Don't throw System.Exception directly!

在寫下列這一段程式時，發現 System.Drawing.Graphics.FromImage 居然會直接拋出 System.Exception：

public class AddImageInfo : BaseImageOp
{
    private Font _drawFont = new Font( "Verdana", 10);
    private SolidBrush _drawBrush = new SolidBrush(Color.White);

    public AddImageInfo(ImageOp innerOp) : base(innerOp)
    {
    }

    public override Image Execute(Image image)
    {
        using (Graphics g = Graphics.FromImage(_innerOp.Execute(image)))
        {
            string info = String.Format( "{0}x{1}", image.Width, image.Height);
            g.DrawString( info, _drawFont, _drawBrush, 10, 10);
        }

        return image;
    }
}

用 Reflector 查了 System.Drawing.Graphics.FromImage() 的實作：

public static Graphics FromImage(Image image)
{
    if (image == null)
    {
        throw new ArgumentNullException("image");
    }
    if ((image.PixelFormat &amp; PixelFormat.Indexed) != PixelFormat.Undefined)
    {
        throw new Exception(
        SR.GetString("GdiplusCannotCreateGraphicsFromIndexedPixelFormat"));
    }

    IntPtr ptr1 = IntPtr.Zero;
    int num1 = SafeNativeMethods.GdipGetImageGraphicsContext(new HandleRef(image, image.nativeImage), out ptr1);
    if (num1 != 0)
    {
        throw SafeNativeMethods.StatusException(num1);
    }
    return new Graphics(ptr1);
}

這段程式碼的作者好歹也用個 ArgumentException 吧。只好暫時用下列方式避免：

public override Image Execute(Image image)
{
    if ((image.PixelFormat & PixelFormat.Indexed) != PixelFormat.Undefined)
    {
        return image; // do nothing.
    }

    ...
}

因為我不想寫這麼醜的code：

public override Image Execute(Image image)
{
    try
    {
        ...
    }
    catch (Exception ex)
    {
        // We will ignore ALL EXCEPTIONS (synchronous or asynchronous).
    }

    return image;
}