Fix GDI+ crash in FillRectangle with AntiAlias and 24bppRgb #14115

merriemcgaw · 2025-12-05T18:00:35Z

Description

This PR addresses a crash (AccessViolationException in .NET 8, ExecutionEngineException in .NET 9+) that occurs when calling \Graphics.FillRectangle\ with a specific combination of parameters:

\PixelFormat.Format24bppRgb\
\SmoothingMode.AntiAlias\
Drawing outside the bitmap bounds

This is a known GDI+ bug where it writes out of bounds. In .NET 9, the runtime treats this memory corruption as fatal and terminates the process.

Fix

Added parameter validation in \Graphics.FillRectangle\ to detect this specific unsafe combination and throw a managed \ArgumentException\ instead of allowing the process to crash.

Regression

This is a fix for a crash that became fatal in .NET 9 due to stricter runtime exception handling.

Testing

Verified with a reproduction app that previously crashed the process and now correctly throws an ArgumentException.

Microsoft Reviewers: Open in CodeFlow

Copilot

Pull request overview

This PR fixes a critical crash that occurs when calling Graphics.FillRectangle with a specific combination of parameters: PixelFormat.Format24bppRgb, SmoothingMode.AntiAlias, and drawing coordinates outside the bitmap bounds. This GDI+ bug causes an AccessViolationException in .NET 8 and an ExecutionEngineException in .NET 9+, with the latter being fatal and terminating the process.

Key Changes:

Added parameter validation in Graphics.FillRectangle to detect the unsafe combination and throw a managed ArgumentException before GDI+ can crash
Added a test case to verify the fix throws the expected exception instead of crashing

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
src/System.Drawing.Common/src/System/Drawing/Graphics.cs	Added bounds validation check in `FillRectangle` method to prevent GDI+ crash when using AntiAlias with 24bppRgb format
src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs	Added test to verify ArgumentException is thrown for the problematic combination instead of crashing

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs

codecov · 2025-12-05T19:02:37Z

Codecov Report

❌ Patch coverage is 81.20567% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15548%. Comparing base (ae5cd21) to head (ff893a1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14115         +/-   ##
===================================================
- Coverage   77.15789%   77.15548%   -0.00241%     
===================================================
  Files           3279        3279                 
  Lines         645317      645599        +282     
  Branches       47718       47761         +43     
===================================================
+ Hits          497913      498115        +202     
- Misses        143717      143783         +66     
- Partials        3687        3701         +14

Flag	Coverage Δ
Debug	`77.15548% <81.20567%> (-0.00241%)`	⬇️
integration	`18.97260% <1.35747%> (-0.01322%)`	⬇️
production	`52.03427% <76.01810%> (+0.00903%)`	⬆️
test	`97.40793% <100.00000%> (+0.00043%)`	⬆️
unit	`49.48328% <76.01810%> (+0.00681%)`	⬆️

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KlausLoeffelmann

Looks good to me!

… AntiAlias

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 9 comments.

Copilot · 2025-12-06T00:36:33Z

src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs

+        RectangleF[] rects = new[]
+        {
+            new RectangleF(190.5f, 180.5f, 100, 100), // Issue repro
+            new RectangleF(-100, 50, 50, 50),         // Fully out left
+            new RectangleF(300, 50, 50, 50),          // Fully out right
+            new RectangleF(-10, -10, 50, 50),         // Partial top-left
+            new RectangleF(200, 200, 100, 100),       // Partial bottom-right
+            new RectangleF(50, 50, 50, 50)            // Fully inside
+        };


According to the coding guidelines (section 1.2), prefer modern collection initializers using [] syntax:

RectangleF[] rects = [ new RectangleF(190.5f, 180.5f, 100, 100), // Issue repro new RectangleF(-100, 50, 50, 50), // Fully out left new RectangleF(300, 50, 50, 50), // Fully out right new RectangleF(-10, -10, 50, 50), // Partial top-left new RectangleF(200, 200, 100, 100), // Partial bottom-right new RectangleF(50, 50, 50, 50) // Fully inside ];

Copilot · 2025-12-06T00:36:34Z

src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs

+    public void FillRectangles_AntiAlias_24bppRgb_OutOfBounds_DoesNotThrow()
+    {
+        using Bitmap bmp = new(256, 256, PixelFormat.Format24bppRgb);
+        using Graphics g = Graphics.FromImage(bmp);
+        g.SmoothingMode = SmoothingMode.AntiAlias;


[nitpick] For consistency with the other test methods in this file (e.g., lines 3017, 3033, 3043, 3054), consider using the variable name graphics instead of g for the Graphics instance.

