Writing Tests for GDI+ Rendering APIs

These rules apply when writing tests for existing or new drawing APIs on Graphics, GraphicsPath, Bitmap, Font, and related System.Drawing types. For WinForms control API tests, see the control-api-tests skill. For the drawing API implementation patterns, see the using-and-extending-gdi-plus skill.

Golden rule: Every rendering API needs tests for both integer and float overloads, and must verify that drawing actually produces pixels — not just that no exception is thrown.

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1. Test Project Structure

1.1 Test locations

API area	Test file location
Graphics methods	src\System.Drawing.Common\tests\GraphicsTests.cs
GraphicsPath methods	src\System.Drawing.Common\tests\Drawing2D\GraphicsPathTests.cs
Bitmap methods	src\System.Drawing.Common\tests\BitmapTests.cs
Font / text rendering	src\System.Drawing.Common\tests\FontTests.cs

Add new tests to the existing test file for the class under test. If the file is very large, use a partial file named {Class}Tests.{Feature}.cs.

1.2 Test framework

The project uses xUnit with FluentAssertions. Graphics tests do not require STA threads, so use standard [Fact] and [Theory] attributes (not [WinFormsFact]/[WinFormsTheory]).

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2. .NET Version Guards in Tests

Tests for version-guarded APIs must use the same preprocessor directive as the API itself. Currently, new APIs target at least .NET 11:

#if NET11_0_OR_GREATER
    [Fact]
    public void Graphics_DrawRoundedRectangle_Integer()
    {
        using Bitmap bitmap = new(10, 10);
        using Graphics graphics = Graphics.FromImage(bitmap);
        graphics.DrawRoundedRectangle(Pens.Red, new(0, 0, 10, 10), new(2, 2));
        VerifyBitmapNotEmpty(bitmap);
    }
#endif

Match the guard exactly. If the API is #if NET11_0_OR_GREATER, the tests must also be #if NET11_0_OR_GREATER. This ensures tests compile and run only when the API is available.

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3. Bitmap-Based Rendering Verification

3.1 The verification pattern

The standard approach for testing drawing APIs is:

1. Create a small Bitmap (e.g., 10×10).
2. Obtain a Graphics from the bitmap.
3. Call the drawing API.
4. Verify the bitmap is not empty (contains non-zero pixels).

[Fact]
public void Graphics_DrawMyShape_Integer()
{
    using Bitmap bitmap = new(10, 10);
    using Graphics graphics = Graphics.FromImage(bitmap);
    graphics.DrawMyShape(Pens.Red, new(0, 0, 10, 10));
    VerifyBitmapNotEmpty(bitmap);
}

3.2 VerifyBitmapNotEmpty / VerifyBitmapEmpty helpers

Use the existing unsafe helper methods in GraphicsTests.cs that lock bitmap bits and scan for non-zero pixels:

internal unsafe static void VerifyBitmapNotEmpty(Bitmap bitmap)
{
    BitmapData data = bitmap.LockBits(
        default, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    try
    {
        ReadOnlySpan<byte> bytes = new(
            (byte*)data.Scan0, data.Stride * data.Height);
        bytes.IndexOfAnyExcept((byte)0).Should().NotBe(-1);
    }
    finally
    {
        bitmap.UnlockBits(data);
    }
}

internal unsafe static void VerifyBitmapEmpty(Bitmap bitmap)
{
    BitmapData data = bitmap.LockBits(
        default, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    try
    {
        ReadOnlySpan<byte> bytes = new(
            (byte*)data.Scan0, data.Stride * data.Height);
        bytes.IndexOfAnyExcept((byte)0).Should().Be(-1);
    }
    finally
    {
        bitmap.UnlockBits(data);
    }
}

When to use each:

• VerifyBitmapNotEmpty — after a draw/fill call, to confirm something was actually rendered.
• VerifyBitmapEmpty — to confirm an initial bitmap has no content, or to verify a clear operation.

3.3 Use using declarations for all disposables

Always dispose Bitmap, Graphics, Pen, Brush, GraphicsPath, and Font objects with using declarations:

using Bitmap bitmap = new(10, 10);
using Graphics graphics = Graphics.FromImage(bitmap);
using Pen pen = new(Color.Blue, 2);

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4. Testing Integer and Float Overloads

Every drawing primitive provides both integer (Rectangle, Point, Size) and float (RectangleF, PointF, SizeF) overloads. Write separate tests for each:

#if NET11_0_OR_GREATER
    [Fact]
    public void Graphics_DrawRoundedRectangle_Integer()
    {
        using Bitmap bitmap = new(10, 10);
        using Graphics graphics = Graphics.FromImage(bitmap);
        graphics.DrawRoundedRectangle(Pens.Red, new(0, 0, 10, 10), new(2, 2));
        VerifyBitmapNotEmpty(bitmap);
    }

