Thursday, November 01, 2007

WBXML support in C# or let's make it smaller

Dear visitors, 03 01 6A 00 00 01 41 42 03 57 42 58 4D 4C 20 50 72 6F 67 72 61 6D 6D 65 72 00 01 43 03 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 00 01 01. Do you know what I'm talking about? Those 45 bytes string means following 208 bytes XML

<greeter>WBXML Programmer</greeter>
<greeting>Hello, World!</greeting>

Basically, XML is very easy language. It's easy to understand, parse and use. However, XML is very inefficient for transport. In order to send this small XML file, you have to transfer more, then 200 bytes, when actually meanful information is two strings. This is not critical issue, when we are working in LAN environment. However it becomes very critical while working with mobile customers. That's the reason of inventing WAP Binary XML (or WBXML).

This language uses name tables (tokens) to convert nodes and attributes into one byte value and this way it saves a lot of unnecessary information to be transferred. WBXML specification is very easy, however .NET (even Compact) framework lack of XmlDocument, that supports this content format.

Is it common enough to lean and use? Well, this is tricky question. From one hand, all of mobile content providers (even those, who dealing with mobile synchronization) widely uses this format. From the other hand, if you are no really "in" mobile (or any other bandwidth efficient) programming, you do not need it.

I think, that one of most important things in programming (in general) is to be efficient, so I decided to write XmlDocument derived class to provide WBXML support.

How to use it?

- First create name tables (you do not have to, if you are using only one token)

Dictionary<int, Dictionary<byte, string>> tokens = new Dictionary<int, Dictionary<byte, string>>();

Dictionary<byte, string> token = new Dictionary<byte, string>();
token.Add(1, "hello-world");
token.Add(2, "greeter");
token.Add(3, "greeting");

tokens.Add(1, token);

- Then create new WBXmlDocument instance with those tables

System.Xml.WBXmlDocument doc = new System.Xml.WBXmlDocument(tokens);

- Now you can either use is as normal XmlDocument

doc.LoadXml("<hello-world><greeter>WBXML Programmer</greeter><greeting>Hello, World!</greeting></hello-world>");

- And then create it's binary representation

byte[] bytes = doc.GetBytes();

- Or load incoming byte array


- And get your XmlDocument

string xml = doc.DocumentElement.OuterXml;

Pretty easy, isn't it?  Download WBXmlDocument.cs

Please note:

  • This is not final version
  • It tests not enough
  • There are known issues with attributes parsing
  • It does not implements full specification (I did it for my own needs)
  • Each company might have their own protocol, that can bi slight different from official specification (SyncML for example)
  • You can know token tables only if you'll get full specification of target protocol. Private tokens are not transferred according the protocol

If you want to help and enhance this solution, feel free to do it, however, know, that I release the source under restrictive CPL. You can use this code or code, derived from this code in either open source, free or commercial application, however, you should clearly provide my name "Tamir Khason" and link to my blog or my private web site within any distribution of the software. For more information, contact me.

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