如何从C#中的Graphics对象获取位图/图像？

Bitmap bitmap = new Bitmap(width, height);

Graphics graphics = Graphics.FromImage(bitmap);

graphics.DrawRectangle(new Pen(Color.Black), 50, 50, 100, 100);
graphics.DrawString("Hello, World!", new Font("Arial", 16), new SolidBrush(Color.Blue), new PointF(60, 80));

bitmap.Save("output.png", ImageFormat.Png);

using (Bitmap bitmap = new Bitmap(300, 300))
{
    using (Graphics graphics = Graphics.FromImage(bitmap))
    {
        graphics.DrawRectangle(new Pen(Color.Black), 50, 50, 100, 100);
        graphics.DrawString("Hello, World!", new Font("Arial", 16), new SolidBrush(Color.Blue), new PointF(60, 80));
    }

    bitmap.Save("output.png", ImageFormat.Png);
}