C#/.NET - Custom Binary File Formats - Where to Start?

Question

I need to be able to store some data in a custom binary file format. I've never designed my own file format before. It needs to be a friendly format for traveling between the C#, Java and Ruby/Perl/Python worlds.

To start with the file will consist of records. A GUID field and a JSON/YAML/XML packet field. I'm not sure what to use as delimiters. A comma, tab or newline kind of thing seems too fragile. What does Excel do? or the pre-XML OpenOffice formats? Should you use ASCII chars 0 or 1. Not sure where to begin. Any articles or books on the topic?

This file format may expand later to include a "header section".

Note: To start with I'll be working in .NET, but I'd like the format to be easily portable.

UPDATE:
The processing of the "packets" can be slow, but navigation within the file format cannot. So I think XML is off the table.

Answer 1

How about looking at using "protocol buffers"? Designed as an efficient, portable, version-tolerant general purpose binary format, it gives you C++, Java and Python in the google library, and C#, Perl, Ruby and others in the community ports?

Note that Guid doesn't have a specific data type, but you can shim it as a message with (essentially) a byte[].

Normally for .NET work, I'd recommend protobuf-net (but as the author, I'm somewhat biased) - however, if you intend to use other languages later you might do better (long term) using Jon's dotnet-protobufs; that'll give you a familiar API accross the platforms (where-as protobuf-net uses .NET idioms).

Answer 2

I'll try to add some general hints for creating a portable binary file format.

Note that to invent a binary file format means to document, how the bits in it must go and what they mean. It's not coding, but documentation.

Now the hints:

1. Decide what to do with endianess. Good and simple way to go is to decide it once and forever. The choice would be preferably little endian when used on common PC (that is x86) to save conversions (performance).

2. Create header. Yes, it is good idea to always have a header. First bytes of the file should be able to tell you, what format you are messing with.

   • Start with magic to be able to recognize your format (ASCII string will do the trick)
   • Add version. Version of your file format will not hurt to add and it will allow you to do backward compatibility later.

3. Finally, add the data. Now, the format of the data will be specific and it will always be based on your exact needs. Basically, the data will be stored in a binary image of some data structure. The data structure is what you need to come up with.

If you need random access to your data by some sort of indices, B-Trees are way to go, while if you just need a lot of numbers to write them all and then read them all an "array" will do the trick.

Additionally, you might use a TLV (Type-Length-Value) concept for forward compatibility.