diff --git a/src/System.Windows.Forms.Primitives/src/Interop/ComCtl32/Interop.TTOOLINFOW.cs b/src/System.Windows.Forms.Primitives/src/Interop/ComCtl32/Interop.TTOOLINFOW.cs
index 413090c91a9..b78d5406002 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/ComCtl32/Interop.TTOOLINFOW.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/ComCtl32/Interop.TTOOLINFOW.cs
@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.InteropServices;
 
 internal static partial class Interop
@@ -27,10 +28,11 @@ public struct ToolInfoWrapper<T>
             where T : IHandle
         {
             public TTOOLINFOW Info;
-            public string Text { get; set; }
+            public string? Text { get; set; }
+            [MaybeNull]
             private readonly T _handle;
 
-            public unsafe ToolInfoWrapper(T handle, TTF flags = default, string text = null)
+            public unsafe ToolInfoWrapper(T handle, TTF flags = default, string? text = null)
             {
                 Info = new TTOOLINFOW
                 {
@@ -42,7 +44,7 @@ public unsafe ToolInfoWrapper(T handle, TTF flags = default, string text = null)
                 _handle = handle;
             }
 
-            public unsafe ToolInfoWrapper(T handle, IntPtr id, TTF flags = default, string text = null, RECT rect = default)
+            public unsafe ToolInfoWrapper(T handle, IntPtr id, TTF flags = default, string? text = null, RECT rect = default)
             {
                 Info = new TTOOLINFOW
                 {
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Gdi32/Interop.BITMAPINFO.cs b/src/System.Windows.Forms.Primitives/src/Interop/Gdi32/Interop.BITMAPINFO.cs
index 95c894032f5..311e3739f0f 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Gdi32/Interop.BITMAPINFO.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Gdi32/Interop.BITMAPINFO.cs
@@ -16,7 +16,7 @@ public struct BITMAPINFO
             public BITMAPINFOHEADER bmiHeader;
 
             [MarshalAs(UnmanagedType.ByValArray, SizeConst = MaxColorSize * 4)]
-            public byte[] bmiColors; // RGBQUAD structs: repeating Blue-Green-Red-Reserved
+            public byte[]? bmiColors; // RGBQUAD structs: repeating Blue-Green-Red-Reserved
         }
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.OLEVERB.cs b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.OLEVERB.cs
index e265d3e1655..4877b2a6467 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.OLEVERB.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.OLEVERB.cs
@@ -13,7 +13,7 @@ public class OLEVERB
         {
             public OLEIVERB lVerb;
             [MarshalAs(UnmanagedType.LPWStr)]
-            public string lpszVerbName;
+            public string? lpszVerbName;
             public User32.MF fuFlags;
             public OLEVERBATTRIB grfAttribs;
         }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.QACONTAINER.cs b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.QACONTAINER.cs
index 0b37eed6373..fd22bf652d4 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.QACONTAINER.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.QACONTAINER.cs
@@ -14,20 +14,20 @@ internal static partial class Ole32
         public sealed class QACONTAINER
         {
             public uint cbSize;
-            public IOleClientSite pClientSite;
-            public IAdviseSink pAdviseSink;
-            public IPropertyNotifySink pPropertyNotifySink;
+            public IOleClientSite? pClientSite;
+            public IAdviseSink? pAdviseSink;
+            public IPropertyNotifySink? pPropertyNotifySink;
             [MarshalAs(UnmanagedType.Interface)]
-            public object pUnkEventSink;
+            public object? pUnkEventSink;
             public QACONTAINERFLAGS dwAmbientFlags;
             public uint colorFore;
             public uint colorBack;
-            public IFont pFont;
+            public IFont? pFont;
             public IntPtr pUndoMgr;
             public uint dwAppearance;
             public int lcid;
             public IntPtr hpal;
-            public IServiceProvider pBindHost;
+            public IServiceProvider? pBindHost;
         }
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.STATSTG.cs b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.STATSTG.cs
index 705df8d4a53..5c56823fa7d 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.STATSTG.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.STATSTG.cs
@@ -60,7 +60,7 @@ public struct STATSTG
             public uint grfStateBits;
             public uint reserved;
 
-            public string GetName() => Marshal.PtrToStringUni(pwcsName);
+            public string? GetName() => Marshal.PtrToStringUni(pwcsName);
 
             /// <summary>
             ///  Caller is responsible for freeing the name memory.
@@ -76,7 +76,7 @@ public void FreeName()
             /// <summary>
             ///  Callee is repsonsible for allocating the name memory.
             /// </summary>
-            public void AllocName(string name)
+            public void AllocName(string? name)
             {
                 pwcsName = Marshal.StringToCoTaskMemUni(name);
             }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.VARIANT.cs b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.VARIANT.cs
index c473e9c7526..90606cbcaea 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.VARIANT.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Ole32/Interop.VARIANT.cs
@@ -42,7 +42,7 @@ public void Clear()
 
             ~VARIANT() => Clear();
 
-            public object ToObject()
+            public object? ToObject()
             {
                 IntPtr val = data1;
                 long longVal;
@@ -165,8 +165,8 @@ public object ToObject()
                         return (val != IntPtr.Zero);
 
                     case VARENUM.VARIANT:
-                        VARIANT varStruct = Marshal.PtrToStructure<VARIANT>(val);
-                        return varStruct.ToObject();
+                        VARIANT? varStruct = Marshal.PtrToStructure<VARIANT>(val);
+                        return varStruct?.ToObject();
 
