C# 缩小图片尺寸

public static Image ReduceImage(string imageFile, int toWidth, int toHeight)
{
    Image originalImage = GetImage(imageFile);
    if (toWidth <= 0 && toHeight <= 0)
    {
        return originalImage;
    }
    else if (toWidth > 0 && toHeight > 0)
    {
        if (originalImage.Width < toWidth && originalImage.Height < toHeight)
            return originalImage;
    }
    else if (toWidth <= 0 && toHeight > 0)
    {
        if (originalImage.Height < toHeight)
            return originalImage;
        toWidth = originalImage.Width * toHeight / originalImage.Height;
    }
    else if (toHeight <= 0 && toWidth > 0)
    {
        if (originalImage.Width < toWidth)
            return originalImage;
        toHeight = originalImage.Height * toWidth / originalImage.Width;
    }
    Image toBitmap = new Bitmap(toWidth, toHeight);
    using (Graphics g = Graphics.FromImage(toBitmap))
    {
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        g.Clear(Color.Transparent);
        g.DrawImage(originalImage,
                    new Rectangle(0, 0, toWidth, toHeight),
                    new Rectangle(0, 0, originalImage.Width, originalImage.Height),
                    GraphicsUnit.Pixel);
        originalImage.Dispose();
        return toBitmap;
    }
}