Catching expected exceptions

Question

I've got code that looks like this because the only reliable way for me to check if some data is an image is to actually try and load it like an image.

    static void DownloadCompleted(HttpConnection conn) {
        Image img;
        HtmlDocument doc;

        try {
            img = Image.FromStream(conn.Stream);
        } catch {
            try {
                doc = new HtmlDocument();
                doc.Load(conn.Stream);
            } catch { return; }
            ProcessDocument(doc);
            return;
        }
        ProcessImage(img);
        return;
    }

Which looks down right terrible!

What's a nice way of handling these situations? Where you're basically forced to use an exception like an if statement?

Answer 1

Your logical structure is

if( /* Loading Image Fails */ )
    /* Try Loading HTML */

so I would try to make the code read that way. It would probably be cleanest (though admittedly annoyingly verbose) to introduce helper methods:

bool LoadImage()
{
    Image img;
    try 
    {
        img = Image.FromStream(conn.Stream);
    } 
    catch( NotAnImageException /* or whatever it is */ )
    {
        return false;
    }

    ProcessImage(img);
    return true;
}


bool LoadDocument()
{
   // etc
}

So you can write

if( !LoadImage() )
   LoadDocument();

or extend it to:

if( !LoadImage() && !LoadDocument() )
{
     /* Complain */
}