Objects created within methods are called local variables, as we discussed earlier. They are local to the method, as opposed to belonging to the whole object, as member variables are. The object is created within the method, used within the method, and then destroyed sometime after the method ends. Local objects are not part of the object’s state—they are temporary value holders, useful only within the particular method.
Local variables of intrinsic types such as int
are created on a portion of memory known as the stack. The stack is allocated and de-allocated as methods are invoked. When you start a method, all its local variables are created on the stack. When the method ends, local variables are destroyed.
These variables are referred to as local because they exist (and are visible) only during the lifetime of the method. They are said to have local scope. When the method ends, the variable goes out of scope and is destroyed.
C# divides the world of types into value types and reference types. Value types are created on the stack. All the intrinsic types (int, long
) are value types (as are structs, discussed later in this chapter), and thus are created on the stack.
Objects, on the other hand, are reference types. Reference types are created on an undifferentiated block of memory known as the heap. When you declare an instance of a reference type, what you are actually declaring is a reference, which is a variable that refers to another object. The reference acts like an alias for the object.
That is, when you write:
Dog milo = new Dog( );
the new
operator creates a Dog
object on the heap and returns a reference to it. That reference is assigned to milo
. Thus, milo
is a reference object that refers to a Dog
object on the heap. It is common to say that milo
is a reference to a Dog
, or even that milo
is a "Dog
object,” but technically that is incorrect. milo
is actually a reference that refers to an (unnamed) Dog
object on the heap.
The reference milo
acts as an alias for that unnamed object. For all practical purposes, however, you can treat milo
as though it were the Dog
object itself. In other words, it’s fine to go on referring to milo
as a Dog
object. He won’t mind.
The implication of using references is that you can have more than one reference to the same object. To see this difference between creating value types and reference types, examine Example 7-7. A complete analysis follows the output.
Example 7-7. Value types are created on the stack, and reference types are created on the heap
using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace Example_7_7_ _ _ _Value_and_Reference_Types { public class Dog { public int weight; } class Tester { public void Run( ) { // create an integer int firstInt = 5; // create a second integer int secondInt = firstInt; // display the two integers Console.WriteLine( "firstInt: {0} secondInt: {1}", firstInt, secondInt ); // modify the second integer secondInt = 7; // display the two integers Console.WriteLine( "firstInt: {0} secondInt: {1}", firstInt, secondInt ); // create a dog Dog milo = new Dog( ); // assign a value to weight milo.weight = 5; // create a second reference to the dog Dog fido = milo; // display their values Console.WriteLine( "milo: {0}, fido: {1}", milo.weight, fido.weight ); // assign a new weight to the second reference fido.weight = 7; // display the two values Console.WriteLine( "milo: {0}, fido: {1}", milo.weight, fido.weight ); } static void Main( ) { Tester t = new Tester( ); t.Run( ); } } }
The output looks like this:
firstInt: 5 secondInt: 5 firstInt: 5 secondInt: 7 Milo: 5, fido: 5 Milo: 7, fido: 7
The program begins by creating an integer, firstInt
, and initializing it with the value 5. The second integer, secondInt
, is then created and initialized with the value in firstInt
. Their values are displayed as output:
firstInt: 5 secondInt: 5
These values are identical. Because int
is a value type, a copy of the firstInt
value is made and assigned to secondInt
; secondInt
is an independent second variable, as illustrated in Figure 7-2.
Then the program assigns a new value to secondInt
:
secondInt = 7;
Because these variables are value types, independent of one another, the first variable is unaffected. Only the copy is changed, as illustrated in Figure 7-3.
When the values are displayed, they are different:
firstInt: 5 secondInt: 7
Your next step is to create a simple Dog
class with only one member variable, called weight
. Note that this field is given an access modifier of public
, which specifies that any method of any class can access this field. (Generally, you will not make member variables public. The weight
field was made public to simplify this example.)
You instantiate a Dog
object and save a reference to that dog in the reference milo
:
Dog milo = new Dog( );
You assign the value 5 to milo
’s weight
field:
milo.weight = 5;
You commonly say that you’ve set milo
’s weight to 5, but actually you’ve set the weight
field of the unnamed object on the heap to which milo
refers, as shown in Figure 7-4.
Next, you create a second reference to Dog
and initialize it by setting it equal to milo
. This creates a new reference to the same object on the heap.
Dog fido = milo;
Notice that this is syntactically similar to creating a second int
variable and initializing it with an existing int
, as you did before:
int secondInt = firstInt; Dog fido = milo;
The difference is that Dog
is a reference type, so fido
is not a copy of milo
—it is a second reference to the same object to which milo
refers. That is, you now have an object on the heap with two references to it, as illustrated in Figure 7-5.
When you change the weight of that object through the fido
reference:
fido.weight = 7;
you change the weight of the same object to which milo
refers. The output reflects this:
Milo: 7, fido: 7
It isn’t that fido
is changing milo
; it is that by changing the (unnamed) object on the heap to which fido
refers you simultaneously change the value of milo
because they refer to the same unnamed object.
If you had used the keyword new
when creating fido
, you’d have created a new instance of Dog
on the heap, and fido
and milo
would not point to the same Dog
object.
If you need a class that acts as a value object, you can create a struct (see the "Structs” sidebar). The use of structs is so unusual that we do not cover them (beyond the sidebar) for the rest of this book. You should know what they are, but you’ll probably never need to use one.