2 files changed, 11 insertions, 10 deletions
diff --git a/src/System.Private.CoreLib/shared/System/Threading/LazyInitializer.cs b/src/System.Private.CoreLib/shared/System/Threading/LazyInitializer.cs
index 7f28a9fd53..b25db6afaf 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/LazyInitializer.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/LazyInitializer.cs
@@ -48,7 +48,7 @@ namespace System.Threading
         /// if an object was not used and to then dispose of the object appropriately.
         /// </para>
         /// </remarks>
-        public static T EnsureInitialized<T>(ref T target) where T : class =>
+        public static T EnsureInitialized<T>(ref T target) where T : class? =>
             Volatile.Read(ref target) ?? EnsureInitializedCore(ref target);
 
         /// <summary>
@@ -57,7 +57,7 @@ namespace System.Threading
         /// <typeparam name="T">The reference type of the reference to be initialized.</typeparam>
         /// <param name="target">The variable that need to be initialized</param>
         /// <returns>The initialized variable</returns>
-        private static T EnsureInitializedCore<T>(ref T target) where T : class
+        private static T EnsureInitializedCore<T>(ref T target) where T : class?
         {
             try
             {
@@ -100,7 +100,7 @@ namespace System.Threading
         /// if an object was not used and to then dispose of the object appropriately.
         /// </para>
         /// </remarks>
-        public static T EnsureInitialized<T>(ref T target, Func<T> valueFactory) where T : class =>
+        public static T EnsureInitialized<T>(ref T target, Func<T> valueFactory) where T : class? =>
             Volatile.Read(ref target) ?? EnsureInitializedCore(ref target, valueFactory);
 
         /// <summary>
@@ -110,7 +110,7 @@ namespace System.Threading
         /// <param name="target">The variable that need to be initialized</param>
         /// <param name="valueFactory">The delegate that will be executed to initialize the target</param>
         /// <returns>The initialized variable</returns>
-        private static T EnsureInitializedCore<T>(ref T target, Func<T> valueFactory) where T : class
+        private static T EnsureInitializedCore<T>(ref T target, Func<T> valueFactory) where T : class?
         {
             T value = valueFactory();
             if (value == null)
@@ -242,7 +242,7 @@ namespace System.Threading
         /// <paramref name="target"/>. If <paramref name="syncLock"/> is null, a new object will be instantiated.</param>
         /// <param name="valueFactory">The <see cref="T:System.Func{T}"/> invoked to initialize the reference.</param>
         /// <returns>The initialized value of type <typeparamref name="T"/>.</returns>
-        public static T EnsureInitialized<T>(ref T target, ref object? syncLock, Func<T> valueFactory) where T : class =>
+        public static T EnsureInitialized<T>(ref T target, ref object? syncLock, Func<T> valueFactory) where T : class? =>
             Volatile.Read(ref target) ?? EnsureInitializedCore(ref target, ref syncLock, valueFactory);
 
         /// <summary>
@@ -257,7 +257,7 @@ namespace System.Threading
         /// The <see cref="T:System.Func{T}"/> to invoke in order to produce the lazily-initialized value.
         /// </param>
         /// <returns>The initialized object.</returns>
-        private static T EnsureInitializedCore<T>(ref T target, ref object? syncLock, Func<T> valueFactory) where T : class
+        private static T EnsureInitializedCore<T>(ref T target, ref object? syncLock, Func<T> valueFactory) where T : class?
         {
             // Lazily initialize the lock if necessary and then double check if initialization is still required.
             lock (EnsureLockInitialized(ref syncLock))
@@ -284,6 +284,6 @@ namespace System.Threading
         private static object EnsureLockInitialized(ref object? syncLock) =>
             syncLock ??
             Interlocked.CompareExchange(ref syncLock, new object(), null) ??
-            syncLock;
+            syncLock!; // TODO-NULLABLE: https://github.com/dotnet/roslyn/issues/26761
     }
 }
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Volatile.cs b/src/System.Private.CoreLib/shared/System/Threading/Volatile.cs
index 845d2d089a..4ace8f994f 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Volatile.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Volatile.cs
@@ -2,6 +2,7 @@
 // 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.Runtime.CompilerServices;
 using System.Runtime.Versioning;
 using Internal.Runtime.CompilerServices;
@@ -214,16 +215,16 @@ namespace System.Threading
         #endregion
 
         #region T
-        private struct VolatileObject { public volatile object Value; }
+        private struct VolatileObject { public volatile object? Value; }
 
         [Intrinsic]
         [NonVersionable]
-        public static T Read<T>(ref T location) where T : class =>
+        public static T Read<T>(ref T location) where T : class? =>
             Unsafe.As<T>(Unsafe.As<T, VolatileObject>(ref location).Value);
 
         [Intrinsic]
         [NonVersionable]
-        public static void Write<T>(ref T location, T value) where T : class =>
+        public static void Write<T>(ref T location, T value) where T : class? =>
             Unsafe.As<T, VolatileObject>(ref location).Value = value;
         #endregion
     }