Graphics.Clear not working with alpha 0 values

Description

The Clear(System.Drawing.Color color) method of System.Drawing.Graphics does not set the RGB values correctly when a color with alpha 0 is used.

Reproduction Steps

This unit test will fail:

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Drawing;
using System.Drawing.Imaging;

[TestMethod]
public void ClearlyNotWorking()
{
    var image = new Bitmap(3, 3, PixelFormat.Format32bppArgb);
    
    Color initialColor = image.GetPixel(0, 0);
    Assert.AreEqual(0, initialColor.A);
    Assert.AreEqual(0, initialColor.R);
    Assert.AreEqual(0, initialColor.G);
    Assert.AreEqual(0, initialColor.B);
    
    using (var graphics = Graphics.FromImage(image))
    {
        // Color.Transparent is A=0, R=255, G=255, B=255
        graphics.Clear(Color.Transparent);
    }
    
    Color newColor = image.GetPixel(0, 0);
    Assert.AreEqual(Color.Transparent.A, newColor.A);
    // Expecting 255 on the RGB values, but actually they remain 0.
    Assert.AreEqual(Color.Transparent.R, newColor.R);
    Assert.AreEqual(Color.Transparent.G, newColor.G);
    Assert.AreEqual(Color.Transparent.B, newColor.B);
}

Expected behavior

I expect all pixels in the bitmap to have exactly the color specified in the System.Drawing.Graphics.Clear(System.Drawing.Color color) method.

Actual behavior

RGB values remain 0 when a color with alpha 0 is provided to the System.Drawing.Graphics.Clear(System.Drawing.Color color) method.

Regression?

No response

Known Workarounds

You can work around this by:

• Locking the bitmap and getting the BitmapData.
• Manually changing all values in the data
• Unlocking the bitmap

Configuration

Tested and found the issue in:

• .NET 7 console application
• .NET Framework WPF application.

I’m on Windows 10 Pro 21H2 19044.2251 (x64), although I don’t think that matters for this issue.

Other information

No response

[JeremyKuhne]

I’ve made a proposal myself. Closing this issue.

[JeremyKuhne]

@JPasterkampRotec thanks for the sample code! You’re welcome to open a doc PR that clarifies that Clear is effectively “fill with a solid brush”. There is an edit button on the doc page that will let you do so.

I’d suggest looking at submitting a PR to provide the functionality if you think other people will find it useful.

@elachlan We generally caution people on opening PRs like this without an API that there has been consensus on. We don’t want to unnecessarily waste people’s time or disappoint them if there isn’t agreement on moving forward. In this particular case it’s a difficult thing to accomplish as there are so many supported pixel formats.