                     case VARENUM.CLSID:
                         Guid guid = Marshal.PtrToStructure<Guid>(val);
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/OleAut32/Interop.FONTDESC.cs b/src/System.Windows.Forms.Primitives/src/Interop/OleAut32/Interop.FONTDESC.cs
index ec41290df97..462d969fbfa 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/OleAut32/Interop.FONTDESC.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/OleAut32/Interop.FONTDESC.cs
@@ -13,7 +13,7 @@ internal static partial class Oleaut32
         public struct FONTDESC
         {
             public uint cbSizeOfStruct;
-            public string lpstrName;
+            public string? lpstrName;
             public long cySize;
             public short sWeight;
             public short sCharset;
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DragQueryFileW.cs b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DragQueryFileW.cs
index 06e0061fd41..eff5ec82ee8 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DragQueryFileW.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.DragQueryFileW.cs
@@ -11,7 +11,7 @@ internal static partial class Interop
     internal static partial class Shell32
     {
         [DllImport(Libraries.Shell32, ExactSpelling = true, EntryPoint = "DragQueryFileW", CharSet = CharSet.Unicode)]
-        private static extern uint DragQueryFileWInternal(IntPtr hDrop, uint iFile, StringBuilder lpszFile, uint cch);
+        private static extern uint DragQueryFileWInternal(IntPtr hDrop, uint iFile, StringBuilder? lpszFile, uint cch);
 
         public static uint DragQueryFileW(IntPtr hDrop, uint iFile, StringBuilder lpszFile)
         {
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHBrowseForFolder.cs b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHBrowseForFolder.cs
index bf5302b144d..ff14b01570d 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHBrowseForFolder.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHBrowseForFolder.cs
@@ -34,15 +34,15 @@ public unsafe struct BROWSEINFO
         {
             public IntPtr hwndOwner;
 
-            public CoTaskMemSafeHandle pidlRoot;
+            public CoTaskMemSafeHandle? pidlRoot;
 
             public char* pszDisplayName;
 
-            public string lpszTitle;
+            public string? lpszTitle;
 
             public uint ulFlags;
 
-            public BrowseCallbackProc lpfn;
+            public BrowseCallbackProc? lpfn;
 
             public IntPtr lParam;
 
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHGetPathFromIDListLongPath.cs b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHGetPathFromIDListLongPath.cs
index 3ce069b5548..c8b338eeb82 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHGetPathFromIDListLongPath.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/Shell32/Interop.SHGetPathFromIDListLongPath.cs
@@ -12,7 +12,7 @@ internal static partial class Shell32
         [DllImport(Libraries.Shell32, ExactSpelling = true)]
         private static extern BOOL SHGetPathFromIDListEx(IntPtr pidl, IntPtr pszPath, int cchPath, int flags);
 
-        public static bool SHGetPathFromIDListLongPath(IntPtr pidl, out string path)
+        public static bool SHGetPathFromIDListLongPath(IntPtr pidl, out string? path)
         {
             IntPtr pszPath = Marshal.AllocHGlobal((Kernel32.MAX_PATH + 1) * sizeof(char));
             int length = Kernel32.MAX_PATH;
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.DEVMODEW.cs b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.DEVMODEW.cs
index a825ccfbe10..7f61dec940c 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.DEVMODEW.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.DEVMODEW.cs
@@ -17,7 +17,7 @@ public struct DEVMODEW
             private const int CCHFORMNAME = 32;
 
             [MarshalAs(UnmanagedType.ByValTStr, SizeConst = CCHDEVICENAME)]
-            public string dmDeviceName;
+            public string? dmDeviceName;
             public ushort dmSpecVersion;
             public ushort dmDriverVersion;
             public ushort dmSize;
@@ -33,7 +33,7 @@ public struct DEVMODEW
             public short dmTTOption;
             public short dmCollate;
             [MarshalAs(UnmanagedType.ByValTStr, SizeConst = CCHFORMNAME)]
-            public string dmFormName;
+            public string? dmFormName;
             public ushort dmLogPixels;
             public uint dmBitsPerPel;
             public uint dmPelsWidth;
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumChildWindows.cs b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumChildWindows.cs
index 873c299fd85..5b62cc42c76 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumChildWindows.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumChildWindows.cs
@@ -50,7 +50,7 @@ public static BOOL EnumChildWindows(HandleRef hwndParent, EnumChildWindowsCallba
 
         private static BOOL HandleEnumChildWindowsNativeCallback(IntPtr hWnd, IntPtr lParam)
         {
-            return ((EnumChildWindowsCallback)GCHandle.FromIntPtr(lParam).Target)(hWnd);
+            return ((EnumChildWindowsCallback)GCHandle.FromIntPtr(lParam).Target!)(hWnd);
         }
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumThreadWindows.cs b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumThreadWindows.cs
index 7fc58de358f..6865efe11a9 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumThreadWindows.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumThreadWindows.cs
@@ -36,7 +36,7 @@ public static BOOL EnumThreadWindows(uint dwThreadId, EnumThreadWindowsCallback
 
         private static BOOL HandleEnumThreadWindowsNativeCallback(IntPtr hWnd, IntPtr lParam)
         {
-            return ((EnumThreadWindowsCallback)GCHandle.FromIntPtr(lParam).Target)(hWnd);
+            return ((EnumThreadWindowsCallback)GCHandle.FromIntPtr(lParam).Target!)(hWnd);
         }
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumWindows.cs b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumWindows.cs
index 22593def9d1..92d765cf2e3 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumWindows.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.EnumWindows.cs
@@ -36,7 +36,7 @@ public static BOOL EnumWindows(EnumWindowsCallback lpEnumFunc)
 
         private static BOOL HandleEnumWindowsNativeCallback(IntPtr hWnd, IntPtr lParam)
         {
-            return ((EnumWindowsCallback)GCHandle.FromIntPtr(lParam).Target)(hWnd);
+            return ((EnumWindowsCallback)GCHandle.FromIntPtr(lParam).Target!)(hWnd);
         }
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.GetClipboardFormatNameW.cs b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.GetClipboardFormatNameW.cs
index 440ce5ad200..131fb04f74d 100644
--- a/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.GetClipboardFormatNameW.cs
+++ b/src/System.Windows.Forms.Primitives/src/Interop/User32/Interop.GetClipboardFormatNameW.cs
@@ -13,7 +13,7 @@ internal static partial class User32
         [DllImport(Libraries.User32, ExactSpelling = true)]
         private static unsafe extern int GetClipboardFormatNameW(uint format, char* lpszFormatName, int cchMaxCount);
 