Copilot · 2025-12-06T00:36:34Z

src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs

+    public void DrawImage_Float_AntiAlias_24bppRgb_OutOfBounds_DoesNotThrow()
+    {
+        using Bitmap bmp = new(256, 256, PixelFormat.Format24bppRgb);
+        using Graphics g = Graphics.FromImage(bmp);
+        g.SmoothingMode = SmoothingMode.AntiAlias;


[nitpick] For consistency with the other test methods in this file (e.g., lines 3017, 3033, 3043, 3054), consider using the variable name graphics instead of g for the Graphics instance.

Copilot · 2025-12-06T00:36:34Z

src/System.Drawing.Common/tests/System/Drawing/GraphicsTests.cs

+    public void DrawImage_RectF_AntiAlias_24bppRgb_OutOfBounds_DoesNotThrow()
+    {
+        using Bitmap bmp = new(256, 256, PixelFormat.Format24bppRgb);
+        using Graphics g = Graphics.FromImage(bmp);
+        g.SmoothingMode = SmoothingMode.AntiAlias;


[nitpick] For consistency with the other test methods in this file (e.g., lines 3017, 3033, 3043, 3054), consider using the variable name graphics instead of g for the Graphics instance.

Copilot · 2025-12-06T00:36:34Z

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

+        // GDI+ has a bug where it writes out of bounds when using AntiAlias smoothing mode
+        // on a 24bppRgb bitmap, causing an AccessViolationException (or ExecutionEngineException in .NET 9+).
+        // We validate the parameters here to prevent the crash.
+        if (SmoothingMode == Drawing2D.SmoothingMode.AntiAlias &&
+            _backingImage is { PixelFormat: PixelFormat.Format24bppRgb })
+        {
+            float left = x;
+            float right = x + width;
+            float top = y;
+            float bottom = y + height;
+
+            if (width < 0)
+            {
+                left = right;
+                right = x;
+            }
+
+            if (height < 0)
+            {
+                top = bottom;
+                bottom = y;
+            }
+
+            if (left < 0)
+            {
+                left = 0;
+            }
+
+            if (right > _backingImage.Width)
+            {
+                right = _backingImage.Width;
+            }
+
+            if (top < 0)
+            {
+                top = 0;
+            }
+
+            if (bottom > _backingImage.Height)
+            {
+                bottom = _backingImage.Height;
+            }
+
+            if (left >= right || top >= bottom)
+            {
+                // The rectangle is completely outside the image bounds.
+                return;
+            }
+
+            x = left;
+            y = top;
+            width = right - left;
+            height = bottom - top;
+        }


This clipping logic is duplicated across four methods (FillRectangle, FillRectangles, and two DrawImage overloads). Consider extracting this into a private helper method to improve maintainability. For example:

private bool ShouldClipForAntiAlias24bppRgb(out int imageWidth, out int imageHeight) { imageWidth = 0; imageHeight = 0; if (SmoothingMode != Drawing2D.SmoothingMode.AntiAlias || _backingImage is not { PixelFormat: PixelFormat.Format24bppRgb }) { return false; } imageWidth = _backingImage.Width; imageHeight = _backingImage.Height; return true; } private static void NormalizeRectangle(float x, float y, float width, float height, out float left, out float right, out float top, out float bottom) { left = x; right = x + width; top = y; bottom = y + height; if (width < 0) { left = right; right = x; } if (height < 0) { top = bottom; bottom = y; } } private static void ClipRectangle(ref float left, ref float right, ref float top, ref float bottom, int maxWidth, int maxHeight) { if (left < 0) left = 0; if (right > maxWidth) right = maxWidth; if (top < 0) top = 0; if (bottom > maxHeight) bottom = maxHeight; }

This would reduce duplication and make the code more maintainable.

