Basic Game Objects - Learning C# by Programming Games - page 116

And now the following instructions:

float someNumber = 10.0f;
Square(someNumber);

After executing these instructions the value of someNumber still is 10.0f (and not
100.0f ). Why is this? It is because when the Square method is called, the float pa-
rameter is passed by value . The variable f is a local variable inside the method that
initially contains the value of the someNumber variable. Inside the method, the local
variable f is changed to contain f

f , but this does not change the someNumber vari-
able, because it is another location in memory. Because class objects are passed by
reference , the following example will change the object's color:

∗

void ChangeColor(Cannon cannon)
{

cannon.Color = Color.Red;

}
...
Cannon cannon1 = new Cannon(cannonSprite);
ChangeColor(cannon1);
// The object referred to by cannon1 now has a red color.

7.6.2 The null Keyword

So if variables whose type is a class contain references to objects, instead of direct
values, what happens when we declare a variable of that type? Suppose that we
declare a member variable of type Cannon :

Cannon anotherCannon;

At this point, we have not yet created an object (using the new keyword) that this
variable points to. So what does the memory look like? It looks like this:

The variable is not yet pointing to anything. We indicate this with the keyword
null . It is even possible to check in a C# program whether a variable is pointing to
an object or not, like this:

if (anotherCannon == null )
anotherCannon = new Cannon(cannonSprite);

In this example, we check if the variable is equal to null (not pointing to an object).
Ifso,wecreatea Cannon instance using the new keyword. After that, the memory
looks like this: