Apply my feedback to Klaus' dark mode feature

- Move Experimental URL to DiagnosticIds
- Move special dark mode logic from CreateBrushScope to GetSysColorBrush
- Remove unnecessary logic in FindNearestColor
- Update brushes and pens by poking SystemEvents static directly

src/System.Drawing.Common/src/System/Drawing/SystemBrushes.cs

@@ -5,11 +5,6 @@ namespace System.Drawing;
 
 public static class SystemBrushes
 {
-#if NET9_0_OR_GREATER
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-    private static bool s_colorSetOnLastAccess = SystemColors.UseAlternativeColorSet;
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-#endif
     private static readonly object s_systemBrushesKey = new();
 
     public static Brush ActiveBorder => FromSystemColor(SystemColors.ActiveBorder);
@@ -62,18 +57,6 @@ public static Brush FromSystemColor(Color c)
             throw new ArgumentException(SR.Format(SR.ColorNotSystemColor, c.ToString()));
         }
 
-#if NET9_0_OR_GREATER
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-        if (s_colorSetOnLastAccess != SystemColors.UseAlternativeColorSet)
-        {
-            s_colorSetOnLastAccess = SystemColors.UseAlternativeColorSet;
-
-            // We need to clear the SystemBrushes cache, when the ColorMode had changed.
-            Gdip.ThreadData.Remove(s_systemBrushesKey);
-        }
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-#endif
-
         if (!Gdip.ThreadData.TryGetValue(s_systemBrushesKey, out object? tempSystemBrushes)
             || tempSystemBrushes is not Brush[] systemBrushes)
         {

src/System.Drawing.Common/src/System/Drawing/SystemPens.cs

@@ -5,12 +5,6 @@ namespace System.Drawing;
 
 public static class SystemPens
 {
-#if NET9_0_OR_GREATER
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-    private static bool s_colorSetOnLastAccess = SystemColors.UseAlternativeColorSet;
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-#endif
-
     private static readonly object s_systemPensKey = new();
 
     public static Pen ActiveBorder => FromSystemColor(SystemColors.ActiveBorder);
@@ -64,18 +58,6 @@ public static Pen FromSystemColor(Color c)
             throw new ArgumentException(SR.Format(SR.ColorNotSystemColor, c.ToString()));
         }
 
-#if NET9_0_OR_GREATER
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-        if (s_colorSetOnLastAccess != SystemColors.UseAlternativeColorSet)
-        {
-            s_colorSetOnLastAccess = SystemColors.UseAlternativeColorSet;
-
-            // We need to clear the SystemBrushes cache, when the ColorMode had changed.
-            Gdip.ThreadData.Remove(s_systemPensKey);
-        }
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-#endif
-
         if (!Gdip.ThreadData.TryGetValue(s_systemPensKey, out object? tempSystemPens)
             || tempSystemPens is not Pen[] systemPens)
         {

src/System.Windows.Forms.Analyzers/src/System/Windows/Forms/Analyzers/Diagnostics/DiagnosticIDs.cs

@@ -19,7 +19,6 @@ internal static class DiagnosticIDs
     public const string DisposeModalDialog = "WFO2000";
 
     // Experimental, number group 5000+
-    public const string ExperimentalVisualStyles = "WFO5000";
     public const string ExperimentalDarkMode = "WFO5001";
     public const string ExperimentalAsync = "WFO5002";
 }

src/System.Windows.Forms.Primitives/src/NativeMethods.txt

@@ -599,6 +599,7 @@ SelectPalette
 SendDlgItemMessage
 SendInput
 SendMessage
+SendMessageCallback
 SendMessageTimeout
 SetActiveWindow
 SetBkColor

src/System.Windows.Forms.Primitives/src/System/Windows/Forms/DeviceContextExtensions.cs

@@ -116,22 +116,7 @@ internal static unsafe void DrawLines(this HDC hdc, HPEN hpen, ReadOnlySpan<int>
     /// </remarks>
     internal static Color FindNearestColor(this HDC hdc, Color color)
     {
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-        if (color.IsSystemColor && SystemColors.UseAlternativeColorSet)
-        {
-            // We need to "un-system" the color for nearest color to work correctly in dark mode,
-            // since GetNearestColor doesn't understand dark mode system colors and internally works
-            // with the light mode colors.
-            color = Color.FromArgb(color.ToArgb());
-            Debug.Assert(!color.IsSystemColor, "Color should not be a system color.");
-        }
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-
-        Color newColor = ColorTranslator.FromWin32(
-            (int)PInvoke.GetNearestColor(
-                hdc: hdc,
-                color: (COLORREF)(uint)ColorTranslator.ToWin32(color)).Value);
-
+        Color newColor = ColorTranslator.FromWin32((int)PInvoke.GetNearestColor(hdc, (COLORREF)(uint)ColorTranslator.ToWin32(color)).Value);
         return newColor.ToArgb() == color.ToArgb() ? color : newColor;
     }
 

