diff --git a/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.cs b/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.cs
index 56fe570dd5de..ca93fb8195ea 100644
--- a/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.cs
+++ b/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.cs
@@ -14,40 +14,40 @@ public partial interface IThreadPoolWorkItem
     public sealed partial class RegisteredWaitHandle : System.MarshalByRefObject
     {
         internal RegisteredWaitHandle() { }
-        public bool Unregister(System.Threading.WaitHandle waitObject) { throw null; }
+        public bool Unregister(System.Threading.WaitHandle? waitObject) { throw null; }
     }
     public static partial class ThreadPool
     {
+        public static long CompletedWorkItemCount { get { throw null; } }
+        public static long PendingWorkItemCount { get { throw null; } }
+        public static int ThreadCount { get { throw null; } }
         [System.ObsoleteAttribute("ThreadPool.BindHandle(IntPtr) has been deprecated.  Please use ThreadPool.BindHandle(SafeHandle) instead.", false)]
         public static bool BindHandle(System.IntPtr osHandle) { throw null; }
         public static bool BindHandle(System.Runtime.InteropServices.SafeHandle osHandle) { throw null; }
-        public static long CompletedWorkItemCount { get { throw null; } }
         public static void GetAvailableThreads(out int workerThreads, out int completionPortThreads) { throw null; }
         public static void GetMaxThreads(out int workerThreads, out int completionPortThreads) { throw null; }
         public static void GetMinThreads(out int workerThreads, out int completionPortThreads) { throw null; }
-        public static long PendingWorkItemCount { get { throw null; } }
         public static bool QueueUserWorkItem(System.Threading.WaitCallback callBack) { throw null; }
-        public static bool QueueUserWorkItem(System.Threading.WaitCallback callBack, object state) { throw null; }
+        public static bool QueueUserWorkItem(System.Threading.WaitCallback callBack, object? state) { throw null; }
         public static bool QueueUserWorkItem<TState>(System.Action<TState> callBack, TState state, bool preferLocal) { throw null; }
-        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, int millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
-        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, long millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
-        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, System.TimeSpan timeout, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, int millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, long millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, System.TimeSpan timeout, bool executeOnlyOnce) { throw null; }
         [System.CLSCompliantAttribute(false)]
-        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, uint millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle RegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, uint millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
         public static bool SetMaxThreads(int workerThreads, int completionPortThreads) { throw null; }
         public static bool SetMinThreads(int workerThreads, int completionPortThreads) { throw null; }
-        public static int ThreadCount { get { throw null; } }
         [System.CLSCompliantAttribute(false)]
         public unsafe static bool UnsafeQueueNativeOverlapped(System.Threading.NativeOverlapped* overlapped) { throw null; }
         public static bool UnsafeQueueUserWorkItem(System.Threading.IThreadPoolWorkItem callBack, bool preferLocal) { throw null; }
-        public static bool UnsafeQueueUserWorkItem(System.Threading.WaitCallback callBack, object state) { throw null; }
+        public static bool UnsafeQueueUserWorkItem(System.Threading.WaitCallback callBack, object? state) { throw null; }
         public static bool UnsafeQueueUserWorkItem<TState>(System.Action<TState> callBack, TState state, bool preferLocal) { throw null; }
-        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, int millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
-        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, long millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
-        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, System.TimeSpan timeout, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, int millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, long millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, System.TimeSpan timeout, bool executeOnlyOnce) { throw null; }
         [System.CLSCompliantAttribute(false)]
-        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object state, uint millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
+        public static System.Threading.RegisteredWaitHandle UnsafeRegisterWaitForSingleObject(System.Threading.WaitHandle waitObject, System.Threading.WaitOrTimerCallback callBack, object? state, uint millisecondsTimeOutInterval, bool executeOnlyOnce) { throw null; }
     }
-    public delegate void WaitCallback(object state);
-    public delegate void WaitOrTimerCallback(object state, bool timedOut);
+    public delegate void WaitCallback(object? state);
+    public delegate void WaitOrTimerCallback(object? state, bool timedOut);
 }
diff --git a/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.csproj b/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.csproj
index 2aea7fcf7edb..e215f11b0afa 100644
--- a/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.csproj
+++ b/src/System.Threading.ThreadPool/ref/System.Threading.ThreadPool.csproj
@@ -3,6 +3,7 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <ProjectGuid>{3A5A0D64-7796-4BC2-8288-4B04B97C4A70}</ProjectGuid>
     <Configurations>netcoreapp-Debug;netcoreapp-Release;uap-Debug;uap-Release</Configurations>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="System.Threading.ThreadPool.cs" />