Gameplay Programming - Learning C# by Programming Games - page 226

16.2.4 Recursion: A Method Calling Itself
There is one thing that we did not take into account. It is, of course, possible that
one or more of the game objects in the list is itself of the type GameObjectList . If that
game object contained a game object with the ID that we seek, then this method
would not find it, since it only checks the game objects that are stored in the list of
the current object ( this ). So how can we solve this? First, we need to check if an
object is of a certain type. For that, we can use the is keyword:
if (obj is GameObjectList)
dosomething
Before the is keyword, we put the object to be checked, and after the keyword we
place the type. If the object is of the given type, then the expression yields true .If
not, the result is false . Therefore, we can use it in an if -instruction like in the example
above. If we know that the object is of type GameObjectList , we can cast it and then
try to find the game object that we are looking for in the casted game object list. The
following code does exactly that:
foreach (GameObject obj in gameObjects)
{
if (obj.ID == id)
return obj;
if (obj is GameObjectList)
{
GameObjectList objlist = obj as GameObjectList;
foreach (GameObject obj2 in objlist.gameObjects)
if (obj2.ID == id)
return obj2;
}
}
return null ;
So, now we check for each game object to determine if it is of type GameObjectList .If
so, we traverse that list's gameObjects variable and look for the game object in there.
Are we done now? Well, not really. What if one of the game objects in objlist also is
of type GameObjectList ? It means that we have to add another layer that checks if one
of the game objects in that list perhaps corresponds to the ID that we are looking for.
But one of those game objects could also be of type GameObjectList . Obviously, this
approach is not ideal. However, we can do something to avoid this kind of infinite
search problem. Why not use the Find method in objlist ? Look at the following code:
foreach (GameObject obj in gameObjects)
{
if (obj.ID == id)
return obj;