Refactoring Day 18 : Replace exception with conditional

Today’s refactoring doesn’t come from any place specifically, just something I’ve picked up over time that I find myself using often. Any variations/comments would be appreciated to this approach. I think there’s some other good refactorings around these type of problems.

A common code smell that I come across from time to time is using exceptions to control program flow. You may see something to this effect:

   1: public class Microwave

   2: {

   3:     private IMicrowaveMotor Motor { get; set;}

   4:  

   5:     public bool Start(object food)

   6:     {

   7:         bool foodCooked = false;

   8:         try

   9:         {

  10:             Motor.Cook(food);

  11:             foodCooked = true;

  12:         }

  13:         catch(InUseException)

  14:         {

  15:             foodcooked = false;

  16:         }

  17:  

  18:         return foodCooked;

  19:     }

  20: }

Exceptions should only be there to do exactly what they are for, handle exceptional behavior. Most of the time you can replace this type of code with a proper conditional and handle it properly. This is called design by contract in the after example because we are ensuring a specific state of the Motor class before performing the necessary work instead of letting an exception handle it.

   1: public class Microwave

   2: {

   3:     private IMicrowaveMotor Motor { get; set; }

   4:  

   5:     public bool Start(object food)

   6:     {

   7:         if (Motor.IsInUse)

   8:             return false;

   9:  

  10:         Motor.Cook(food);

  11:  

  12:         return true;

  13:     }

  14: }

This is part of the 31 Days of Refactoring series. For a full list of Refactorings please see the original introductory post.