"Normal" Upcasting and Downcasting of Reference Types
For reference types, casting variables doesn't change the type of the object already allocated on the heap, it just affects the type of the variable which references the object.
So no, there isn't any additional heap overhead with casting reference types (i.e. object instances from classes) provided that there are no custom conversion operators involved (See below, tolanj's comment).
Consider the following class hierarchy:
public class Fruit
{
public Color Colour {get; set;}
public bool Edible {get; set;}
}
public class Apple : Fruit
{
public Apple { Color = Green; Edible = true; KeepsDoctorAtBay = true;}
public bool KeepsDoctorAtBay{get; set;}
}
Which, when used with both upcasting and downcasting:
There is only ever one allocation on the heap, which is the initial var foo = new Apple().
After the various variable assignments, all three variables, foo, bar and baz point to the same object (an Apple instance on the heap).
Upcasting (Fruit bar = foo) will simply restrict the variable's available access to only Fruit methods and properties, and if the (Apple)bar downcast is successful all methods, properties and events of the downcast type will be available to the variable. If the downcast fails, an InvalidCastException will be thrown, as the type system will check the type of the heap object's compatability with the variable's type at run time.
Conversion Operators
As per tolanj's comment, all bets about the heap are off if an explicit conversion operator replaces the default casting of reference types.
For instance, if we add an unrelated class:
public class WaxApple // Not inherited from Fruit or Apple
{
public static explicit operator Apple(WaxApple wax)
{
return new Apple
{
Edible = false,
Colour = Color.Green,
KeepsDoctorAtBay = false
};
}
}
As you can imagine, WaxApple's explicit operator Apple can do whatever it likes, including allocate new objects on the heap.
var wax = new WaxApple();
var fakeApple = (Apple)wax;
// Explicit cast operator called, new heap allocation as per the conversion code.
