Lesson 7. Events, Delegates, Generics. Programming in C# Lesson 7. Events, Delegates, Generics.

Delegates

What is a delegate? A delegate is a type safe pointer to a method (or a chain of methods) All delegates are classes that inherited from MulticastDelegate class Delegates are useful in callback scenarios Delegates are used in APM (Asynchronous Programming Model) Delegates are the base of events

Declaring a delegate delegate keyword Name Return value type Parameters (number and type)

Using a delegate

Using a delegate as a callback method

Using a delegate as a callback method

Delegates chaining

Delegates chaining

Using delegates in APM

Anonymous Methods and Lambda Expressions

Events Events provide a way for an object to be notified about something that has happened in another object

Designing a Type That Exposes an Event Define a type that will hold any additional information that should be sent to subscribers Define the event member (based on existing or custom delegate) Define a method responsible for raising the event Call the method responsible for raising the event from the appropriate place

Defining a type that will hold additional information

Defining the event member

Defining a method responsible for raising the event

Calling the method responsible for raising the event

Subscribing/Unsubscribing to events

Method GetInvocationList Method GetInvocationList allows us to iterate manually through the list of subscribers and handle them correspondingly:

EventHandler and EventHandler<TEventArgs> delegates In order to avoid the necessity to create custom delegates each time we need to expose an event, these standard delegates should be used:

Generics

What are generics? Generics provide another form of code reuse: algorithm reuse Generics allow us to encapsulate an algorithm within a class, interface or method without specifying the data type(s) that class or method works on Generics provide type safety Generics provide better performance (especially for collection data structures) as they prevent the necessity of boxing/unboxing and type casting There are generic classes, interfaces, methods and delegates

A generic class sample

Operator default Operator default specifies the default value of the type parameter. This will be null for reference types and zero for value types

Constraints on Type Parameters where T: struct – The type argument must be a value type where T: class – The type argument must be a reference type where T: new() – The type argument must have a public parameterless constructor where T: <base class name> – The type argument must be or derive from the specified base class where T: <interface name> – The type argument must be or implement the specified interface

Generic Class with Constraints Sample

Generic Method with Constraints Sample