Copilot · 2025-12-06T00:36:35Z

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

    public void DrawImage(Image image, RectangleF destRect, RectangleF srcRect, GraphicsUnit srcUnit)
    {
        ArgumentNullException.ThrowIfNull(image);



According to the coding guidelines (section 1.11), an empty line should be inserted after structure blocks. Add an empty line between ArgumentNullException.ThrowIfNull(image); and the start of the clipping logic to improve readability.

Copilot · 2025-12-06T00:36:35Z

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

+                float destW = right - left;
+                float destH = bottom - top;
+
+                if (Math.Abs(destW) > float.Epsilon && Math.Abs(destH) > float.Epsilon)


The check Math.Abs(destW) > float.Epsilon may not be the most appropriate for graphics dimensions. float.Epsilon is the smallest positive value greater than zero (approximately 1.4e-45), which is extremely small. For graphics operations, consider using a more reasonable threshold for zero comparisons, such as 0.001f or simply checking for exact zero if negative dimensions are normalized earlier. The current check would allow extremely small non-zero values that could still cause issues in division operations.

Copilot · 2025-12-06T00:36:35Z

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

+                float destW = right - left;
+                float destH = bottom - top;
+
+                if (Math.Abs(destW) > float.Epsilon && Math.Abs(destH) > float.Epsilon)


The check Math.Abs(destW) > float.Epsilon may not be the most appropriate for graphics dimensions. float.Epsilon is the smallest positive value greater than zero (approximately 1.4e-45), which is extremely small. For graphics operations, consider using a more reasonable threshold for zero comparisons, such as 0.001f or simply checking for exact zero if negative dimensions are normalized earlier. The current check would allow extremely small non-zero values that could still cause issues in division operations.

Copilot · 2025-12-06T00:36:35Z

src/System.Drawing.Common/src/System/Drawing/Graphics.cs

    public void DrawImage(Image image, float x, float y, float width, float height)
    {
        ArgumentNullException.ThrowIfNull(image);
+


According to the coding guidelines (section 1.11), an empty line should be inserted after structure blocks. Add an empty line between ArgumentNullException.ThrowIfNull(image); and the start of the clipping logic to improve readability.

KlausLoeffelmann · 2025-12-06T09:54:43Z

@copilot: I am wondering. If we defined a clipping region temporarily, would that have a chance to mitigate the underlying access violation?

Copilot · 2025-12-06T09:54:52Z

@KlausLoeffelmann I've opened a new pull request, #14117, to work on those changes. Once the pull request is ready, I'll request review from you.

Fix GDI+ crash in FillRectangle with AntiAlias and 24bppRgb

c91c408

merriemcgaw requested a review from a team as a code owner December 5, 2025 18:00

github-actions bot added the area-Infrastructure label Dec 5, 2025

dotnet-policy-service bot assigned merriemcgaw Dec 5, 2025

Add regression test for GDI+ crash in FillRectangle

d0c4e1d

merriemcgaw requested a review from Copilot December 5, 2025 18:30

Copilot started reviewing on behalf of merriemcgaw December 5, 2025 18:30 View session

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merriemcgaw added 2 commits December 5, 2025 11:03

Address PR feedback: localize error message, simplify enum, expand tests

05d1d10

Address PR feedback: handle negative width/height in bounds check

528f02f

KlausLoeffelmann previously approved these changes Dec 5, 2025

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Fix GDI+ crash by clipping out-of-bounds drawing for Format24bppRgb +…

ff893a1

… AntiAlias

merriemcgaw dismissed KlausLoeffelmann’s stale review via ff893a1 December 6, 2025 00:15

merriemcgaw requested review from KlausLoeffelmann and Copilot December 6, 2025 00:21

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Copilot AI mentioned this pull request Dec 6, 2025

Fix GDI+ crash in FillRectangle with AntiAlias and 24bppRgb #14117

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Fix GDI+ crash in FillRectangle with AntiAlias and 24bppRgb #14115

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Fix GDI+ crash in FillRectangle with AntiAlias and 24bppRgb #14115

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