-        public static unsafe string GetClipboardFormatNameW(uint format)
+        public static unsafe string? GetClipboardFormatNameW(uint format)
         {
             // The max length of the name of clipboard formats is equal to the max length
             // of a Win32 Atom of 255 chars. An additional null terminator character is added,
diff --git a/src/System.Windows.Forms.Primitives/src/System.Windows.Forms.Primitives.csproj b/src/System.Windows.Forms.Primitives/src/System.Windows.Forms.Primitives.csproj
index d0f3929ac43..f8489af8aff 100644
--- a/src/System.Windows.Forms.Primitives/src/System.Windows.Forms.Primitives.csproj
+++ b/src/System.Windows.Forms.Primitives/src/System.Windows.Forms.Primitives.csproj
@@ -4,6 +4,7 @@
     <AssemblyName>System.Windows.Forms.Primitives</AssemblyName>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <CLSCompliant>true</CLSCompliant>
+    <Nullable>enable</Nullable>
     <!--the obsolete usage in public surface can't be removed-->
     <NoWarn>$(NoWarn);618</NoWarn>
     <Deterministic>true</Deterministic>
diff --git a/src/System.Windows.Forms.Primitives/src/System/ComponentModel/CompModSwitches.cs b/src/System.Windows.Forms.Primitives/src/System/ComponentModel/CompModSwitches.cs
index 60e71c1319a..dd78fd8c0b8 100644
--- a/src/System.Windows.Forms.Primitives/src/System/ComponentModel/CompModSwitches.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/ComponentModel/CompModSwitches.cs
@@ -8,39 +8,39 @@ namespace System.ComponentModel
 {
     internal static class CompModSwitches
     {
-        private static TraceSwitch activeX;
-        private static TraceSwitch flowLayout;
-        private static TraceSwitch dataCursor;
-        private static TraceSwitch dataGridCursor;
-        private static TraceSwitch dataGridEditing;
-        private static TraceSwitch dataGridKeys;
-        private static TraceSwitch dataGridLayout;
-        private static TraceSwitch dataGridPainting;
-        private static TraceSwitch dataGridParents;
-        private static TraceSwitch dataGridScrolling;
-        private static TraceSwitch dataGridSelection;
-        private static TraceSwitch dataObject;
-        private static TraceSwitch dataView;
-        private static TraceSwitch debugGridView;
-        private static TraceSwitch dgCaptionPaint;
-        private static TraceSwitch dgEditColumnEditing;
-        private static TraceSwitch dgRelationShpRowLayout;
-        private static TraceSwitch dgRelationShpRowPaint;
-        private static TraceSwitch dgRowPaint;
-        private static TraceSwitch dragDrop;
-        private static TraceSwitch imeMode;
-        private static TraceSwitch msaa;
-        private static TraceSwitch msoComponentManager;
-        private static TraceSwitch layoutPerformance;
-        private static TraceSwitch layoutSuspendResume;
-        private static TraceSwitch richLayout;
-        private static TraceSwitch setBounds;
-
-        private static BooleanSwitch lifetimeTracing;
-
-        private static TraceSwitch s_handleLeak;
-        private static BooleanSwitch s_traceCollect;
-        private static BooleanSwitch s_commonDesignerServices;
+        private static TraceSwitch? activeX;
+        private static TraceSwitch? flowLayout;
+        private static TraceSwitch? dataCursor;
+        private static TraceSwitch? dataGridCursor;
+        private static TraceSwitch? dataGridEditing;
+        private static TraceSwitch? dataGridKeys;
+        private static TraceSwitch? dataGridLayout;
+        private static TraceSwitch? dataGridPainting;
+        private static TraceSwitch? dataGridParents;
+        private static TraceSwitch? dataGridScrolling;
+        private static TraceSwitch? dataGridSelection;
+        private static TraceSwitch? dataObject;
+        private static TraceSwitch? dataView;
+        private static TraceSwitch? debugGridView;
+        private static TraceSwitch? dgCaptionPaint;
+        private static TraceSwitch? dgEditColumnEditing;
+        private static TraceSwitch? dgRelationShpRowLayout;
+        private static TraceSwitch? dgRelationShpRowPaint;
+        private static TraceSwitch? dgRowPaint;
+        private static TraceSwitch? dragDrop;
+        private static TraceSwitch? imeMode;
+        private static TraceSwitch? msaa;
+        private static TraceSwitch? msoComponentManager;
+        private static TraceSwitch? layoutPerformance;
+        private static TraceSwitch? layoutSuspendResume;
+        private static TraceSwitch? richLayout;
+        private static TraceSwitch? setBounds;
+
+        private static BooleanSwitch? lifetimeTracing;
+
+        private static TraceSwitch? s_handleLeak;
+        private static BooleanSwitch? s_traceCollect;
+        private static BooleanSwitch? s_commonDesignerServices;
 
         public static TraceSwitch ActiveX
         {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlace.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlace.cs
index c10e0b37132..f2f97edcdce 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlace.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlace.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.
 
+using System.Diagnostics.CodeAnalysis;
 using static Interop;
 
 namespace System.Windows.Forms
@@ -18,7 +19,7 @@ public class FileDialogCustomPlace
         private string _path = string.Empty;
         private Guid _knownFolderGuid = Guid.Empty;
 
-        public FileDialogCustomPlace(string path)
+        public FileDialogCustomPlace(string? path)
         {
             Path = path;
         }
@@ -28,6 +29,7 @@ public FileDialogCustomPlace(Guid knownFolderGuid)
             KnownFolderGuid = knownFolderGuid;
         }
 
