1. 15: Object
Object
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Introduce the Object class concept references methods
overriding methods
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. Unified inheritance hierarchy
15: Object
Unified inheritance hierarchy
Type system is unified
all types derive from System.Object
root of type
hierarchy
Object
Person
Shape
Student
Employee
Circle
Rectangle
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. Object class alias C# provides a convenient alias for Object
lower case object
object a;
System.Object b;
equivalent
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Motivation Unified inheritance hierarchy is useful
15: Object
Motivation
Unified inheritance hierarchy is useful
Object reference is generic handle
Object methods universally supported
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. Object reference Object reference can refer to anything
legal because all types are derived from Object
object a = new Person ();
object b = new Student ();
object c = new Stock ();
object d = new Rational();
use object
references
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. Access through Object reference
Access through Object reference limited
can only access Object members
object o = new Person();
o.Name = "Bob";
error, not
Object member
class Person {
public string Name;
public int Age;
... }
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. Cast Can cast to recover more specific reference
15: Object
Cast
Can cast to recover more specific reference
allows access to entire object
void Rename(object o) {
Person p = (Person)o;
p.Name = "Bob";
... }
cast
access Person member
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. Object methods Object class provides several methods
provide basic services to all types
class Object {
public Type GetType ()
public virtual string ToString ()
public virtual int GetHashCode ()
public virtual bool Equals (object o)
public static bool Equals (object a, object b) ...
public static bool ReferenceEquals(object a, object b) ...
protected void Finalize ()
protected object MemberwiseClone() }
Object
methods
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. GetType GetType gives access to type metadata returns Type object
not virtual, cannot be overridden
Person p = new Person("Bob", 32);
Type t = p.GetType();
get Type object
for Person
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. Type object Type object contains complete information about the type
runtime type information facility called reflection
public class Type {
public string FullName { get; }
public bool IsAbstract { get; }
public bool IsClass { get; }
public bool IsPrimitive { get; }
public bool IsInterface { get; }
public Type BaseType { get; }
public ConstructorInfo[] GetConstructors() ...
public FieldInfo [] GetFields () ...
public MethodInfo [] GetMethods () ...
public Type [] GetInterfaces () ...
... }
properties
methods
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. Unique Type object One Type object per type shared by all instances
Person a = new Person("Ann", 27);
Person b = new Person("Bob", 32);
Type ta = a.GetType();
Type tb = b.GetType();
references are to
same Type object
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. 15: Object
Object ToString
Object version of ToString returns fully qualified type name
Person p = new Person("Bob", 32);
string s = p.ToString();
returns "Person"
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. Simple type string conversion
15: Object
Simple type string conversion
Simple types have overridden ToString method
returns string representation of data
implicitly called when simple type used in string context
int age = 32;
string s = age.ToString();
string t = "Bob " + age;
call explicitly
called implicitly
when used in
string context
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. Override ToString Common to override ToString virtual method
15: Object
Override ToString
Common to override ToString
virtual method
return more specific string containing object data
class Person {
public string Name;
public int Age;
public override string ToString()
return Name + " " + Age; }
...
override ToString
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

16. Automatic use of ToString
15: Object
Automatic use of ToString
ToString sometimes called automatically
when object used in string context
inside WriteLine(object) method
Person p = new Person("Bob", 32);
string s = "Hello " + p;
Console.WriteLine(p);
calls ToString
calls ToString
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

17. Dynamic binding ToString is dynamically bound
15: Object
Dynamic binding
ToString is dynamically bound
derived class version called through Object reference
object o = new Person("Bob", 32);
string s = o.ToString();
calls Person.ToString
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

18. Hashing Hash table is efficient way to store key/value pairs
15: Object
Hashing
Hash table is efficient way to store key/value pairs
Hash code computed on key to determine placement in table
Person bob = new Person("Bob", 32);
House home = new House(...);
Hashtable addresses = new Hashtable();
addresses.Add(bob, home);
hash code computed
to position the new
entry in the table
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

19. Object GetHashCode Object supplies GetHashCode method
called by library hash table when entry added
few guarantees given about exact behavior of Object version
public class Object {
public virtual int GetHashCode()
... }
object GetHashCode
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

20. Override GetHashCode Can override virtual method GetHashCode
15: Object
Override GetHashCode
Can override virtual method GetHashCode
typical motivation is to increase performance of hash tables
goal is to get a random distribution of hash codes
class Person {
public override int GetHashCode()
return Name.GetHashCode() * Age;
} ... }
override GetHashCode
hash on Person data
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

21. Summary Object class is root of type system
all types are derived from Object
Object reference is generic handle
all types automatically convert to Object reference
cast required when recovering original type
Object methods provide basic operations
supported by all types
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003