Longevity of using the Delphi text DFM format for my own store and retrieve

Question

Over time I've rolled my own formats for saving and loading object properties but on having to revisit this I'm wondering about using Delphi's own text DFM format. I know tha this is really an 'internal' format, but the reader for it now seems pretty well defined and it copes with all types of property. Has anyone any comments about possible pitfalls?

Upwind

Answer 1

I wouldn't really say that DFM is an 'internal format'. Sure Delphi uses it internally for forms and datamodules, but TReader and TWriter classes that perform streaming are publicly accessible and even documented. So they are clearly intended for end users as well.

Now, the possible problem is when you save a stream and later one of the classes in the stream changes so that the stream is not compatible any more. You may have seen this in Delphi if you attempt to open a form saved in D2007+ in D7 (missing property). But even if it happens, it's not too hard to resolve. You will get an exception that will report the exact property that is causing the problem. You also have to register all classes that you want to stream with RegisterClass.

DFM can be stored in binary or text format. Even if you store it Binary you can convert it to Text (using ObjectBinaryToText), once in text format, it's easy to fix.

So, the problems you may get happen due to incompatible changes in the structure, but those have nothing to do with DFM mechanism itself, and would also happen using any other streaming mechanism.

As for longevity, you can still open DFM's saved with D1 in the latest Delphi. So as long as you keep backward compatibility in mind, you have nothing to fear.

In conclusion, the choice of any particular format, DFM, XML, JSON, your own... doesn't really affect longevity. They all require same level of compatibility.

The reasons for choosing the format have more to do with decisions regarding:

• interoperability with other apps/services
• size/speed/human readability

But you didn't mention any of those in the question.

So I suggest using DFM over roll your own, as it would mean less code to maintain.

Mongolian

Good answer, and you have my upvote. But I think that "DFM streaming is more future-compatible than your own format" (which you imply) is not strictly correct. Highly flexible and universal formats like XML, JSON or lists of textual key-value pairs have superior forward compatibility, at the expense of being a bit cumbersome and space-consuming. Still, I'd only recommend those if future compatibility of complex objects was a big issue. – Virginiavirginie