1. Properties 7: Properties Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Describe properties purpose definition use 7: Properties
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. Public data Public data can be both good and bad
7: Properties
Public data
Public data can be both good and bad
good because it allows clear and simple access syntax
bad because it exposes implementation details of class
class Person {
public string name;
... }
public field
Person p = new Person();
p.name = "Bob";
string n = p.name;
write
read
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. 7: Properties
Private data
Class typically has private fields and public access methods
clients use methods to access fields
class Person {
private string name;
public string GetName()
return name; }
public void SetName(string newName)
name = newName;
private field
public read method
public write method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Advantage: access control
7: Properties
Advantage: access control
Private data lets class offer only desired access methods
read-only
write-only
read and write
class Person {
private string name;
public string GetName()
return name; }
supply only read method
for read-only access
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. Advantage: data validation
7: Properties
Advantage: data validation
Private data allows class to perform data validation
methods can do error checking
clients can not avoid validation code by accessing data directly
class Person {
public void SetName(string newName)
if (newName == "")
// error ...
name = newName; }
...
error checking
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. Disadvantage: access method names
7: Properties
Disadvantage: access method names
Typical to prefix access method names with Get and Set
follows common wisdom to make method names verb phrases
but conflicts with concept of field as representing data property
Person p = new Person();
p.SetName("Bob");
string n = p.GetName();
write
read
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. Disadvantage: access method usage
7: Properties
Disadvantage: access method usage
Syntax of access methods can be undesirable
set does not use = so is not obviously a write operation
get typically has awkward looking empty parentheses
Person p = new Person();
p.SetName("Bob");
string n = p.GetName();
no assignment operator
empty parentheses
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. 7: Properties
Properties
Properties combine the advantages of public and private data
have clean access syntax like public fields
provide error checking opportunity like private fields
Person has Name property
Person p = new Person();
p.Name = "Ann";
string n = p.Name;
write
read
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. Structure of property definition
7: Properties
Structure of property definition
Property definition has
declaration of access level, type, and name
code block with implementation
class Person {
public string Name
... }
declaration of
Name property
implementation
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. Properties storage Property does not provide any needed data storage
typically define field to store data
class Person {
private string name;
public string Name
... }
field to store data
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. Property accessors Property can define set and get accessors
7: Properties
Property accessors
Property can define set and get accessors
definitions go inside implementation code block
no explicit arguments or return types
class Person {
public string Name
set
... }
get
write method
read method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. Set implementation Set typically records new data for property
7: Properties
Set implementation
Set typically records new data for property
new data passed in hidden argument called value
class Person {
private string name;
public string Name
set
name = value; }
...
use value
argument
Person p = new Person();
p.Name = "Ann";
calls set
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. Get implementation Get typically returns current value of property
7: Properties
Get implementation
Get typically returns current value of property
uses return to return value
class Person {
private string name;
public string Name
get
return name; }
...
return value
of property
Person p = new Person();
...
string n = p.Name;
calls get
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. Validation Property accessors can contain validation code class Person
7: Properties
Validation
Property accessors can contain validation code
class Person {
private string name;
public string Name
set
if (value == "")
// error ...
name = value; }
...
validation
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

16. Access control Property may provide any access desired read-only
7: Properties
Access control
Property may provide any access desired
read-only
write-only
read and write
class Person {
public int Age
get
... }
Age property
supply get and omit
set for read-only
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

17. Data storage Property does not need to be backed by matching field
7: Properties
Data storage
Property does not need to be backed by matching field
value may be produced in any way that is appropriate
class Person {
private DateTime dob;
public int Age
get
int age = DateTime.Today.Year - dob.Year;
DateTime birthday = dob.AddYears(age);
if (birthday > DateTime.Today)
age--;
return age; }
...
store date
of birth
calculate age
birthday occurred
yet this year?
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

18. Static property Property can be static
7: Properties
Static property
Property can be static
must be accessed through type name
class Item {
private static int revenue;
public static int Revenue
get { return revenue; }
set { revenue = value; } }
...
define static
property
Item.Revenue = 5;
access using
type name
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

19. Access level Property accessors have same access level
7: Properties
Access level
Property accessors have same access level
both public, both private, etc.
class Person {
public string Name
set { ... }
get { ... } }
...
public property
public read method
public write method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

20. Summary Properties model traits of a class or object
provide advantages of private field / public accessor pattern
with clean user syntax
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003