    [Fact]
    public void Graphics_DrawRoundedRectangle_Float()
    {
        using Bitmap bitmap = new(10, 10);
        using Graphics graphics = Graphics.FromImage(bitmap);
        graphics.DrawRoundedRectangle(
            Pens.Red, new RectangleF(0, 0, 10, 10), new(2, 2));
        VerifyBitmapNotEmpty(bitmap);
    }
#endif

4.1 Test naming convention

Follow the pattern:

{ClassName}_{MethodName}_{Integer|Float}

For additional scenario tests, extend with a scenario suffix:

Graphics_DrawRoundedRectangle_Integer
Graphics_DrawRoundedRectangle_Float
Graphics_FillRoundedRectangle_Integer
Graphics_FillRoundedRectangle_Float
GraphicsPath_AddRoundedRectangle_Integer
GraphicsPath_AddRoundedRectangle_Float

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5. Testing GraphicsPath Operations

5.1 Path point verification

For GraphicsPath.Add[Shape] methods, verify the resulting path points using FluentAssertions' approximate comparison:

#if NET11_0_OR_GREATER
    [Fact]
    public void GraphicsPath_AddRoundedRectangle_Integer()
    {
        using GraphicsPath path = new();
        path.AddRoundedRectangle(
            new Rectangle(10, 10, 20, 20), new(5, 5));
        path.PathPoints.Should().BeApproximatelyEquivalentTo(
            new PointF[]
            {
                new(27.499994f, 10),
                new(28.880707f, 9.999997f),
                // … remaining expected points …
            },
            precision: 0.000001f);
    }
#endif

5.2 How to obtain expected path points

1. Write the test with the API call but without assertions.
2. Run the test under a debugger or add a temporary output.
3. Capture the actual PathPoints array.
4. Use those values as the expected data with an appropriate floating-point tolerance (typically 0.000001f).

5.3 Test both overloads separately

Even though the integer overload delegates to the float overload, test both independently — the cast from Rectangle → RectangleF must be verified:

[Fact]
public void GraphicsPath_AddRoundedRectangle_Integer() { /* … */ }

[Fact]
public void GraphicsPath_AddRoundedRectangle_Float() { /* … */ }

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6. Testing Draw and Fill Pairs

When a new shape has both Draw and Fill methods, write tests for all four combinations (Draw×Integer, Draw×Float, Fill×Integer, Fill×Float):

#if NET11_0_OR_GREATER
    [Fact]
    public void Graphics_DrawRoundedRectangle_Integer()
    {
        using Bitmap bitmap = new(10, 10);
        using Graphics graphics = Graphics.FromImage(bitmap);
        graphics.DrawRoundedRectangle(Pens.Red, new(0, 0, 10, 10), new(2, 2));
        VerifyBitmapNotEmpty(bitmap);
    }

    [Fact]
    public void Graphics_FillRoundedRectangle_Integer()
    {
        using Bitmap bitmap = new(10, 10);
        using Graphics graphics = Graphics.FromImage(bitmap);
        graphics.FillRoundedRectangle(
            Brushes.Green, new(0, 0, 10, 10), new(2, 2));
        VerifyBitmapNotEmpty(bitmap);
    }

    // … and Float variants for each …
#endif

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7. Testing Pen and Brush Variations

For more thorough coverage, consider additional tests with:

• Different pen widths.
• Different brush types (SolidBrush, LinearGradientBrush).
• Anti-aliased vs. default smoothing modes.
• Edge cases: zero-size rectangles, very large corner radii, negative coordinates.

[Fact]
public void Graphics_DrawRoundedRectangle_ThickPen()
{
    using Bitmap bitmap = new(20, 20);
    using Graphics graphics = Graphics.FromImage(bitmap);
    using Pen pen = new(Color.Blue, 3);
    graphics.DrawRoundedRectangle(pen, new RectangleF(2, 2, 16, 16), new(4, 4));
    VerifyBitmapNotEmpty(bitmap);
}

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8. Testing Font and Text Rendering

For font and text rendering APIs, use the same bitmap-based approach:

[Fact]
public void Graphics_DrawString_RendersText()
{
    using Bitmap bitmap = new(100, 30);
    using Graphics graphics = Graphics.FromImage(bitmap);
    using Font font = new("Arial", 12);
    graphics.DrawString("Test", font, Brushes.Black, 0, 0);
    VerifyBitmapNotEmpty(bitmap);
}

For font metric APIs, verify returned values are within expected ranges rather than exact pixel matching, since font rendering can vary across environments.

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9. Checklist for GDI+ Rendering Tests

• Tests guarded with matching #if NET11_0_OR_GREATER (or appropriate version)
• Separate tests for integer and float overloads
• Tests for both Draw and Fill variants (if applicable)
• GraphicsPath tests verify path points with approximate comparison
• Bitmap-based verification using VerifyBitmapNotEmpty
• All disposables use using declarations
• Test naming follows {Class}_{Method}_{Overload} convention
• Tests added to existing test files (not new files, unless partials)