Fix dark mode issues.

Add plausibility unit tests.
Apply backwards-compatibility (10->9) fix.
Refactor System-handling for SetColor.
Fix XML comments and property value handling in Formxxx properties.
Fix bug where in Visual Basic VisualStyles no longer get applied.
Fix raising wrong event on changing FormCaptionTextColor.

Author: KlausLoeffelmann

src/Microsoft.VisualBasic.Forms/src/Microsoft/VisualBasic/ApplicationServices/WindowsFormsApplicationBase.vb

@@ -550,6 +550,10 @@ Namespace Microsoft.VisualBasic.ApplicationServices
             Debug.Assert(dpiSetResult, "We could net set the HighDpiMode.")
 
             ' Now, let's set VisualStyles and ColorMode:
+            If (_enableVisualStyles) Then
+                Application.EnableVisualStyles()
+            End If
+
             Application.SetColorMode(_colorMode)
 
 #Enable Warning WFO5001 ' Type is for evaluation purposes only and is subject to change or removal in future updates.

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

@@ -45,12 +45,12 @@ public sealed partial class Application
     private static bool s_useWaitCursor;
 
 #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.
-    private static SystemColorMode? s_systemColorMode;
+    private static SystemColorMode? s_colorMode;
 #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.
 
     private const string DarkModeKeyPath = "HKEY_CURRENT_USER\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
     private const string DarkModeKey = "AppsUseLightTheme";
-    private const int DarkModeNotAvailable = -1;
+    private const int SystemDarkModeDisabled = 1;
 
     /// <summary>
     ///  Events the user can hook into
@@ -247,27 +247,51 @@ internal static bool CustomThreadExceptionHandlerAttached
         => ThreadContext.FromCurrent().CustomThreadExceptionHandlerAttached;
 
     /// <summary>
-    ///  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"/>.
+    ///  Gets the default color mode (dark mode) for the application.
     /// </summary>
+    /// <remarks>
+    ///  <para>
+    ///    This is the <see cref="SystemColorMode"/> which either has been set by <see cref="SetColorMode(SystemColorMode)"/>
+    ///    or its default value <see cref="SystemColorMode.Classic"/>. If it has been set to <see cref="SystemColorMode.System"/>,
+    ///    then the actual color mode is determined by the system settings (which can be retrieved by the
+    ///    static (shared in VB) <see cref="Application.SystemColorMode"/> property.
+    ///  </para>
+    /// </remarks>
     [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static SystemColorMode ColorMode =>
-        !s_systemColorMode.HasValue
-            ? SystemColorMode.Classic
-            : s_systemColorMode.Value == SystemColorMode.System
-                ? SystemColorMode
-                : s_systemColorMode.Value;
+        s_colorMode ?? SystemColorMode.Classic;
 
     /// <summary>
-    ///  Sets the default dark mode for the application.
+    ///  Sets the default color mode (dark mode) for the application.
     /// </summary>
-    /// <param name="systemColorMode">The default dark mode to set.</param>
+    /// <param name="systemColorMode">The application's default color mode (dark mode) to set.</param>
+    /// <remarks>
+    ///  <para>
+    ///    You should use this method to set the default color mode (dark mode) for the application. Set it,
+    ///    before creating any UI elements, to ensure that the correct color mode is used. You can set it to
+    ///    dark mode (<see cref="SystemColorMode.Dark"/>), light mode (<see cref="SystemColorMode.Classic"/>)
+    ///    or to the system setting (<see cref="SystemColorMode.System"/>).
+    ///  </para>
+    ///  <para>
+    ///    If you set it to <see cref="SystemColorMode.System"/>, the actual color mode is determined by the
+    ///    Windows system settings. If the system setting is changed, the application will not automatically
+    ///    adapt to the new setting.
+    ///  </para>
+    ///  <para>
+    ///    Note that the dark color mode is only available from Windows 11 on or later versions. If the system
+    ///    is set to a high contrast mode, the dark mode is not available.
+    ///  </para>
+    ///  <para>
+    ///    <b>Note for Visual Basic:</b> If you are using the Visual Basic Application Framework, you should set the
+    ///    color mode by handling the Application Events (see "WindowsFormsApplicationBase.ApplyApplicationDefaults").
+    ///  </para>
+    /// </remarks>
     [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static void SetColorMode(SystemColorMode systemColorMode)
     {
         try
         {
-            // Can't use the Generator here, since it cannot deal with experimentals.
+            // Can't use the Generator here, since it cannot deal with [Experimental].
             _ = systemColorMode switch
             {
                 SystemColorMode.Classic => systemColorMode,
@@ -276,18 +300,12 @@ public static void SetColorMode(SystemColorMode systemColorMode)
                 _ => throw new ArgumentOutOfRangeException(nameof(systemColorMode))
             };
 
-            if (systemColorMode == s_systemColorMode)
+            if (systemColorMode == s_colorMode)
             {
                 return;
             }
 
-            if (GetSystemColorModeInternal() > -1)
-            {
-                s_systemColorMode = systemColorMode;
-                return;
-            }
-
-            s_systemColorMode = SystemColorMode.Classic;
+            s_colorMode = systemColorMode;
         }
         finally
         {
@@ -315,6 +333,7 @@ static void NotifySystemEventsOfColorChange()
             bool complete = false;
             bool success = PInvoke.SendMessageCallback(hwnd, PInvoke.WM_SYSCOLORCHANGE + MessageId.WM_REFLECT, () => complete = true);
             Debug.Assert(success);
+
             if (!success)
             {
                 return;
@@ -357,25 +376,21 @@ private static int GetSystemColorModeInternal()
     {
         if (SystemInformation.HighContrast)
         {
-            return DarkModeNotAvailable;
+            return SystemDarkModeDisabled;
         }
 
-        int systemColorMode = DarkModeNotAvailable;
+        int systemColorMode = SystemDarkModeDisabled;
 
-        // Dark mode is supported when we are >= W11/22000
-        // Technically, we could go earlier, but then the APIs we're using weren't officially public.
-        if (OsVersion.IsWindows11_OrGreater())
+        try
+        {
+            // 0 for dark mode and |1| for light mode.
+            systemColorMode = Math.Abs((Registry.GetValue(
+                keyName: DarkModeKeyPath,
+                valueName: DarkModeKey,
+                defaultValue: SystemDarkModeDisabled) as int?) ?? systemColorMode);
+        }
+        catch (Exception ex) when (!ex.IsCriticalException())
         {
-            try
-            {
-                systemColorMode = (Registry.GetValue(
-                    keyName: DarkModeKeyPath,
-                    valueName: DarkModeKey,
-                    defaultValue: DarkModeNotAvailable) as int?) ?? systemColorMode;
-            }
-            catch (Exception ex) when (!ex.IsCriticalException())
-            {
-            }
         }
 
         return systemColorMode;
@@ -388,7 +403,8 @@ private static int GetSystemColorModeInternal()
     [Experimental(DiagnosticIDs.ExperimentalDarkMode, UrlFormat = DiagnosticIDs.UrlFormat)]
     public static bool IsDarkModeEnabled =>
         !SystemInformation.HighContrast
-        && (ColorMode == SystemColorMode.Dark);
+        && (ColorMode == SystemColorMode.Dark
+            || (ColorMode == SystemColorMode.System && SystemColorMode == SystemColorMode.Dark));
 
     /// <summary>
     ///  Gets the path for the executable file that started the application.