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;
    }
}