src/System.Windows.Forms.Primitives/src/Windows/Win32/CreateBrushScope.cs

@@ -28,11 +28,9 @@ internal readonly ref struct CreateBrushScope
     /// </summary>
     public CreateBrushScope(Color color)
     {
-#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-        HBRUSH = color.IsSystemColor && !SystemColors.UseAlternativeColorSet
+        HBRUSH = color.IsSystemColor
             ? PInvoke.GetSysColorBrush(color)
             : PInvoke.CreateSolidBrush(color);
-#pragma warning restore SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
 
         ValidateBrushHandle();
     }

src/System.Windows.Forms.Primitives/src/Windows/Win32/PInvoke.GetSysColorBrush.cs

@@ -10,9 +10,19 @@ internal static partial class PInvoke
     /// <inheritdoc cref="GetSysColorBrush(SYS_COLOR_INDEX)"/>
     public static HBRUSH GetSysColorBrush(Color systemColor)
     {
+#pragma warning disable SYSLIB5002 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
+        bool useSolidBrush = SystemColors.UseAlternativeColorSet;
+#pragma warning restore SYSLIB5002
+
+        if (useSolidBrush)
+        {
+            // We don't have a real system color, so we'll just create a solid brush.
+            return CreateSolidBrush(systemColor);
+        }
+
         Debug.Assert(systemColor.IsSystemColor);
 
         // Extract the COLOR value
-        return PInvoke.GetSysColorBrush((SYS_COLOR_INDEX)(ColorTranslator.ToOle(systemColor) & 0xFF));
+        return GetSysColorBrush((SYS_COLOR_INDEX)(ColorTranslator.ToOle(systemColor) & 0xFF));
     }
 }

src/System.Windows.Forms.Primitives/src/Windows/Win32/PInvoke.SendMessageCallback.cs

@@ -0,0 +1,33 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using System.Runtime.CompilerServices;
+
+namespace Windows.Win32;
+
+internal static partial class PInvoke
+{
+    internal static unsafe BOOL SendMessageCallback<T>(
+       T hWnd,
+       MessageId Msg,
+       Action callback,
+       WPARAM wParam = default,
+       LPARAM lParam = default)
+       where T : IHandle<HWND>
+    {
+        GCHandle gcHandle = GCHandle.Alloc(callback);
+        BOOL result = SendMessageCallback(hWnd.Handle, Msg, wParam, lParam, &NativeCallback, (nuint)(nint)gcHandle);
+        GC.KeepAlive(hWnd.Wrapper);
+        return result;
+    }
+
+    [UnmanagedCallersOnly(CallConvs = [typeof(CallConvStdcall)])]
+    private static void NativeCallback(HWND hwnd, uint Msg, nuint dwData, LRESULT lResult)
+    {
+        GCHandle gcHandle = (GCHandle)(nint)dwData;
+        Action action = (Action)gcHandle.Target!;
+        gcHandle.Free();
+        action();
+    }
+}

src/System.Windows.Forms/src/System/Windows/Forms/Application.cs

@@ -48,8 +48,6 @@ public sealed partial class Application
     private static SystemColorMode? s_systemColorMode;
 #pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
 
-    internal const string WinFormsExperimentalUrl = "https://aka.ms/winforms-experimental/{0}";
-
     private const string DarkModeKeyPath = "HKEY_CURRENT_USER\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
     private const string DarkModeKey = "AppsUseLightTheme";
     private const int DarkModeNotAvailable = -1;
@@ -252,7 +250,7 @@ internal static bool CustomThreadExceptionHandlerAttached
     ///  Gets the default dark mode for the application. This is the SystemColorMode which either has been set
     ///  by <see cref="SetColorMode(SystemColorMode)"/> or its default value <see cref="SystemColorMode.Classic"/>.
     /// </summary>
-    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = WinFormsExperimentalUrl)]
+    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static SystemColorMode ColorMode =>
         !s_systemColorMode.HasValue
             ? SystemColorMode.Classic
@@ -264,7 +262,7 @@ internal static bool CustomThreadExceptionHandlerAttached
     ///  Sets the default dark mode for the application.
     /// </summary>
     /// <param name="systemColorMode">The default dark mode to set.</param>
-    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = WinFormsExperimentalUrl)]
+    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static void SetColorMode(SystemColorMode systemColorMode)
     {
         try
@@ -278,6 +276,11 @@ public static void SetColorMode(SystemColorMode systemColorMode)
                 _ => throw new ArgumentOutOfRangeException(nameof(systemColorMode))
             };
 
