Avoid downcasting
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).
