Fix issue 14807: CheckBox/RadioButton: ToggleSwitch renders black background when BackColor is Transparent

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/CheckBoxModernAdapter.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Drawing;
@@ -91,8 +91,9 @@ private void PaintCore(PaintEventArgs e)
         PaintBackgroundImage(e);
 
         Color? customOnColor = Control.ShouldSerializeBackColor()
-            ? Control.BackColor
-            : null;
+            && Control.BackColor.A == byte.MaxValue
+                ? Control.BackColor
+                : null;
 
         Color? customBorderColor = Control.FlatAppearance.BorderColor.IsEmpty
             ? null

src/System.Windows.Forms/System/Windows/Forms/Controls/Buttons/ButtonInternal/RadioButtonModernAdapter.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Drawing;
@@ -90,8 +90,9 @@ private void PaintCore(PaintEventArgs e)
         PaintBackgroundImage(e);
 
         Color? customOnColor = Control.ShouldSerializeBackColor()
-            ? Control.BackColor
-            : null;
+            && Control.BackColor.A == byte.MaxValue
+                ? Control.BackColor
+                : null;
 
         Color? customBorderColor = Control.FlatAppearance.BorderColor.IsEmpty
             ? null

src/System.Windows.Forms/System/Windows/Forms/Rendering/CheckBox/AnimatedToggleSwitchRenderer.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Drawing;
@@ -88,7 +88,7 @@ public override void RenderControl(Graphics graphics)
             metrics,
             textSize);
 
-        graphics.Clear(Control.BackColor);
+        PaintControlBackground(graphics);
 
         if (contentBounds.Width <= 0 || contentBounds.Height <= 0)
         {
@@ -218,7 +218,7 @@ private void RenderSwitch(Graphics graphics, Rectangle rect, ToggleSwitchMetrics
         Color circleColor = Control.Enabled
             ? highContrast
                 ? highContrastForeground
-                : PopupButtonColorMath.GetReadableForeColor(offColor, onColor)
+                : PopupButtonColorMath.GetReadableForeColor(backgroundColor)
             : SystemColors.GrayText;
         circleColor = ApplyInteractionShade(circleColor, focus);
         borderColor = ApplyInteractionShade(borderColor, focus);
@@ -329,6 +329,25 @@ private void EnsurePositionInitialized()
         _positionInitialized = true;
     }
 
+    private void PaintControlBackground(Graphics graphics)
+    {
+        if (Control.BackgroundImage is null && !Control.BackColor.HasTransparency())
+        {
+            graphics.Clear(Control.BackColor);
+            return;
+        }
+
+        Rectangle clipRectangle = Rectangle.Ceiling(graphics.ClipBounds);
+        clipRectangle.Intersect(Control.ClientRectangle);
+        if (clipRectangle.Width <= 0 || clipRectangle.Height <= 0)
+        {
+            return;
+        }
+
+        using PaintEventArgs paintEventArgs = new(graphics, clipRectangle);
+        Control.PaintBackground(paintEventArgs, clipRectangle);
+    }
+
     private static float EaseOut(float value)
         => 1 - ((1 - value) * (1 - value));
 

src/System.Windows.Forms/System/Windows/Forms/Rendering/RadioButton/AnimatedRadioGlyphRenderer.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Drawing;
@@ -123,6 +123,15 @@ internal void DrawGlyph(
         borderColor = ApplyInteractionShade(borderColor, focus);
         backColor = ApplyInteractionShade(backColor, focus);
 
+        // Blend the outer circle toward the accent based on animated checked progress
+        // (_dotScaleCurrent), so check and uncheck transitions stay smooth and symmetric.
+        float checkedProgress = Math.Clamp(
+            _dotScaleCurrent,
+            0f,
+            1f);
+        Color activeBackColor = LerpColor(backColor, onColor, checkedProgress);
+        Color activeBorderColor = LerpColor(borderColor, onColor, checkedProgress);
+
         float normalOuterScale = 1f / HoverGrowth;
         float outerScale = Lerp(normalOuterScale, 1f, enabled ? _hoverCurrent : 0f);
         RectangleF outerBounds = ScaleFromCenter(bounds, outerScale);
@@ -133,7 +142,7 @@ internal void DrawGlyph(
         {
             graphics.SmoothingMode = SmoothingMode.AntiAlias;
 
