[Dark Mode] Button with FlatStyle 'Standard' is not in dark mode on 100% dpi after enabled SystemColorMode.Dark

[Olina-Zhang]

.NET version

9.0.0-rc.1.24418.2 + private dlls built from winforms repo main branch

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

This is a new feature Dark Mode: #11857

Issue description

On 100% DPI, Button with FlatStyle 'Standard' is not in dark mode after enabled SystemColorMode.Dark, it cannot repro when monitor's dpi is greater than 100%.

on 100%DPI:

on 150%DPI:

Steps to reproduce

1. Create a WinForms .NET application with button control, set button's FlatStyle == Standard
2. Enable DarkMode by Application.SetColorMode(SystemColorMode.Dark) in Program.cs file
3. Run the application to observe

[dotnet-policy-service]

This issue is now marked as "help wanted", and we’re looking for a community volunteer to work on this issue. If we receive no interest in 180 days, we will close the issue. To learn more about how we handle feature requests, please see our documentation.

Happy Coding!

[gerhardol]

A fix for this (and similar) issues is to set the back color when loading the form

In the (base) form constructor

Load += (s, e) => ((Form)s!).FixVisualStyle();

public static class ThemeFix
{
    private static readonly ConditionalWeakTable<IWin32Window, IWin32Window> AlreadyFixedControls =
        [];

    private static readonly ConditionalWeakTable<IWin32Window, IWin32Window> AlreadyFixedContextMenuOwners =
        [];

    public static ThemeSettings ThemeSettings { private get; set; } = ThemeSettings.Default;

    public static void FixVisualStyle(this Control container)
    {
        container.DescendantsToFix<Button>()
            .ForEach(SetupButton);
    }

    private static IEnumerable<TControl> DescendantsToFix<TControl>(this Control c)
        where TControl : Control
    {
        return c.FindDescendantsOfType<TControl>(SkipThemeAware)
            .Where(control => TryAddToWeakTable(control, AlreadyFixedControls));
    }

    private static bool SkipThemeAware(Control c) =>
        c.GetType().GetCustomAttribute<ThemeAwareAttribute>() is not null;
        private static void SetupButton(this Button button)
        {
            // .net9 fix for https://github.com/dotnet/winforms/issues/11949 (only supposed to occur for 100%)
#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 (Application.IsDarkModeEnabled && button.FlatStyle == FlatStyle.Standard)
            {
                button.TouchBackColor();
            }
#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 static void TouchBackColor(this Control c)
        {
            c.BackColor = c.BackColor;
        }

        private static void TouchForeColor(this Control c)
        {
            c.ForeColor = c.ForeColor;
        }

        private static bool TryAddToWeakTable(IWin32Window element, ConditionalWeakTable<IWin32Window, IWin32Window> weakTable)
        {
            if (weakTable.TryGetValue(element, out _))
            {
                return false;
            }

            weakTable.Add(element, element);
            return true;
        }
}