+        [AllowNull]
         public string Path
         {
             get => _path ?? string.Empty;
@@ -58,7 +60,7 @@ public override string ToString()
         ///  to an actual filesystem directory.
         ///  The caller is responsible for handling these situations.
         /// </remarks>
-        internal FileDialogNative.IShellItem GetNativePath()
+        internal FileDialogNative.IShellItem? GetNativePath()
         {
             string filePathString;
             if (!string.IsNullOrEmpty(_path))
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlacesCollection.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlacesCollection.cs
index 9ee2816c1aa..2742636796c 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlacesCollection.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/FileDialogCustomPlacesCollection.cs
@@ -18,7 +18,7 @@ internal void Apply(FileDialogNative.IFileDialog dialog)
 
                 try
                 {
-                    FileDialogNative.IShellItem shellItem = customPlace.GetNativePath();
+                    FileDialogNative.IShellItem? shellItem = customPlace.GetNativePath();
                     if (null != shellItem)
                     {
                         dialog.AddPlace(shellItem, 0);
@@ -32,7 +32,7 @@ internal void Apply(FileDialogNative.IFileDialog dialog)
             }
         }
 
-        public void Add(string path) => Add(new FileDialogCustomPlace(path));
+        public void Add(string? path) => Add(new FileDialogCustomPlace(path));
 
         public void Add(Guid knownFolderGuid) => Add(new FileDialogCustomPlace(knownFolderGuid));
     }
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/ClientUtils.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/ClientUtils.cs
index d73f0327537..92e3fdcb55e 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/ClientUtils.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/ClientUtils.cs
@@ -4,6 +4,7 @@
 
 using System.Collections;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Numerics;
 
@@ -146,7 +147,7 @@ public static int GetWordBoundaryStart(char[] text, int endIndex)
 
 #if DEBUG
         [ThreadStatic]
-        private static Hashtable enumValueInfo;
+        private static Hashtable? enumValueInfo;
         public const int MAXCACHE = 300;  // we think we're going to get O(100) of these, put in a tripwire if it gets larger.
 
         private class SequentialEnumInfo
@@ -157,7 +158,9 @@ public SequentialEnumInfo(Type t)
                 int actualMaximum = int.MinValue;
                 int countEnumVals = 0;
 
+#pragma warning disable CS8605 // Unboxing a possibly null value.
                 foreach (int iVal in Enum.GetValues(t))
+#pragma warning restore CS8605 // Unboxing a possibly null value.
                 {
                     actualMinimum = Math.Min(actualMinimum, iVal);
                     actualMaximum = Math.Max(actualMaximum, iVal);
@@ -184,7 +187,7 @@ private static void Debug_SequentialEnumIsDefinedCheck(Enum value, int minVal, i
                 enumValueInfo = new Hashtable();
             }
 
-            SequentialEnumInfo sequentialEnumInfo = null;
+            SequentialEnumInfo? sequentialEnumInfo = null;
 
             if (enumValueInfo.ContainsKey(t))
             {
@@ -218,7 +221,9 @@ private static void Debug_ValidateMask(Enum value, uint mask)
         {
             Type t = value.GetType();
             uint newmask = 0;
+#pragma warning disable CS8605 // Unboxing a possibly null value.
             foreach (int iVal in Enum.GetValues(t))
+#pragma warning restore CS8605 // Unboxing a possibly null value.
             {
                 newmask |= (uint)iVal;
             }
@@ -232,7 +237,9 @@ private static void Debug_NonSequentialEnumIsDefinedCheck(Enum value, int minVal
                int checkedValue = Convert.ToInt32(value, CultureInfo.InvariantCulture);
                int maxBitsFound = 0;
                bool foundValue = false;
+#pragma warning disable CS8605 // Unboxing a possibly null value.
                foreach (int iVal in Enum.GetValues(t)){
+#pragma warning restore CS8605 // Unboxing a possibly null value.
                    actualMinimum = Math.Min(actualMinimum, iVal);
                    actualMaximum = Math.Max(actualMaximum, iVal);
                    maxBitsFound = Math.Max(maxBitsFound, BitOperations.PopCount((uint)iVal));
@@ -295,7 +302,7 @@ public WeakRefCollection(int size)
             /// </summary>
             public int RefCheckThreshold { get; set; } = int.MaxValue; // this means this is disabled by default.
 
-            public object this[int index]
+            public object? this[int index]
             {
                 get
                 {
@@ -315,7 +322,7 @@ public void ScavengeReferences()
                 int currentCount = Count;
                 for (int i = 0; i < currentCount; i++)
                 {
-                    object item = this[currentIndex];
+                    object? item = this[currentIndex];
 
                     if (item == null)
                     {
@@ -329,9 +336,9 @@ public void ScavengeReferences()
                 }
             }
 
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
-                WeakRefCollection other = obj as WeakRefCollection;
+                WeakRefCollection? other = obj as WeakRefCollection;
                 if (other == this)
                 {
                     return true;
@@ -346,7 +353,7 @@ public override bool Equals(object obj)
                 {
                     if (InnerList[i] != other.InnerList[i])
                     {
-                        if (InnerList[i] == null || !InnerList[i].Equals(other.InnerList[i]))
+                        if (InnerList[i] == null || !InnerList[i]!.Equals(other.InnerList[i]))
                         {
                             return false;
                         }
@@ -359,7 +366,7 @@ public override bool Equals(object obj)
             public override int GetHashCode()
             {
                 var hash = new HashCode();
-                foreach (object o in InnerList)
+                foreach (object? o in InnerList)
                 {
                     hash.Add(o);
                 }
@@ -367,7 +374,8 @@ public override int GetHashCode()
                 return hash.ToHashCode();
             }
 
-            private WeakRefObject CreateWeakRefObject(object value)
+            [return: NotNullIfNotNull("value")]
+            private WeakRefObject? CreateWeakRefObject(object? value)
             {
                 if (value == null)
                 {
@@ -416,7 +424,7 @@ public void RemoveByHashCode(object value)
                 int hash = value.GetHashCode();
                 for (int idx = 0; idx < InnerList.Count; idx++)
                 {
-                    if (InnerList[idx] != null && InnerList[idx].GetHashCode() == hash)
+                    if (InnerList[idx] != null && InnerList[idx]!.GetHashCode() == hash)
                     {
                         RemoveAt(idx);
                         return;
@@ -428,17 +436,17 @@ public void RemoveByHashCode(object value)
 
             public bool IsFixedSize => InnerList.IsFixedSize;
 
