Avoid downcasting

Revision as of 03:26, 6 August 2009

Typically one downcasts when one has a collection of mixed objects of a certain superclass (for instance, several Cars, several Bicycles and several Trains, all subclasses of Vehicle). This is almost certainly the sub-optimal technique to accomplish your chosen task, because it's an indication (code smell) that you're not using polymorphism properly. The resulting code will inevitably have a Switch statement smell, another reason you should probably be restructuring your code.

Using our example, here's a pseudocode method that uses bad downcasting:

method turnonallvehicles()
{
 foreach (Vehicle v in vehicleCollection)
 {
  if v is Car
   Car c = (Car)v
   c.turnKey()
  else if v is Bicycle 
   Bicycle b = (Bicycle) v
   b.startPedalling()
  else if v is Train
   Train t = (Train) v
   t.lightCoal()
 }
}

This clearly is disgusting. The problem here is that we are manually engaging a different method depending on the type, which means there is a problem relating to the superclass.

This means we are not taking advantage of polymorphism. The better way to go is this:

foreach (Vehicle v in vehicleCollection)
{
 v.start()
}

This would be the obvious technique if your superclass was defined correctly.

Thus downcasting strongly indicates problems with your class hierarchy (or interface).

Double Dispatch as used in the Visitor pattern can be a useful way of removing downcasts.

Downcasting can be a problem when trying to conform to Design by contract. By downcasting you are not conforming to the contract of a design.

See Also

• Refactoring: Encapsulate Downcast
• Double Dispatch