Chapte r10 - Delegate, Events and Lambdas pdf

Chapter 10. Delegates,Events, and Lambdas Hoang Anh Viet VietHA@it-hut.edu.vn HaNoi University of Technology 1 MicrosoftMicrosoft Objectives 2 “The purpose of Chapter 10 is to demystify the delegate type. Simply put, a .NET delegate is an object that “points” to other methods in your application. Using this pattern, you are able to build systems that allow multiple objects to engage in a two-way onversation. After you have examined the use of .NET delegates, you will then be introduced to the C# event keyword, which is used to implify the manipulation of raw delegate programming. You wrap up by investigating the role of the C# 2008 lambda operator (=>) and exploring the connection between delegates, anonymous methods, and lambda expressions.” MicrosoftMicrosoft Roadmap 10.1 Understanding the .NET Delegate Type 10.2 Defining a Delegate in C# 10.3 The System.MulticastDelegate and System.Delegate Base class. 10.4 The Simplest Possible Delegate Example 10.5 Retrofiting the Car Type with Delegates 10.6 A more Elaborate Delegate Example 3 MicrosoftMicrosoft Roadmap 10.7 Understanding Delegate Covarance 10.8 Greating Generic Delegates 10.9 Understanding C# Events. 10.10 The Generic C# EventHandler<T> Delegate. 10.11 Understanding C# Anonymouns Methods 10.12 Understanding Method Group Conversions 10.13 The C# 2008 Lambdas Operator. 4 MicrosoftMicrosoft 10.1 Understanding the .NET Delegate Type  A delegate is a type that references a method.  Once a delegate is assigned a method, it behaves exactly like that method .  The delegate method can be invoked like any other method, with parameters and a return value.  Such as : 5 public delegate int PerformCalculation(int x, int y); MicrosoftMicrosoft Kinds of Methods  Any method from any accessible class or struct that matches the delegate's signature, which consists of the return type and parameters, can be assigned to the delegate .  The method can be either static or an instance method .  Note : • In the context of method overloading:  The signature of a method does not include the return value. • In the context of delegates:  The signature does include the return value. MicrosoftMicrosoft Delegates Overview  Delegates have the following properties: • Delegates are like C++ function pointers but are type safe. • Delegates allow methods to be passed as parameters. • Delegates can be used to define callback methods. • Delegates can be chained together; for example, multiple methods can be called on a single event. • Methods do not have to match the delegate signature exactly • Allow code blocks to be passed as parameters in place of a separately defined method. MicrosoftMicrosoft Efficency of Delegates  Plug new code into existing classes.  Makes it possible to programmatically change method calls.  Defines callback methods. Allows for an algorithm to be written in a more general way. MicrosoftMicrosoft Roadmap 10.1 Understanding the .NET Delegate Type 10.2 Defining a Delegate in C# 10.3 The System.MulticastDelegate and System.Delegate Base class. 10.4 The Simplest Possible Delegate Example 10.5 Retrofiting the Car Type with Delegates 10.6 A more Elaborate Delegate Example 9 MicrosoftMicrosoft 10.2 Definding a Delegate in C#  Declare  Instantiate  and Use a Delegate [...]... In C# 1.0 and later, delegates can be declared as shown here: public delegate void Del(T item); public void Notify(int i) { } Del d1 = new Del(Notify);  In C# 2.0 and later, it is also possible to use an anonymous method to declare and initialize a delegate by using this simplified syntax: Del d2 = Notify; Microsoft Microsoft 10.2 Definding a Delegate in C#   In C# 3.0 and later,... private member variables (one for each delegate type) and two helper functions (OnAboutToBlow() and OnExploded()), that allow the client to add a method to the delegate’s invocation list Microsoft Microsoft Updating the Accelerate() method  we need to update the Accelerate() method to invoke each delegate, rather than iterate over an ArrayList of client-side sinks public void Accelerate(int delta) { //... d:"); d("D"); Console.Read(); Roadmap 10.1 Understanding the NET Delegate Type 10.2 Defining a Delegate in C# 10.3 The System.MulticastDelegate and System.Delegate Base class 10.4 The Simplest Possible Delegate Example 10.5 Retrofiting the Car Type with Delegates 10.6 A more Elaborate Delegate Example Microsoft Microsoft 16 10.3 The System.MulticastDelegate and System.Delegate Base class   The System.MulticastDelegate... MulticastDelegate : Delegate  Remarks • MulticastDelegate is a special class • Compilers and other tools can derive from this class • But we cannot derive from it explicitly Microsoft Microsoft Delegate Class   Represents a delegate, which is a data structure that refers to a static method or to a class instance and an instance method of that class Form: [SerializableAttribute] [ComVisibleAttribute(true)]... reference to the method's entry point and • A reference to a target object assignable to the type of the method's first argument  When the delegate is invoked, the first argument of the static method receives the target object Microsoft Microsoft Roadmap 10.1 Understanding the NET Delegate Type 10.2 Defining a Delegate in C# 10.3 The System.MulticastDelegate and System.Delegate Base class 10.4 The... “match the pattern,”, receive a compile-time error public class SimpleMath { public static int SquareNumber(int a) { return a * a; } } // Error! Method does not match delegate pattern! BinaryOp b2 = new BinaryOp(SimpleMath.SquareNumber); Microsoft Microsoft Roadmap 10.1 Understanding the NET Delegate Type 10.2 Defining a Delegate in C# 10.3 The System.MulticastDelegate and System.Delegate Base class 10.4... = Hello; Example (2) // Create the delegate object b that references // the method Goodbye: b = Goodbye; // The two delegates, a and b, are composed to form c: c = a + b; } } // Remove a from the composed delegate, leaving d, // which calls only the method Goodbye: d = c - a; System.Console.WriteLine("Invoking delegate a:"); a("A"); System.Console.WriteLine("Invoking delegate b:"); b("B"); System.Console.WriteLine("Invoking... currSpeed += delta; // Almost dead? if (10 == maxSpeed - currSpeed && almostDeadList != null) { almostDeadList("Careful buddy! Gonna blow!"); } // Still OK! if (currSpeed >= maxSpeed) carIsDead = true; else Console.WriteLine( "-> CurrSpeed = {0}", currSpeed); } } //end of Accelerate Remark    Before we invoke the methods maintained by the almostDeadList and explodedList member variables, we are checking... almostDeadList and explodedList member variables, we are checking them against a null value If the caller does not call the OnAboutToBlow() and OnExploded() helper methods, and we attempt to invoke the delegate’s invocation list, we will trigger a NullReferenceException and bomb at runtime Observe the updates to the Program class: Microsoft Microsoft ... instance of a delegate type that has references to: • An instance method of a type and a target object • • • Microsoft assignable to that type An instance method of a type, with the hidden this parameter exposed in the formal parameter list The delegate is said to be an open instance delegate A static method A static method and a target object assignable to the first parameter of the method The delegate . Chapter 10. Delegates ,Events, and Lambdas Hoang Anh Viet VietHA@it-hut.edu.vn HaNoi University of Technology 1 MicrosoftMicrosoft Objectives 2 “The purpose of Chapter 10 is to. Example 3 MicrosoftMicrosoft Roadmap 10.7 Understanding Delegate Covarance 10.8 Greating Generic Delegates 10.9 Understanding C# Events. 10.10 The Generic C# EventHandler<T> Delegate. 10.11 Understanding C# Anonymouns. 2008 lambda operator (=>) and exploring the connection between delegates, anonymous methods, and lambda expressions.” MicrosoftMicrosoft Roadmap 10.1 Understanding the .NET Delegate Type 10.2