-            public bool Contains(object value) => InnerList.Contains(CreateWeakRefObject(value));
+            public bool Contains(object? value) => InnerList.Contains(CreateWeakRefObject(value));
 
             public void RemoveAt(int index) => InnerList.RemoveAt(index);
 
-            public void Remove(object value) => InnerList.Remove(CreateWeakRefObject(value));
+            public void Remove(object? value) => InnerList.Remove(CreateWeakRefObject(value));
 
-            public int IndexOf(object value) => InnerList.IndexOf(CreateWeakRefObject(value));
+            public int IndexOf(object? value) => InnerList.IndexOf(CreateWeakRefObject(value));
 
-            public void Insert(int index, object value) => InnerList.Insert(index, CreateWeakRefObject(value));
+            public void Insert(int index, object? value) => InnerList.Insert(index, CreateWeakRefObject(value));
 
-            public int Add(object value)
+            public int Add(object? value)
             {
                 if (Count > RefCheckThreshold)
                 {
@@ -483,7 +491,7 @@ internal bool IsAlive
                     get { return weakHolder.IsAlive; }
                 }
 
-                internal object Target
+                internal object? Target
                 {
                     get
                     {
@@ -493,9 +501,9 @@ internal object Target
 
                 public override int GetHashCode() => hash;
 
-                public override bool Equals(object obj)
+                public override bool Equals(object? obj)
                 {
-                    WeakRefObject other = obj as WeakRefObject;
+                    WeakRefObject? other = obj as WeakRefObject;
 
                     if (other == this)
                     {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/DpiHelper.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/DpiHelper.cs
index 5e4e2088271..63d3853bbf8 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/DpiHelper.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/DpiHelper.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.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Drawing;
 using System.Drawing.Drawing2D;
 using static Interop;
@@ -288,7 +289,8 @@ public static Size LogicalToDeviceUnits(Size logicalSize, int deviceDpi = 0)
         /// </summary>
         /// <param name="logicalImage">The image to scale from logical units to device units</param>
         /// <param name="targetImageSize">The size to scale image to</param>
-        public static Bitmap CreateResizedBitmap(Bitmap logicalImage, Size targetImageSize)
+        [return: NotNullIfNotNull("logicalImage")]
+        public static Bitmap? CreateResizedBitmap(Bitmap? logicalImage, Size targetImageSize)
         {
             if (logicalImage == null)
             {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/FileDialog_Vista_Interop.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/FileDialog_Vista_Interop.cs
index 148cacf0a09..3da56211f49 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/FileDialog_Vista_Interop.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/FileDialog_Vista_Interop.cs
@@ -12,7 +12,7 @@ internal static class FileDialogNative
     {
         internal static IShellItem GetShellItemForPath(string path)
         {
-            IShellItem ret = null;
+            IShellItem? ret = null;
             IntPtr pidl = IntPtr.Zero;
             uint zero = 0;
             if (UnsafeNativeMethods.Shell32.SHILCreateFromPath(path, out pidl, ref zero) >= 0)
@@ -412,9 +412,9 @@ public enum SIGDN : uint
         public struct COMDLG_FILTERSPEC
         {
             [MarshalAs(UnmanagedType.LPWStr)]
-            public string pszName;
+            public string? pszName;
             [MarshalAs(UnmanagedType.LPWStr)]
-            public string pszSpec;
+            public string? pszSpec;
         }
 
         public enum CDCONTROLSTATE
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/MessageDecoder.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/MessageDecoder.cs
index 08caf7dc3fa..8062cf03fdf 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/MessageDecoder.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/MessageDecoder.cs
@@ -17,9 +17,9 @@ internal static class MessageDecoder
         ///  Returns the symbolic name of the msg value, or null if it isn't one of the
         ///  existing constants.
         /// </summary>
-        private static string MsgToString(int msg)
+        private static string? MsgToString(int msg)
         {
-            string text;
+            string? text;
             switch (msg)
             {
                 case (int)WM.NULL:
@@ -839,7 +839,7 @@ private static string MsgToString(int msg)
             return text;
         }
 
-        private static string Parenthesize(string input)
+        private static string Parenthesize(string? input)
         {
             if (input == null)
             {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/NativeMethods.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/NativeMethods.cs
index 827d62e8f68..b7075ba70c4 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/NativeMethods.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/NativeMethods.cs
@@ -183,11 +183,11 @@ public class HH_AKLINK
         {
             internal int cbStruct = Marshal.SizeOf<HH_AKLINK>();
             internal bool fReserved = false;
-            internal string pszKeywords = null;
-            internal string pszUrl = null;
-            internal string pszMsgText = null;
-            internal string pszMsgTitle = null;
-            internal string pszWindow = null;
+            internal string? pszKeywords = null;
+            internal string? pszUrl = null;
+            internal string? pszMsgText = null;
+            internal string? pszMsgTitle = null;
+            internal string? pszWindow = null;
             internal bool fIndexOnFail = false;
         }
 
@@ -202,7 +202,7 @@ public class HH_POPUP
             internal int clrForeground = -1;
             internal int clrBackground = -1;
             internal RECT rcMargins = new RECT(-1, -1, -1, -1);     // amount of space between edges of window and text, -1 for each member to ignore
-            internal string pszFont = null;
+            internal string? pszFont = null;
         }
 
         public const int HH_FTS_DEFAULT_PROXIMITY = -1;
@@ -213,13 +213,13 @@ public class HH_FTS_QUERY
             internal int cbStruct = Marshal.SizeOf<HH_FTS_QUERY>();
             internal bool fUniCodeStrings = false;
             [MarshalAs(UnmanagedType.LPStr)]
-            internal string pszSearchQuery = null;
+            internal string? pszSearchQuery = null;
             internal int iProximity = NativeMethods.HH_FTS_DEFAULT_PROXIMITY;
             internal bool fStemmedSearch = false;
             internal bool fTitleOnly = false;
             internal bool fExecute = true;
             [MarshalAs(UnmanagedType.LPStr)]
-            internal string pszWindow = null;
+            internal string? pszWindow = null;
         }
 
         public delegate int EditStreamCallback(IntPtr dwCookie, IntPtr buf, int cb, out int transferred);
@@ -229,7 +229,7 @@ public class EDITSTREAM
         {
             public IntPtr dwCookie = IntPtr.Zero;
             public int dwError = 0;
-            public EditStreamCallback pfnCallback = null;
+            public EditStreamCallback? pfnCallback = null;
         }
 
