperator.cs
+++ b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Unary/ForAllOperator.cs
@@ -147,7 +147,7 @@ internal ForAllEnumerator(QueryOperatorEnumerator<TInput, TKey> source, Action<T
             // element action for each element.
             //
 
-            internal override bool MoveNext([MaybeNull, AllowNull] ref TInput currentElement, ref int currentKey)
+            internal override bool MoveNext([MaybeNullWhen(false), AllowNull] ref TInput currentElement, ref int currentKey)
             {
                 Debug.Assert(_elementAction != null, "expected a compiled operator");
 
@@ -159,14 +159,13 @@ internal override bool MoveNext([MaybeNull, AllowNull] ref TInput currentElement
                 TInput element = default(TInput)!;
                 TKey keyUnused = default(TKey)!;
                 int i = 0;
-                while (_source.MoveNext(ref element!, ref keyUnused))
+                while (_source.MoveNext(ref element, ref keyUnused))
                 {
                     if ((i++ & CancellationState.POLL_INTERVAL) == 0)
                         CancellationState.ThrowIfCanceled(_cancellationToken);
                     _elementAction(element);
                 }
 
-
                 return false;
             }
 
diff --git a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/HashLookup.cs b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/HashLookup.cs
index fca8b05d10d735..2bfa32b04bd3f3 100644
--- a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/HashLookup.cs
+++ b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/HashLookup.cs
@@ -64,9 +64,7 @@ internal TValue this[TKey key]
         private int GetKeyHashCode(TKey key)
         {
             return HashCodeMask &
-                (comparer == null ?
-                    (key == null ? 0 : key.GetHashCode()) :
-                    comparer.GetHashCode(key));
+                (key == null ? 0 : (comparer?.GetHashCode(key) ?? key.GetHashCode()));
         }
 
         private bool AreKeysEqual(TKey key1, TKey key2)
diff --git a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/ReverseComparer.cs b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/ReverseComparer.cs
index e484479269e960..a5c847575b96c0 100644
--- a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/ReverseComparer.cs
+++ b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/ReverseComparer.cs
@@ -9,6 +9,7 @@
 // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Linq.Parallel
 {
@@ -26,7 +27,7 @@ internal ReverseComparer(IComparer<T> comparer)
             _comparer = comparer;
         }
 
-        public int Compare(T x, T y)
+        public int Compare([AllowNull] T x, [AllowNull] T y)
         {
             return _comparer.Compare(y, x);
         }
diff --git a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/WrapperEqualityComparer.cs b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/WrapperEqualityComparer.cs
index 4f447720e260c6..b203042f5791ad 100644
--- a/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/WrapperEqualityComparer.cs
+++ b/src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/WrapperEqualityComparer.cs
@@ -43,7 +43,8 @@ public bool Equals(Wrapper<T> x, Wrapper<T> y)
         public int GetHashCode(Wrapper<T> x)
         {
             Debug.Assert(_comparer != null);
-            return _comparer.GetHashCode(x.Value);
+            T value = x.Value;
+            return value == null ? 0 : _comparer.GetHashCode(value);
         }
     }
 }
diff --git a/src/libraries/System.Linq/src/System/Linq/Partition.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/Partition.SpeedOpt.cs
index 4c59749a7dea15..2f61a5d61670d9 100644
--- a/src/libraries/System.Linq/src/System/Linq/Partition.SpeedOpt.cs
+++ b/src/libraries/System.Linq/src/System/Linq/Partition.SpeedOpt.cs
@@ -36,7 +36,6 @@ private EmptyPartition()
         public bool MoveNext() => false;
 
         [ExcludeFromCodeCoverage] // Shouldn't be called, and as undefined can return or throw anything anyway.
-        [MaybeNull]
         public TElement Current => default!;
 
         [ExcludeFromCodeCoverage] // Shouldn't be called, and as undefined can return or throw anything anyway.
diff --git a/src/libraries/System.Private.CoreLib/src/System/Action.cs b/src/libraries/System.Private.CoreLib/src/System/Action.cs
index 54ca7aaf5323d5..1c4771a1b64ff7 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Action.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Action.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.CodeAnalysis;
+
 namespace System
 {
     public delegate void Action();
@@ -24,7 +26,7 @@ namespace System
     public delegate TResult Func<in T1, in T2, in T3, in T4, in T5, in T6, in T7, out TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7);
     public delegate TResult Func<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, out TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8);
 
-    public delegate int Comparison<in T>(T x, T y);
+    public delegate int Comparison<in T>([AllowNull] T x, [AllowNull] T y);
 
     public delegate TOutput Converter<in TInput, out TOutput>(TInput input);
 