-            using (var brush = backColor.GetCachedSolidBrushScope())
+            using (var brush = activeBackColor.GetCachedSolidBrushScope())
             {
                 graphics.FillEllipse(brush, outerBounds);
             }
@@ -142,7 +151,7 @@ internal void DrawGlyph(
                 1,
                 Control.LogicalToDeviceUnits(flatStyle == FlatStyle.Popup ? 2 : 1));
 
-            using (var pen = new Pen(borderColor, borderThickness))
+            using (var pen = new Pen(activeBorderColor, borderThickness))
             {
                 graphics.DrawEllipse(pen, outerBounds);
             }
@@ -156,22 +165,11 @@ internal void DrawGlyph(
                     dotDiameter,
                     dotDiameter);
 
-                Color dotOutlineColor = highContrast
+                Color dotColor = highContrast
                     ? SystemColors.HighlightText
-                    : PopupButtonColorMath.GetReadableForeColor(onColor, backColor);
-                int outlineThickness = Math.Max(1, Control.LogicalToDeviceUnits(1));
-                using var outlineBrush = dotOutlineColor.GetCachedSolidBrushScope();
-                graphics.FillEllipse(outlineBrush, dotRectangle);
-
-                RectangleF accentRectangle = RectangleF.Inflate(
-                    dotRectangle,
-                    -outlineThickness,
-                    -outlineThickness);
-                if (accentRectangle.Width > 0 && accentRectangle.Height > 0)
-                {
-                    using var dotBrush = onColor.GetCachedSolidBrushScope();
-                    graphics.FillEllipse(dotBrush, accentRectangle);
-                }
+                    : PopupButtonColorMath.GetReadableForeColor(onColor);
+                using var dotBrush = dotColor.GetCachedSolidBrushScope();
+                graphics.FillEllipse(dotBrush, dotRectangle);
             }
         }
         finally
@@ -226,6 +224,20 @@ private static float EaseOut(float progress)
     private static float Lerp(float from, float to, float progress)
         => from + ((to - from) * Math.Clamp(progress, 0f, 1f));
 
+    private static Color LerpColor(Color from, Color to, float progress)
+    {
+        progress = Math.Clamp(progress, 0f, 1f);
+
+        return Color.FromArgb(
+            LerpChannel(from.A, to.A, progress),
+            LerpChannel(from.R, to.R, progress),
+            LerpChannel(from.G, to.G, progress),
+            LerpChannel(from.B, to.B, progress));
+
+        static int LerpChannel(int from, int to, float progress)
+            => from + (int)((to - from) * progress);
+    }
+
     private static RectangleF ScaleFromCenter(Rectangle bounds, float scale)
     {
         float width = bounds.Width * scale;

src/test/unit/System.Windows.Forms/System/Windows/Forms/CheckBoxTests.cs

@@ -625,7 +625,43 @@ public void CheckBox_ModernGlyph_DefaultCheckedColorUsesWindowsAccent()
     }
 
     [WinFormsFact]
-    public void CheckBox_ModernGlyph_BrightAccentUsesReadableCheckmark()
+    public void CheckBox_ModernGlyph_TransparentBackColor_CheckedUsesWindowsAccent()
+    {
+        if (SystemInformation.HighContrast)
+        {
+            return;
+        }
+
+        using Panel parent = new() { BackColor = Color.White };
+        using CheckBox box = new()
+        {
+            BackColor = Color.Transparent,
+            CheckState = CheckState.Checked,
+            Size = new Size(40, 24),
+            VisualStylesMode = VisualStylesMode.Net11
+        };
+        parent.Controls.Add(box);
+
+        using Bitmap bitmap = new(box.Width, box.Height);
+        using Graphics graphics = Graphics.FromImage(bitmap);
+        PaintEventArgs e = new(graphics, box.ClientRectangle);
+
+        box.CreateStandardAdapter().PaintUp(e, box.CheckState);
+
+        Assert.True(CountPixels(bitmap, Application.SystemVisualSettings.AccentColor) > 0);
+    }
+
+    [WinFormsTheory]
+    [InlineData(0xFF, 0xB9, 0x00, 0, 0, 0)]
+    [InlineData(0x00, 0x66, 0xCC, 255, 255, 255)]
+    [InlineData(0x4D, 0x4D, 0x4D, 255, 255, 255)]
+    public void CheckBox_ModernGlyph_AccentUsesReadableCheckmark(
+        int accentR,
+        int accentG,
+        int accentB,
+        int expectedR,
+        int expectedG,
+        int expectedB)
     {
         if (SystemInformation.HighContrast)
         {
@@ -637,7 +673,7 @@ public void CheckBox_ModernGlyph_BrightAccentUsesReadableCheckmark()
         renderer.NotifyCheckStateChanged(CheckState.Checked);
         using Bitmap bitmap = new(24, 24);
         using Graphics graphics = Graphics.FromImage(bitmap);
-        Color brightAccent = Color.FromArgb(0xFF, 0xB9, 0x00);
+        Color accent = Color.FromArgb(accentR, accentG, accentB);
 