         [StructLayout(LayoutKind.Sequential)]
@@ -266,11 +266,11 @@ public interface PRINTDLG
             IntPtr hInstance { get; set; }
             IntPtr lCustData { get; set; }
 
-            WndProc lpfnPrintHook { get; set; }
-            WndProc lpfnSetupHook { get; set; }
+            WndProc? lpfnPrintHook { get; set; }
+            WndProc? lpfnSetupHook { get; set; }
 
-            string lpPrintTemplateName { get; set; }
-            string lpSetupTemplateName { get; set; }
+            string? lpPrintTemplateName { get; set; }
+            string? lpSetupTemplateName { get; set; }
 
             IntPtr hPrintTemplate { get; set; }
             IntPtr hSetupTemplate { get; set; }
@@ -297,11 +297,11 @@ public class PRINTDLG_32 : PRINTDLG
             IntPtr m_hInstance;
             IntPtr m_lCustData;
 
-            WndProc m_lpfnPrintHook;
-            WndProc m_lpfnSetupHook;
+            WndProc? m_lpfnPrintHook;
+            WndProc? m_lpfnSetupHook;
 
-            string m_lpPrintTemplateName;
-            string m_lpSetupTemplateName;
+            string? m_lpPrintTemplateName;
+            string? m_lpSetupTemplateName;
 
             IntPtr m_hPrintTemplate;
             IntPtr m_hSetupTemplate;
@@ -324,11 +324,11 @@ public class PRINTDLG_32 : PRINTDLG
             public IntPtr hInstance { get { return m_hInstance; } set { m_hInstance = value; } }
             public IntPtr lCustData { get { return m_lCustData; } set { m_lCustData = value; } }
 
-            public WndProc lpfnPrintHook { get { return m_lpfnPrintHook; } set { m_lpfnPrintHook = value; } }
-            public WndProc lpfnSetupHook { get { return m_lpfnSetupHook; } set { m_lpfnSetupHook = value; } }
+            public WndProc? lpfnPrintHook { get { return m_lpfnPrintHook; } set { m_lpfnPrintHook = value; } }
+            public WndProc? lpfnSetupHook { get { return m_lpfnSetupHook; } set { m_lpfnSetupHook = value; } }
 
-            public string lpPrintTemplateName { get { return m_lpPrintTemplateName; } set { m_lpPrintTemplateName = value; } }
-            public string lpSetupTemplateName { get { return m_lpSetupTemplateName; } set { m_lpSetupTemplateName = value; } }
+            public string? lpPrintTemplateName { get { return m_lpPrintTemplateName; } set { m_lpPrintTemplateName = value; } }
+            public string? lpSetupTemplateName { get { return m_lpSetupTemplateName; } set { m_lpSetupTemplateName = value; } }
 
             public IntPtr hPrintTemplate { get { return m_hPrintTemplate; } set { m_hPrintTemplate = value; } }
             public IntPtr hSetupTemplate { get { return m_hSetupTemplate; } set { m_hSetupTemplate = value; } }
@@ -355,11 +355,11 @@ public class PRINTDLG_64 : PRINTDLG
             IntPtr m_hInstance;
             IntPtr m_lCustData;
 
-            WndProc m_lpfnPrintHook;
-            WndProc m_lpfnSetupHook;
+            WndProc? m_lpfnPrintHook;
+            WndProc? m_lpfnSetupHook;
 
-            string m_lpPrintTemplateName;
-            string m_lpSetupTemplateName;
+            string? m_lpPrintTemplateName;
+            string? m_lpSetupTemplateName;
 
             IntPtr m_hPrintTemplate;
             IntPtr m_hSetupTemplate;
@@ -382,11 +382,11 @@ public class PRINTDLG_64 : PRINTDLG
             public IntPtr hInstance { get { return m_hInstance; } set { m_hInstance = value; } }
             public IntPtr lCustData { get { return m_lCustData; } set { m_lCustData = value; } }
 
-            public WndProc lpfnPrintHook { get { return m_lpfnPrintHook; } set { m_lpfnPrintHook = value; } }
-            public WndProc lpfnSetupHook { get { return m_lpfnSetupHook; } set { m_lpfnSetupHook = value; } }
+            public WndProc? lpfnPrintHook { get { return m_lpfnPrintHook; } set { m_lpfnPrintHook = value; } }
+            public WndProc? lpfnSetupHook { get { return m_lpfnSetupHook; } set { m_lpfnSetupHook = value; } }
 
-            public string lpPrintTemplateName { get { return m_lpPrintTemplateName; } set { m_lpPrintTemplateName = value; } }
-            public string lpSetupTemplateName { get { return m_lpSetupTemplateName; } set { m_lpSetupTemplateName = value; } }
+            public string? lpPrintTemplateName { get { return m_lpPrintTemplateName; } set { m_lpPrintTemplateName = value; } }
+            public string? lpSetupTemplateName { get { return m_lpSetupTemplateName; } set { m_lpSetupTemplateName = value; } }
 
             public IntPtr hPrintTemplate { get { return m_hPrintTemplate; } set { m_hPrintTemplate = value; } }
             public IntPtr hSetupTemplate { get { return m_hSetupTemplate; } set { m_hSetupTemplate = value; } }
@@ -418,9 +418,9 @@ public class PRINTDLGEX
 
             public IntPtr hInstance;
             [MarshalAs(UnmanagedType.LPStr)]
-            public string lpPrintTemplateName;
+            public string? lpPrintTemplateName;
 