+            if (systemColorMode == s_systemColorMode)
+            {
+                return;
+            }
+
             if (GetSystemColorModeInternal() > -1)
             {
                 s_systemColorMode = systemColorMode;
@@ -288,7 +291,40 @@ public static void SetColorMode(SystemColorMode systemColorMode)
         }
         finally
         {
-            SystemColors.UseAlternativeColorSet = IsDarkModeEnabled;
+            bool useAlternateColorSet = SystemColors.UseAlternativeColorSet;
+            bool darkModeEnabled = IsDarkModeEnabled;
+
+            if (useAlternateColorSet != darkModeEnabled)
+            {
+                SystemColors.UseAlternativeColorSet = darkModeEnabled;
+                NotifySystemEventsOfColorChange();
+            }
+        }
+
+        static void NotifySystemEventsOfColorChange()
+        {
+            string s_systemTrackerWindow = $".NET-BroadcastEventWindow.{AppDomain.CurrentDomain.GetHashCode():x}.0";
+
+            HWND hwnd = PInvoke.FindWindow(s_systemTrackerWindow, s_systemTrackerWindow);
+            if (hwnd.IsNull)
+            {
+                // Haven't created the window yet, so no need to notify.
+                return;
+            }
+
+            bool complete = false;
+            bool success = PInvoke.SendMessageCallback(hwnd, PInvoke.WM_SYSCOLORCHANGE + MessageId.WM_REFLECT, () => complete = true);
+            Debug.Assert(success);
+            if (!success)
+            {
+                return;
+            }
+
+            while (!complete)
+            {
+                DoEvents();
+                Thread.Yield();
+            }
         }
     }
 
@@ -310,7 +346,7 @@ public static void SetColorMode(SystemColorMode systemColorMode)
     ///   SystemColorModes is not supported, if the Windows OS <c>High Contrast Mode</c> has been enabled in the system settings.
     ///  </para>
     /// </remarks>
-    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = WinFormsExperimentalUrl)]
+    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static SystemColorMode SystemColorMode =>
         GetSystemColorModeInternal() == 0
             ? SystemColorMode.Dark
@@ -349,7 +385,7 @@ private static int GetSystemColorModeInternal()
     ///  Gets a value indicating whether the application is running in a dark system color context.
     ///  Note: In a high contrast mode, this will always return <see langword="false"/>.
     /// </summary>
-    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = WinFormsExperimentalUrl)]
+    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static bool IsDarkModeEnabled =>
         !SystemInformation.HighContrast
         && (ColorMode == SystemColorMode.Dark);

src/System.Windows.Forms/src/System/Windows/Forms/Control.cs

@@ -716,8 +716,7 @@ internal HBRUSH BackColorBrush
             Color color = BackColor;
             HBRUSH backBrush;
 
-#pragma warning disable WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
-            if (color.IsSystemColor && !Application.IsDarkModeEnabled)
+            if (color.IsSystemColor)
             {
                 backBrush = PInvoke.GetSysColorBrush(color);
                 SetState(States.OwnCtlBrush, false);
@@ -727,7 +726,6 @@ internal HBRUSH BackColorBrush
                 backBrush = PInvoke.CreateSolidBrush((COLORREF)(uint)ColorTranslator.ToWin32(color));
                 SetState(States.OwnCtlBrush, true);
             }
-#pragma warning restore WFO5001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
 
             Debug.Assert(!backBrush.IsNull, "Failed to create brushHandle");
             Properties.SetObject(s_backBrushProperty, backBrush);

src/System.Windows.Forms/src/System/Windows/Forms/ControlStyles.cs

@@ -144,6 +144,6 @@ public enum ControlStyles
     ///  from this setting. Note that using this settings will cause some
     ///  win32 control theming renderers to become inactive for a specific theme.
     /// </summary>
-    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = Application.WinFormsExperimentalUrl)]
+    [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     ApplyThemingImplicitly = 0x00080000
 }