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Comparer.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Comparer.cs
index ec2535e2010ec7..31cc4ece91b1d1 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Comparer.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Comparer.cs
@@ -43,7 +43,7 @@ public ComparisonComparer(Comparison<T> comparison)
             _comparison = comparison;
         }
 
-        public override int Compare(T x, T y)
+        public override int Compare([AllowNull] T x, [AllowNull] T y)
         {
             return _comparison(x, y);
         }
diff --git a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs
index 61a6bd1e7422f8..29fde08191e2ed 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs
@@ -8,7 +8,7 @@ namespace System.Collections.Generic
 {
     // The generic IEqualityComparer interface implements methods to if check two objects are equal
     // and generate Hashcode for an object.
-    // It is use in Dictionary class.
+    // It is used in Dictionary class.
     public interface IEqualityComparer<in T>
     {
         bool Equals([AllowNull] T x, [AllowNull] T y);
diff --git a/src/libraries/System.Private.CoreLib/src/System/Diagnostics/DebugProvider.cs b/src/libraries/System.Private.CoreLib/src/System/Diagnostics/DebugProvider.cs
index cf4dfe1effdb81..6d199d7f786d9a 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Diagnostics/DebugProvider.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Diagnostics/DebugProvider.cs
@@ -5,6 +5,8 @@
 // Do not remove this, it is needed to retain calls to these conditional methods in release builds
 #define DEBUG
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace System.Diagnostics
 {
     /// <summary>
@@ -12,6 +14,7 @@ namespace System.Diagnostics
     /// </summary>
     public partial class DebugProvider
     {
+        [DoesNotReturn]
         public virtual void Fail(string? message, string? detailMessage)
         {
             string stackTrace;
@@ -25,7 +28,9 @@ public virtual void Fail(string? message, string? detailMessage)
             }
             WriteAssert(stackTrace, message, detailMessage);
             FailCore(stackTrace, message, detailMessage, "Assertion failed.");
+#pragma warning disable 8763 // "A method marked [DoesNotReturn] should not return."
         }
+#pragma warning restore 8763
 
         internal void WriteAssert(string stackTrace, string? message, string? detailMessage)
         {
diff --git a/src/libraries/System.Private.CoreLib/src/System/HashCode.cs b/src/libraries/System.Private.CoreLib/src/System/HashCode.cs
index 0ff94adf17b981..6ea2c67d0e1eb9 100644
--- a/src/libraries/System.Private.CoreLib/src/System/HashCode.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/HashCode.cs
@@ -303,7 +303,7 @@ public void Add<T>(T value)
 
         public void Add<T>(T value, IEqualityComparer<T>? comparer)
         {
-            Add(comparer != null ? comparer.GetHashCode(value) : (value?.GetHashCode() ?? 0));
+            Add(value is null ? 0 : (comparer?.GetHashCode(value) ?? value.GetHashCode()));
         }
 
         private void Add(int value)
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
index 16d431cc2d2fd7..6fbb2f8cb9791e 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs
@@ -307,7 +307,9 @@ public static bool TryGetMemoryManager<T, TManager>(ReadOnlyMemory<T> memory, [N
         {
             TManager? localManager; // Use register for null comparison rather than byref
             manager = localManager = memory.GetObjectStartLength(out _, out _) as TManager;
+#pragma warning disable 8762 // "Parameter 'manager' may not have a null value when exiting with 'true'."
             return localManager != null;
+#pragma warning restore 8762
         }
 
         /// <summary>
@@ -335,7 +337,9 @@ public static bool TryGetMemoryManager<T, TManager>(ReadOnlyMemory<T> memory, [N
                 length = default;
                 return false;
             }
+#pragma warning disable 8762 // "Parameter 'manager' may not have a null value when exiting with 'true'."
             return true;
+#pragma warning restore 8762
         }
 
         /// <summary>
diff --git a/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.BinarySearch.cs b/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.BinarySearch.cs
index 385992fe45aeeb..b50bed9581f993 100644
--- a/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.BinarySearch.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.BinarySearch.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -75,7 +76,7 @@ public ComparerComparable(T value, TComparer comparer)
             }
 
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
-            public int CompareTo(T other) => _comparer.Compare(_value, other);
+            public int CompareTo([AllowNull] T other) => _comparer.Compare(_value, other);
         }
     }
 }
diff --git a/src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ByteSequenceComparer.cs b/src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ByteSequenceComparer.cs
index cd89135d49b20e..b97d68de6960ef 100644
--- a/src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ByteSequenceComparer.cs
+++ b/src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/ByteSequenceComparer.cs
@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Reflection.Internal
 {
@@ -62,7 +63,7 @@ internal static bool Equals(byte[] left, int leftStart, byte[] right, int rightS
             return true;
         }
 
