Function to rotate an image using Cairo in C#/Mono.

ImageTools.cs

public enum ImageRotation 
{
    None,
    Left,
    Right,
    UpsideDown
}

public class ImageTools
{
    public static void RotatePng(ParsedPath pngPath, ImageRotation rotation)
    {
        if (rotation == ImageRotation.None)
            return;

        using (ImageSurface originalImage = new ImageSurface(pngPath))
        {
            int w;
            int h;
            
            if (rotation == ImageRotation.Left || rotation == ImageRotation.Right)
            {
                w = originalImage.Height;
                h = originalImage.Width;
            }
            else
            {
                w = originalImage.Width;
                h = originalImage.Height;
            }
            
            double[] rotationRadians = {0, -Math.PI / 2, Math.PI / 2, Math.PI };
            
            using (ImageSurface rotatedImage = new ImageSurface(Format.Argb32, w, h))
            {
                using (Cairo.Context g = new Cairo.Context(rotatedImage))
                {
                    g.Translate(rotatedImage.Width / 2.0, rotatedImage.Height / 2.0);
                    g.Rotate(rotationRadians[(int)rotation]);
                    g.Translate(-originalImage.Width / 2.0, -originalImage.Height / 2.0);
                    
                    g.SetSourceSurface(originalImage, 0, 0);
                    g.Paint();
                }
                
                rotatedImage.WriteToPng(pngPath);
            }
        }
    }
}