Monday, January 10, 2011

Replacing inheritance by closures

For a long time I have been using inheritance to allow other developers to extend my code.

Imagine I have the following class responsible for managing a unit of work:

   1:  public class UnitOfWork:IUnitOfWork
   2:  {
   3:     public virtual ITransaction CreateTransaction()
   4:     {
   5:        return new Transaction(){TimeOut=new Timespan(0,0,60)};
   6:     }
   7:  }

Important here is the CreateTransaction() method which  just creates a transaction with some default time out settings. Now what if you want to change this timeout value. I could change the implementation so that you can pass some extra settings when calling this method. But the next time I want to add another setting, I have to change my class again. Therefore I made this method virtual allowing the developer to override my default implementation.

The problem is that in this case I find it overkill to create a new class just for changing this one small setting. Another option is the usage of closures.

Nested closure is one of those fancy patterns Martin Fowler first coined and published on his DSL WIP site. His formal definition:

Express statement sub-elements of a function call by putting them into a closure in an argument.

You pass a delegate as a method parameter. The receiving method executes the function represented by that delegate against an object it controls.  Nested closure in C# is easy to spot because the delegate is usually typed as Action<T>, where T is the type of the object that the called method will provide as a parameter to our argument function.

Nested closure is like template method, but instead of deriving from a base class to affect behavior, the additional behavior is provided as an argument.  In the same way that template method is great when you want to communicate behavior from derivations to base classes, nested closure works when you want to communicate behavior as you call a method.

How does this changes my code?

   1:  public class UnitOfWork:IUnitOfWork
   2:  {
   3:     public UnitOfWork()
   4:     {
   5:        this.ConstructTransaction=()=> return new Transaction(){TimeOut=new TimeSpan(0,0,60)};
   6:     }
   8:     Func<ITransaction> ConstructTransaction{get;set;}
  10:     public ITransaction CreateTransaction()
  11:     {
  12:        return ConstructTransaction();
  13:     }
  14:  }

By exposing the internal code of the CreateTransaction method as a Func<ITransaction> it is possible to override the implementation without the need to inherit from this class. Changing the timeout settings will become as simple as:

   1:  var uow=new UnitOfWork();
   2:  uow.ConstructTransaction=()=> new Transaction(){TimeOut=new TimeSpan(0,0,120)};

Note: if you always want to change the same behavior, embedding the code in an inherited class remains a better alternative.

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