         renderer.DrawGlyph(
             graphics,
@@ -646,10 +682,10 @@ public void CheckBox_ModernGlyph_BrightAccentUsesReadableCheckmark()
             enabled: true,
             hovered: false,
             focused: false,
-            customOnColor: brightAccent,
+            customOnColor: accent,
             customBorderColor: null);
 
-        Assert.True(CountPixels(bitmap, Color.Black) > 0);
+        Assert.True(CountPixels(bitmap, Color.FromArgb(expectedR, expectedG, expectedB)) > 0);
     }
 
     [WinFormsFact]
@@ -831,6 +867,35 @@ public void CheckBox_ToggleSwitch_DefaultOnColorUsesWindowsAccent()
         Assert.True(CountPixels(bitmap, Application.SystemVisualSettings.AccentColor) > 0);
     }
 
+    [WinFormsFact]
+    public void CheckBox_ToggleSwitch_DarkAccentUsesReadableThumbColor()
+    {
+        if (SystemInformation.HighContrast)
+        {
+            return;
+        }
+
+        using SystemVisualSettingsTestScope settingsScope =
+            new(clientAreaAnimationEnabled: true, accentColor: Color.FromArgb(0x4D, 0x4D, 0x4D));
+        using CheckBox box = new()
+        {
+            Appearance = Appearance.ToggleSwitch,
+            BackColor = Color.Black,
+            Checked = true,
+            Size = new Size(60, 24),
+            VisualStylesMode = VisualStylesMode.Net11
+        };
+        Rendering.CheckBox.AnimatedToggleSwitchRenderer renderer =
+            box.TestAccessor.Dynamic.ToggleSwitchRenderer;
+        renderer.SynchronizeState();
+        using Bitmap bitmap = new(box.Width, box.Height);
+        using Graphics graphics = Graphics.FromImage(bitmap);
+
+        renderer.RenderControl(graphics);
+
+        Assert.True(CountPixels(bitmap, Color.White) > 0);
+    }
+
     [WinFormsFact]
     public void CheckBox_ToggleSwitch_HoverAndFocusAnimateWithoutChangingPreferredSize()
     {

src/test/unit/System.Windows.Forms/System/Windows/Forms/CheckBoxToggleSwitchTests.cs

@@ -226,4 +226,30 @@ public void CheckBox_VisualStylesMode_DefaultIsAmbient()
 
         Assert.Equal(VisualStylesMode.Inherit, checkBox.VisualStylesMode);
     }
+
+    [WinFormsTheory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void CheckBox_ToggleSwitch_TransparentBackColor_DoesNotThrow(bool useBackgroundImage)
+    {
+        using SystemVisualSettingsTestScope settingsScope = new(
+            clientAreaAnimationEnabled: false,
+            highContrastEnabled: false);
+        using Bitmap backgroundImage = new(10, 10);
+        using Panel parent = new() { BackColor = Color.LightBlue, Size = new Size(200, 100) };
+        using CheckBox checkBox = new()
+        {
+            Parent = parent,
+            Appearance = Appearance.ToggleSwitch,
+            VisualStylesMode = VisualStylesMode.Net11,
+            BackColor = Color.Transparent,
+            BackgroundImage = useBackgroundImage ? backgroundImage : null,
+            Size = new Size(140, 36),
+            Text = "Matrix"
+        };
+        parent.CreateControl();
+        using Bitmap bitmap = new(checkBox.Width, checkBox.Height);
+
+        checkBox.DrawToBitmap(bitmap, new Rectangle(Point.Empty, checkBox.Size));
+    }
 }