-            public WndProc lpCallback = null;
+            public WndProc? lpCallback = null;
 
             public int nPropertyPages;
 
@@ -448,14 +448,14 @@ public class NOTIFYICONDATA
             public int uCallbackMessage;
             public IntPtr hIcon;
             [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
-            public string szTip;
+            public string? szTip;
             public int dwState = 0;
             public int dwStateMask = 0;
             [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
-            public string szInfo;
+            public string? szInfo;
             public int uTimeoutOrVersion;
             [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
-            public string szInfoTitle;
+            public string? szInfoTitle;
             public int dwInfoFlags;
         }
 
@@ -464,7 +464,7 @@ public struct MSAAMENUINFO
         {
             public int dwMSAASignature;
             public int cchWText;
-            public string pszWText;
+            public string? pszWText;
 
             public MSAAMENUINFO(string text)
             {
@@ -480,7 +480,7 @@ public class OPENFILENAME_I
             public int lStructSize = Marshal.SizeOf<OPENFILENAME_I>(); //ndirect.DllLib.sizeOf(this);
             public IntPtr hwndOwner;
             public IntPtr hInstance;
-            public string lpstrFilter;   // use embedded nulls to separate filters
+            public string? lpstrFilter;   // use embedded nulls to separate filters
             public IntPtr lpstrCustomFilter = IntPtr.Zero;
             public int nMaxCustFilter = 0;
             public int nFilterIndex;
@@ -488,15 +488,15 @@ public class OPENFILENAME_I
             public int nMaxFile = Kernel32.MAX_PATH;
             public IntPtr lpstrFileTitle = IntPtr.Zero;
             public int nMaxFileTitle = Kernel32.MAX_PATH;
-            public string lpstrInitialDir;
-            public string lpstrTitle;
+            public string? lpstrInitialDir;
+            public string? lpstrTitle;
             public int Flags;
             public short nFileOffset = 0;
             public short nFileExtension = 0;
-            public string lpstrDefExt;
+            public string? lpstrDefExt;
             public IntPtr lCustData = IntPtr.Zero;
-            public WndProc lpfnHook;
-            public string lpTemplateName = null;
+            public WndProc? lpfnHook;
+            public string? lpTemplateName = null;
             public IntPtr pvReserved = IntPtr.Zero;
             public int dwReserved = 0;
             public int FlagsEx;
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/UnsafeNativeMethods.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/UnsafeNativeMethods.cs
index 872d8dff1f4..50cb79c4662 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/UnsafeNativeMethods.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Internals/UnsafeNativeMethods.cs
@@ -154,9 +154,9 @@ public static StringBuilder GetModuleFileNameLongPath(HandleRef hModule)
         [DllImport(ExternDll.Gdi32, SetLastError = true, ExactSpelling = true, CharSet = System.Runtime.InteropServices.CharSet.Auto)]
         public static extern int GetRegionData(HandleRef hRgn, int size, IntPtr lpRgnData);
 
-        public unsafe static RECT[] GetRectsFromRegion(IntPtr hRgn)
+        public unsafe static RECT[]? GetRectsFromRegion(IntPtr hRgn)
         {
-            RECT[] regionRects = null;
+            RECT[]? regionRects = null;
             IntPtr pBytes = IntPtr.Zero;
             try
             {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Message.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Message.cs
index 7eea3f97f8f..2b2e501815f 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Message.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Message.cs
@@ -30,7 +30,7 @@ public struct Message
         /// <summary>
         ///  Gets the <see cref='LParam'/> value, and converts the value to an object.
         /// </summary>
-        public object GetLParam(Type cls) => Marshal.PtrToStructure(LParam, cls);
+        public object? GetLParam(Type cls) => Marshal.PtrToStructure(LParam, cls);
 
         internal static Message Create(IntPtr hWnd, User32.WM msg, IntPtr wparam, IntPtr lparam)
             => Create(hWnd, (int)msg, wparam, lparam);
@@ -55,7 +55,7 @@ public static Message Create(IntPtr hWnd, int msg, IntPtr wparam, IntPtr lparam)
             return m;
         }
 
-        public override bool Equals(object o)
+        public override bool Equals(object? o)
         {
             if (!(o is Message m))
             {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Padding.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Padding.cs
index 773c467d84e..5de20edf938 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Padding.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/Padding.cs
@@ -130,7 +130,7 @@ public int Top
 
         public static Padding Subtract(Padding p1, Padding p2) => p1 - p2;
 
-        public override bool Equals(object other)
+        public override bool Equals(object? other)
         {
             if (!(other is Padding otherPadding))
             {
diff --git a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/PaddingConverter.cs b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/PaddingConverter.cs
index 7dbf266269d..023b24ed707 100644
--- a/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/PaddingConverter.cs
+++ b/src/System.Windows.Forms.Primitives/src/System/Windows/Forms/PaddingConverter.cs
@@ -16,7 +16,7 @@ public class PaddingConverter : TypeConverter
         ///  Determines if this converter can convert an object in the given source type to
         ///  the native type of the converter.
         /// </summary>
-        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
+        public override bool CanConvertFrom(ITypeDescriptorContext? context, Type sourceType)
         {
             if (sourceType == typeof(string))
             {
@@ -26,7 +26,7 @@ public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceT
             return base.CanConvertFrom(context, sourceType);
         }
 
-        public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
+        public override bool CanConvertTo(ITypeDescriptorContext? context, Type destinationType)
         {
             if (destinationType == typeof(InstanceDescriptor))
             {
@@ -39,7 +39,7 @@ public override bool CanConvertTo(ITypeDescriptorContext context, Type destinati
         /// <summary>
         ///  Converts the given object to the converter's native type.
         /// </summary>
-        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
+        public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object? value)
         {
             if (value is string stringValue)
             {
@@ -75,7 +75,7 @@ public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo c
             return base.ConvertFrom(context, culture, value);
         }
 
