Writing robust code

Posted: November 14, 2008 in Testing, Thoughts on development

A little more than a week ago I gave a speech on writing robust code to the dev’s of the team Im currently working for. I think I learned more from that speech than did the listeners.

Im used to highly object oriented people and hadn’t realized that most of the team have never developed OO style, so we had a lot of misunderstandings and strange looks but slowing we got closer and closer to the point I was trying to make and in the end we had a plan for the next session.

Having a second seesion gave me the option of integrating the knowledge I had gotten from the first session and rephrasing the goal in none OO terms, we still debated but ended up with a very easy to remember conclusion: “Only implement the needed functionnality”

That might seem very simple but take a look at you own code and see if you in any way can provoke the code to go down an unexpected path. If you have a switch modelling different states in your application with a very high certainty the answer is yes.

As one of the listernes worngly thought Im not advocating never to use switches, but I am advocating they make loosy state machines.

take the switch

switch(state)
   case StepOne:

  break;

  case StepTwo:

  break;

There’s absolutely nothing that enforces that stepone is handled before steptwo is even valid. If that’s the intention, fine no worries.

In our particular case we had a 4 case switch called 3 consecutive times giving a possible 64 paths through the code but only 2 of them was actually valid.

Changing the implementation from a swicth to an simple state machine reduced the possible paths through the system to 2. the state machine was implemented with a simple class
state
   state Next;
   IHandler handler;
   void Enter();
   void Leaver();

 IHandler
    void Execute();

That way it was very easy to link the states to give us the only two possible ways it was actually valid to travers our potentially 64 path execution graph.

The neat thing about the solution comes when you start testing your solution.
If you have an undetermined number of paths you’re at risk that one of the paths you hadn’t realized existed fails.

You can write code to handle those situations but if you forget to do so or just didn’t cover everything of what you didn’t know existed, it’s very unlikely you will spot it.

Wheras if you only implement what you need you will not have to worry about all those cases you dont even know about, instead if you mess up and forget something, you will find it in your test every time. You simply can test all the sunny day scenarios with out realizing that the functionality for one of those scenarios isn’t implemented.

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