-        internal static bool Equals(byte[] left, byte[] right)
+        internal static bool Equals(byte[]? left, byte[]? right)
         {
             if (ReferenceEquals(left, right))
             {
@@ -99,7 +100,7 @@ internal static int GetHashCode(ImmutableArray<byte> x)
             return Hash.GetFNVHashCode(x);
         }
 
-        bool IEqualityComparer<byte[]>.Equals(byte[] x, byte[] y)
+        bool IEqualityComparer<byte[]>.Equals(byte[]? x, byte[]? y)
         {
             return Equals(x, y);
         }
diff --git a/src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/RoAssemblyName.cs b/src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/RoAssemblyName.cs
index 293fb2aee61fb1..904d3cd67ee878 100644
--- a/src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/RoAssemblyName.cs
+++ b/src/libraries/System.Reflection.MetadataLoadContext/src/System/Reflection/TypeLoading/General/RoAssemblyName.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Reflection.TypeLoading
 {
@@ -49,8 +50,9 @@ public RoAssemblyName(string? name, Version? version, string? cultureName, byte[
         // Equality - this compares every bit of data in the RuntimeAssemblyName which is acceptable for use as keys in a cache
         // where semantic duplication is permissible. This method is *not* meant to define ref->def binding rules or
         // assembly binding unification rules.
-        public bool Equals(RoAssemblyName other)
+        public bool Equals(RoAssemblyName? other)
         {
+            Debug.Assert(other is object);
             if (Name != other.Name)
                 return false;
             if (Version != other.Version)
diff --git a/src/libraries/System.Resources.Writer/src/System/Resources/__FastResourceComparer.cs b/src/libraries/System.Resources.Writer/src/System/Resources/__FastResourceComparer.cs
index d884dec2a1e8b5..628e2e4b7e2c4c 100644
--- a/src/libraries/System.Resources.Writer/src/System/Resources/__FastResourceComparer.cs
+++ b/src/libraries/System.Resources.Writer/src/System/Resources/__FastResourceComparer.cs
@@ -6,6 +6,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Resources
 {
@@ -33,16 +34,14 @@ internal static int HashFunction(string key)
             return (int)hash;
         }
 
-        public int Compare(string a, string b)
+        public int Compare(string? a, string? b)
         {
             return string.CompareOrdinal(a, b);
         }
 
-        public bool Equals(string a, string b)
+        public bool Equals(string? a, string? b)
         {
             return string.Equals(a, b);
         }
-
-
     }
 }
diff --git a/src/libraries/System.Runtime.Extensions/ref/System.Runtime.Extensions.cs b/src/libraries/System.Runtime.Extensions/ref/System.Runtime.Extensions.cs
index a06a6075660e67..1414b85ce3145d 100644
--- a/src/libraries/System.Runtime.Extensions/ref/System.Runtime.Extensions.cs
+++ b/src/libraries/System.Runtime.Extensions/ref/System.Runtime.Extensions.cs
@@ -683,12 +683,12 @@ public static partial class Environment
         public static System.Version Version { get { throw null; } }
         public static long WorkingSet { get { throw null; } }
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
-        public static void Exit(int exitCode) { }
+        public static void Exit(int exitCode) => throw null; 
         public static string ExpandEnvironmentVariables(string name) { throw null; }
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
-        public static void FailFast(string? message) { }
+        public static void FailFast(string? message) => throw null;
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
-        public static void FailFast(string? message, System.Exception? exception) { }
+        public static void FailFast(string? message, System.Exception? exception) => throw null; 
         public static string[] GetCommandLineArgs() { throw null; }
         public static string? GetEnvironmentVariable(string variable) { throw null; }
         public static string? GetEnvironmentVariable(string variable, System.EnvironmentVariableTarget target) { throw null; }
diff --git a/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/Formatters/Binary/ObjectProgress.cs b/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/Formatters/Binary/ObjectProgress.cs
index 64c0a193330f26..94febe485f0733 100644
--- a/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/Formatters/Binary/ObjectProgress.cs
+++ b/src/libraries/System.Runtime.Serialization.Formatters/src/System/Runtime/Serialization/Formatters/Binary/ObjectProgress.cs
@@ -76,7 +76,7 @@ internal void Init()
         internal void ArrayCountIncrement(int value) => _count += value;
 
