casting to multiple (unknown types) at runtime

Question

I\'m developing a viewer that will be able to open all of the custom documents that we produce via our software. All of the documents inherit from IDocument, but I\'m not su

Answer 1

Why not just cast to IDocument? What benefit do you believe casting to the exact type would have here?

If you want the caller to get the result in a strongly typed way, here's how I'd write it, using generics:

public T OpenDocument<T>(String filename) where T : IDocument
{
    using (FileStream fs = new FileStream(filename, FileMode.Open))
    {
        BinaryFormatter bFormatter = new BinaryFormatter();    
        return (T) bFormatter.Deserialize(fs);
    }
}

Of course, that relies on the caller knowing the right type at compile-time. If they don't, there's no way they're going to know how to use the right type anyway, so just return IDocument:

public IDocument OpenDocument(String filename)
{
    using (FileStream fs = new FileStream(filename, FileMode.Open))
    {
        BinaryFormatter bFormatter = new BinaryFormatter();    
        return (IDocument) bFormatter.Deserialize(fs);
    }
}

EDIT: To answer the edit to your question, yes the derived property would still exist. Casting doesn't actually change an object - it just means you get a reference which the compiler knows is of an appropriate type.

Answer 2

You are using the BinaryFormatter class which includes type information into the serialized stream, so you don't need the docType variable:

public Boolean OpenDocument(String filename, out IDocument document)
{
    // exception handling etc. removed for brevity

    FileStream fs = null;
    BinaryFormatter bFormatter = new BinaryFormatter();

    fs = new FileStream(filename, FileMode.Open);
    document = (IDocument)bFormatter.Deserialize(fs);

    return true;
}

Answer 3

If you can easily figure out what kind of document it is (ie: it's part of the file header), it might be easiest to have a strategy for deserializing that is specific to each type.