1. Delegates and Events 14: Delegates and Events
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

2. Objectives Introduce delegates and events defining delegate types
invoking through delegates
registering multiple targets
Discuss design benefits of using delegates
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

3. State Objects typically maintain state state changes over time
14: Delegates and Events
State
Objects typically maintain state
state changes over time
class Student {
string name;
double gpa;
int units;
public void RecordClass(int grade)
gpa = (gpa * units + grade) / (units + 1);
units++; }
...
store state
change state
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

4. Notification May want to notify interested parties of state change
14: Delegates and Events
Notification
May want to notify interested parties of state change
notification widely used throughout .NET framework
user interface event handling most common example
Parent
RecordClass
Student
Registrar
new grade causes
gpa to change
notify
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

5. Pattern Notification typically involves registration and callback
14: Delegates and Events
Pattern
Notification typically involves registration and callback
target registers with caller
caller calls back target when state changes
pattern also called publish/subscribe
Student
(caller)
Parent
(target)
register
callback
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

6. 14: Delegates and Events
Delegates
.NET Framework uses delegates to implement callbacks
intermediary between caller and target
declaration defines callback method signature
instance stores object reference and method token
caller
target
object
target
method
callback
delegate
callback
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

7. Information flow delegate
14: Delegates and Events
Information flow
Caller and target need to agree on information flow
data passed through delegate
Student
Parent
object
Parent
method
double
delegate
double
void
void
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

8. Delegate definition Define delegate with delegate keyword
14: Delegates and Events
Delegate definition
Define delegate with delegate keyword
syntax similar to method declaration without body
delegate name placed where method name usually goes
delegate keyword
name of delegate
delegate void StudentCallback(double gpa);
target method
return type
target method
parameter
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

9. Delegate as type Delegate name is type name can declare references
14: Delegates and Events
Delegate as type
Delegate name is type name
can declare references
can create objects
define delegate
delegate void StudentCallback(double gpa);
StudentCallback a = new StudentCallback(...);
reference
object
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

10. 14: Delegates and Events
Target use of delegate
Target defines method with signature specified by delegate
parameters and return type must match
method name not constrained
delegate defines
required signature
delegate void StudentCallback(double gpa);
target
class Parent {
public void Report(double gpa)
... }
method signature
matches delegate
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

11. Caller use of delegate Caller typically defines delegate reference
14: Delegates and Events
Caller use of delegate
Caller typically defines delegate reference
caller
class Student {
public StudentCallback GpaChanged;
... }
delegate reference
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

12. Registration Create delegate object and store in caller to register
14: Delegates and Events
Registration
Create delegate object and store in caller to register
pass target object and method to delegate constructor
void Run() {
Student ann = new Student("Ann");
Parent mom = new Parent();
StudentCallback a = new StudentCallback(mom.Report);
ann.GpaChanged = a;
... }
caller
target
create
store
target
object
target
method
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

13. Invocation Caller invokes callback indirectly through delegate
14: Delegates and Events
Invocation
Caller invokes callback indirectly through delegate
uses method call syntax on delegate
delegate calls target method on target object
class Student {
public StudentCallback GpaChanged;
public void RecordClass(int grade)
// update gpa
...
GpaChanged(gpa); }
invoke callback
through delegate
callback takes
double argument
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

14. Summary of delegate use
14: Delegates and Events
Summary of delegate use
define delegate
delegate void StudentCallback(double gpa);
class Parent {
public void Report(double gpa) { ... } }
class Student
public StudentCallback GpaChanged;
public void RecordClass(int grade)
// update gpa
...
GpaChanged(gpa);
target method
caller stores delegate
caller invokes delegate
Student ann = new Student("Ann");
Parent mom = new Parent();
ann.GpaChanged = new StudentCallback(mom.Report);
ann.RecordClass(4); // 4 == 'A'
create and install delegate
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

15. Null reference Delegate is reference type
14: Delegates and Events
Null reference
Delegate is reference type
defaults to null when used as field
typical to guard invocation
class Student {
public StudentCallback GpaChanged;
public void RecordClass(int grade)
// update gpa
...
if (GpaChanged != null)
GpaChanged(gpa); }
test before call
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

16. Static methods Static method may be target of delegate
14: Delegates and Events
Static methods
Static method may be target of delegate
use class name and method name when creating delegate
no object specified since no this object associated with call
class Registrar {
public static void Log(double gpa)
... }
static method
void Run() {
Student ann = new Student("Ann");
ann.GpaChanged = new StudentCallback(Registrar.Log);
... }
register
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

17. Multiple targets Can combine delegates using operator+= or operator+
14: Delegates and Events
Multiple targets
Can combine delegates using operator+= or operator+
creates invocation list of delegates
all targets called when delegate invoked
targets called in order added
use of += ok even when left-hand-side is null
Parent mom = new Parent();
Parent dad = new Parent();
Student ann = new Student("Ann");
ann.GpaChanged += new StudentCallback(mom.Report);
ann.GpaChanged += new StudentCallback(dad.Report);
...
targets
first
second
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

18. Remove delegate Can remove delegate from invocation list
14: Delegates and Events
Remove delegate
Can remove delegate from invocation list
use operator-= or operator-
identity of target object/method determines which is removed
Parent mom = new Parent();
Parent dad = new Parent();
Student ann = new Student("Ann");
ann.GpaChanged += new StudentCallback(mom.Report);
ann.GpaChanged += new StudentCallback(dad.Report);
...
ann.GpaChanged -= new StudentCallback(dad.Report);
add
remove
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

19. Public delegate Not common to have public delegate field
14: Delegates and Events
Public delegate
Not common to have public delegate field
allows assignment: could overwrite existing registrants
allows external invocation: decision should be made internally
class Student {
public StudentCallback GpaChanged;
... }
public delegate
Parent mom = new Parent();
Parent dad = new Parent();
Student ann = new Student("Ann");
...
ann.GpaChanged = new StudentCallback(mom.Report);
ann.GpaChanged = new StudentCallback(dad.Report);
ann.GpaChanged(4.0);
overwrite mom handler
invoke
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

20. Events Events give private data/public accessor pattern for delegates
14: Delegates and Events
Events
Events give private data/public accessor pattern for delegates
created by applying event keyword to delegate
external code can uses += and -=
no external assignment or invocation
class Student {
public event StudentCallback GpaChanged;
... }
event
Parent mom = new Parent();
Student ann = new Student("Ann");
ann.GpaChanged += new StudentCallback(mom.Report);
ann.GpaChanged = new StudentCallback(mom.Report);
ann.GpaChanged(4.0);
...
ok to use +=
error to use =
error to invoke
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

21. Design delegate Delegates provide clean way to code publish/subscribe
14: Delegates and Events
Design
Delegates provide clean way to code publish/subscribe
publisher independent of type of subscriber
publisher independent of target method name
subscriber constrained only by callback method signature
promotes loosely coupled designs
Publisher
Subscriber
target
method
publish
delegate
publish
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003

22. Summary Delegates are objects that invoke methods on other objects
14: Delegates and Events
Summary
Delegates are objects that invoke methods on other objects
useful to implement callbacks
Events add semantic/syntactic layer to delegates
enforce clean and safe registration/deregistration protocols
Programming C# © 2003 DevelopMentor, Inc.
12/1/2003