Basic Game Objects - Learning C# by Programming Games - page 115

Cannon cannon1;
cannon1 = new Cannon(cannonSprite);
Cannon cannon2 = cannon1;

Looking at the example using the int type, you would expect that there are now two
cannon objects in the memory: one stored in the variable cannon1 , and one stored in
cannon2 . However, this is not the case! Actually, both cannon1 and cannon2 refer to
the same object . After the first instruction (creating the Cannon object), the memory
looks like this:

Here, you see that there is a big difference between how a basic type such as int
or float is represented in memory and more complicated types such as the Cannon
class. In C#, all objects of a class type are stored as references as opposed to values.
This means that a variable such as cannon1 does not directly contain the Cannon
object, but it contains a reference to it. If we now declare the cannon2 variable and
we assign the value of cannon1 to it, the memory looks like this:

The result is that if we would change the color of the cannon as follows:

cannon2.Color = Color.Red;

then the expression cannon1.Color would be Color.Red , since both cannon1 and
cannon2 refer to the same object! This also has an effect on how objects are passed
around in methods. For example, in the Draw method of the Cannon class, we pass
a SpriteBatch object as a parameter. Because SpriteBatch is a class, we pass this pa-
rameter as a reference . The result of this is that we can change the SpriteBatch object
in our method (for example, by drawing in it). Passing basic types such as float as
parameters to method happens by value , so changing the value inside the method
has no effect. Consider the following method:

void Square( float f)
{

∗ f; f=f

}