-        public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
+        public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
         {
             if (value is Padding padding)
             {
@@ -120,7 +120,7 @@ public override object ConvertTo(ITypeDescriptorContext context, CultureInfo cul
             return base.ConvertTo(context, culture, value, destinationType);
         }
 
-        public override object CreateInstance(ITypeDescriptorContext context, IDictionary propertyValues)
+        public override object CreateInstance(ITypeDescriptorContext? context, IDictionary propertyValues)
         {
             if (propertyValues == null)
             {
@@ -140,16 +140,16 @@ public override object CreateInstance(ITypeDescriptorContext context, IDictionar
 
                 if (original is Padding originalPadding)
                 {
-                    int all = (int)propertyValues[nameof(Padding.All)];
+                    int all = (int)propertyValues[nameof(Padding.All)]!;
                     if (originalPadding.All != all)
                         return new Padding(all);
                 }
 
                 return new Padding(
-                    (int)propertyValues[nameof(Padding.Left)],
-                    (int)propertyValues[nameof(Padding.Top)],
-                    (int)propertyValues[nameof(Padding.Right)],
-                    (int)propertyValues[nameof(Padding.Bottom)]
+                    (int)propertyValues[nameof(Padding.Left)]!,
+                    (int)propertyValues[nameof(Padding.Top)]!,
+                    (int)propertyValues[nameof(Padding.Right)]!,
+                    (int)propertyValues[nameof(Padding.Bottom)]!
                 );
             }
             catch (InvalidCastException invalidCast)
@@ -162,14 +162,14 @@ public override object CreateInstance(ITypeDescriptorContext context, IDictionar
             }
         }
 
-        public override bool GetCreateInstanceSupported(ITypeDescriptorContext context) => true;
+        public override bool GetCreateInstanceSupported(ITypeDescriptorContext? context) => true;
 
-        public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
+        public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext? context, object value, Attribute[] attributes)
         {
             PropertyDescriptorCollection props = TypeDescriptor.GetProperties(typeof(Padding), attributes);
             return props.Sort(new string[] { nameof(Padding.All), nameof(Padding.Left), nameof(Padding.Top), nameof(Padding.Right), nameof(Padding.Bottom) });
         }
 
-        public override bool GetPropertiesSupported(ITypeDescriptorContext context) => true;
+        public override bool GetPropertiesSupported(ITypeDescriptorContext? context) => true;
     }
 }
diff --git a/src/System.Windows.Forms.Primitives/tests/Interop/Mso/IMsoComponentManagerTests.cs b/src/System.Windows.Forms.Primitives/tests/Interop/Mso/IMsoComponentManagerTests.cs
index 8ec521c08ec..e3ccfe5ef1b 100644
--- a/src/System.Windows.Forms.Primitives/tests/Interop/Mso/IMsoComponentManagerTests.cs
+++ b/src/System.Windows.Forms.Primitives/tests/Interop/Mso/IMsoComponentManagerTests.cs
@@ -14,8 +14,8 @@ public unsafe class IMsoComponentManagerTests : IClassFixture<ThreadExceptionFix
     {
         private IMsoComponentManager CreateComponentManager()
             => (IMsoComponentManager)Activator.CreateInstance(
-                typeof(Application).Assembly.GetType("System.Windows.Forms.Application+ComponentManager"),
-                nonPublic: true);
+                typeof(Application).Assembly.GetType("System.Windows.Forms.Application+ComponentManager")!,
+                nonPublic: true)!;
 
         [Fact]
         public void FDebugMessage_ReturnsTrue()
diff --git a/src/System.Windows.Forms.Primitives/tests/Interop/User32/GetWindowTextTests.cs b/src/System.Windows.Forms.Primitives/tests/Interop/User32/GetWindowTextTests.cs
index 1f40c99f426..bdc767d3eca 100644
--- a/src/System.Windows.Forms.Primitives/tests/Interop/User32/GetWindowTextTests.cs
+++ b/src/System.Windows.Forms.Primitives/tests/Interop/User32/GetWindowTextTests.cs
@@ -50,13 +50,13 @@ private void CallGetWindowText(bool useBeforeGetTextLengthCallback)
 
         private class ChangeWindowTextForm : Form
         {
-            public Func<string> BeforeGetTextLengthCallback
+            public Func<string>? BeforeGetTextLengthCallback
             {
                 get;
                 set;
             }
 
-            public Func<string> BeforeGetTextCallback
+            public Func<string>? BeforeGetTextCallback
             {
                 get;
                 set;
@@ -66,7 +66,7 @@ protected override void WndProc(ref Message m)
             {
                 if (m.Msg == (int)WM.GETTEXTLENGTH)
                 {
-                    string text = BeforeGetTextLengthCallback?.Invoke();
+                    string? text = BeforeGetTextLengthCallback?.Invoke();
                     if (text != null)
                     {
                         SetWindowTextW(m.HWnd, text);
@@ -74,7 +74,7 @@ protected override void WndProc(ref Message m)
                 }
                 else if (m.Msg == (int)WM.GETTEXT)
                 {
-                    string text = BeforeGetTextCallback?.Invoke();
+                    string? text = BeforeGetTextCallback?.Invoke();
                     if (text != null)
                     {
                         SetWindowTextW(m.HWnd, text);
diff --git a/src/System.Windows.Forms.Primitives/tests/System.Windows.Forms.Primitives.Tests.csproj b/src/System.Windows.Forms.Primitives/tests/System.Windows.Forms.Primitives.Tests.csproj
index e080b1c0b9d..4e6b7b3747b 100644
--- a/src/System.Windows.Forms.Primitives/tests/System.Windows.Forms.Primitives.Tests.csproj
+++ b/src/System.Windows.Forms.Primitives/tests/System.Windows.Forms.Primitives.Tests.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <AssemblyName>System.Windows.Forms.Primitives.Tests</AssemblyName>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>