         // Specifies what is to parsed next from the wire.
-        internal bool GetNext(out BinaryTypeEnum outBinaryTypeEnum, [NotNullWhen(true)] out object? outTypeInformation)
+        internal bool GetNext(out BinaryTypeEnum outBinaryTypeEnum, out object? outTypeInformation)
         {
             //Initialize the out params up here.
             outBinaryTypeEnum = BinaryTypeEnum.Primitive;
diff --git a/src/libraries/System.Runtime/ref/System.Runtime.cs b/src/libraries/System.Runtime/ref/System.Runtime.cs
index 122e0037232134..5ac59a7713aa42 100644
--- a/src/libraries/System.Runtime/ref/System.Runtime.cs
+++ b/src/libraries/System.Runtime/ref/System.Runtime.cs
@@ -608,7 +608,7 @@ public sealed partial class CLSCompliantAttribute : System.Attribute
         public CLSCompliantAttribute(bool isCompliant) { }
         public bool IsCompliant { get { throw null; } }
     }
-    public delegate int Comparison<in T>(T x, T y);
+    public delegate int Comparison<in T>([System.Diagnostics.CodeAnalysis.AllowNullAttribute] T x, [System.Diagnostics.CodeAnalysis.AllowNullAttribute] T y);
     public delegate TOutput Converter<in TInput, out TOutput>(TInput input);
     public readonly partial struct DateTime : System.IComparable, System.IComparable<System.DateTime>, System.IConvertible, System.IEquatable<System.DateTime>, System.IFormattable, System.Runtime.Serialization.ISerializable
     {
@@ -4213,10 +4213,10 @@ public static void Assert([System.Diagnostics.CodeAnalysis.DoesNotReturnIfAttrib
         public static void Close() { }
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
         [System.Diagnostics.ConditionalAttribute("DEBUG")]
-        public static void Fail(string? message) { }
+        public static void Fail(string? message) => throw null; 
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
         [System.Diagnostics.ConditionalAttribute("DEBUG")]
-        public static void Fail(string? message, string? detailMessage) { }
+        public static void Fail(string? message, string? detailMessage) => throw null; 
         [System.Diagnostics.ConditionalAttribute("DEBUG")]
         public static void Flush() { }
         [System.Diagnostics.ConditionalAttribute("DEBUG")]
@@ -7447,9 +7447,9 @@ internal ExceptionDispatchInfo() { }
         public static System.Runtime.ExceptionServices.ExceptionDispatchInfo Capture(System.Exception source) { throw null; }
         public static System.Exception SetCurrentStackTrace(System.Exception source) { throw null; }
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
-        public void Throw() { }
+        public void Throw() => throw null;
         [System.Diagnostics.CodeAnalysis.DoesNotReturnAttribute]
-        public static void Throw(System.Exception source) { }
+        public static void Throw(System.Exception source) => throw null; 
     }
     public partial class FirstChanceExceptionEventArgs : System.EventArgs
     {
diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReferenceEqualsEqualityComparer.cs b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReferenceEqualsEqualityComparer.cs
index 6f3667c2380657..495f4c0d75f4c6 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReferenceEqualsEqualityComparer.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReferenceEqualsEqualityComparer.cs
@@ -4,6 +4,7 @@
 
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Text.Json.Serialization
 {
@@ -15,7 +16,7 @@ internal sealed class ReferenceEqualsEqualityComparer<T> : IEqualityComparer<T>
     {
         public static ReferenceEqualsEqualityComparer<T> Comparer = new ReferenceEqualsEqualityComparer<T>();
 
-        bool IEqualityComparer<T>.Equals(T x, T y)
+        bool IEqualityComparer<T>.Equals([AllowNull] T x, [AllowNull] T y)
         {
             return ReferenceEquals(x, y);
         }
diff --git a/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs b/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs
index a03e0c3e233d31..18723094128d26 100644
--- a/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs
+++ b/src/libraries/System.Text.Json/src/System/Text/Json/ThrowHelper.Serialization.cs
@@ -19,7 +19,6 @@ public static void ThrowArgumentException_DeserializeWrongType(Type type, object
             throw new ArgumentException(SR.Format(SR.DeserializeWrongType, type, value.GetType()));
         }
 
-        [DoesNotReturn]
         [MethodImpl(MethodImplOptions.NoInlining)]
         public static NotSupportedException GetNotSupportedException_SerializationNotSupportedCollection(Type propertyType, Type? parentType, MemberInfo? memberInfo)
         {
diff --git a/src/mono/netcore/System.Private.CoreLib/src/System/TypeIdentifier.cs b/src/mono/netcore/System.Private.CoreLib/src/System/TypeIdentifier.cs
index 0460777d058ff1..24e07936bebda9 100644
--- a/src/mono/netcore/System.Private.CoreLib/src/System/TypeIdentifier.cs
+++ b/src/mono/netcore/System.Private.CoreLib/src/System/TypeIdentifier.cs
@@ -21,6 +21,8 @@
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace System
 {
 	// A TypeName is wrapper around type names in display form
@@ -68,7 +70,7 @@ internal abstract class ATypeName : TypeName
 
 			public abstract TypeName NestedName (TypeIdentifier innerName);
 
-			public bool Equals (TypeName other)
+			public bool Equals ([AllowNull] TypeName other)
 			{
 				return other != null && DisplayName == other.DisplayName;
 			}