Understanding Delegates in C#

A delegate is a reference type that holds the reference of a class method. Any method which has the same signature as delegate can be assigned to delegate. It is very similar to the function pointer but with a difference that delegates are a type-safe. We can say that it is the object-oriented implementation of function pointers.

There are three steps for defining and using delegates:

1. Declaration

   A delegate is declared by using the keyword delegate, otherwise it resembles a method declaration.

2. Instantiation

   To create a delegate instance, we need to assign a method (which has same signature as delegate) to delegate.

3. Invocation

   Invoking a delegate is like as invoking a regular method.

//1. Declaration
public delegate int MyDelagate(int a, int b); //delegates having same signature as method 
 
public class Example
{
 // methods to be assigned and called by delegate
 public int Sum(int a, int b)
 {
 return a + b;
 }
 
 public int Difference(int a, int b)
 {
 return a - b;
 }
 }
 class Program
 {
 static void Main()
 {
 Example obj = new Example();
 
 // 2. Instantiation : As a single cast delegate
 MyDelagate sum = new MyDelagate(obj.Sum); 
 MyDelagate diff = new MyDelagate(obj.Difference);
 
 // 3.Invocation
 Console.WriteLine("Sum of two integer is = " + sum(10, 20)); 
 Console.WriteLine("Difference of two integer is = " + diff(20, 10));
 }
 }
 
/* Out Put 
 
 Sum of two integer is = 30
 Difference of two integer is = 10
*/

Key points about delegates

1. Delegates are like C++ function pointers but are type safe.
2. Delegates allow methods to be passed as parameters.
3. Delegates are used in event handling for defining callback methods.
4. Delegates can be chained together i.e. these allow defining a set of methods that executed as a single unit.
5. Once a delegate is created, the method it is associated will never changes because delegates are immutable in nature.
6. Delegates provide a way to execute methods at run-time.
7. All delegates are implicitly derived from System.MulticastDelegate, class which is inheriting from System.Delegate class.
8. Delegate types are incompatible with each other, even if their signatures are the same. These are considered equal if they have the reference of same method.

Types of delegates

1. Single cast delegate

   A single cast delegate holds the reference of only single method. In previous example, created delegate is a single cast delegate.

2. Multi cast delegate

   A delegate which holds the reference of more than one method is called multi-cast delegate. A multicast delegate only contains the reference of methods which return type is void. The + and += operators are used to combine delegate instances. Multicast delegates are considered equal if they reference the same methods in the same order.

//1. Declaration
public delegate void MyDelagate(int a, int b); 
public class Example
{
 // methods to be assigned and called by delegate
 public void Sum(int a, int b)
 {
 Console.WriteLine("Sum of integers is = " + (a + b));
 }
 
 public void Difference(int a, int b)
 {
 Console.WriteLine("Difference of integer is = " + (a - b));
 }
}
class Program
{
 static void Main()
 {
 Example obj = new Example();
 // 2. Instantiation
 MyDelagate multicastdel = new MyDelagate(obj.Sum); 
 multicastdel += new MyDelagate(obj.Difference);
 
 // 3. Invocation
 multicastdel (50, 20);
 }
}
 
/* Out put
 
 Sum of integers is = 70 
 Difference of integer is = 30
 
*/

Difference between ref and out parameters

Ref and out parameters are used to pass an argument within a method. In this article, you will learn the differences between these two parameters.

Ref

The ref keyword is used to pass an argument as a reference. This means that when value of that parameter is changed in the method, it gets reflected in the calling method. An argument that is passed using a ref keyword must be initialized in the calling method before it is passed to the called method.

Out

The out keyword is also used to pass an argument like ref keyword, but the argument can be passed without assigning any value to it. An argument that is passed using an out keyword must be initialized in the called method before it returns back to calling method.

Program with ref and out keyword

public class Example
{
 public static void Main() //calling method
 {
 int val1 = 0; //must be initialized 
 int val2; //optional
 
 Example1(ref val1);
 Console.WriteLine(val1); // val1=1
 
 Example2(out val2);
 Console.WriteLine(val2); // val2=2
 }
 
 static void Example1(ref int value) //called method
 {
 value = 1;
 }
 static void Example2(out int value) //called method
 {
 value = 2; //must be initialized 
 }
}
 
/* Output
 1
 2
 */

Note

1. Do not be confused with the concept of passing by reference and the concept of reference type. These two concepts are not the same.
2. A value type or a reference type can be passed to method parameter by using ref keyword. There is no boxing of a value type when it is passed by reference.
3. Properties cannot be passed to ref or out parameters since internally they are functions and not members/variables.

Ref and out in method overloading

Both ref and out cannot be used in method overloading simultaneously. However, ref and out are treated differently at run-time but they are treated same at compile time (CLR doesn't differentiates between the two while it created IL for ref and out). Hence methods cannot be overloaded when one method takes a ref parameter and other method takes an out parameter. The following two methods are identical in terms of compilation.

class MyClass
{
 public void Method(out int a) // compiler error “cannot define overloaded”
 {
 // method that differ only on ref and out"
 }
 public void Method(ref int a) 
 {
 // method that differ only on ref and out" 
 }
}

However, method overloading can be done, if one method takes a ref or out argument and the other method takes simple argument. The following example is perfectly valid to be overloaded.

class MyClass
{
 public void Method(int a) 
 {
 
 }
 public void Method(out int a)
 {
 // method differ in signature.
 }
}

Classes and Objects - C++

Thanks for watching 
 Join our Channel  for more

Java - Basic Syntax

Java program  can be defined as a collection of objects that communicate by invoking each other's methods. Let's take a look into class, object, methods and instance variables mean.

Variables : Contains some kind of data , variables might be your name or your age or anything you want.
Methods : A method is basically a function. it does stuff with those variables . a method is simply a sort of following instructions that can make a specific treatment.
Object : An Object include both variables and methods.
Class : a Class is a sort of template that describes the methods and variables that will be in each object.

First Java Program:

This is  simple code that will print  "Hello World" message .

public class MyFirstProgram {

   /* This is my first java program.  
    * This will print 'Hello World' as the output
    */
 
    public static void main(String []args) {
       System.out.println("Hello World"); // print Hello World
    }
} 

Basic Syntax:

Case Sensitivity : Java is case sensitive, which means it makes the  difference between uppercase and lowercase syntax.example "String" and "string"are different in java.

Class Names : The first letter in class name should be in UpperCase           Example class Myclass

Method Names : All method names should start with a Lower Case letter.

Example public void myMethod()
Program File Name : Name of the program file should be the same class .

Example: 'MyProgram' is the class name. Then the file should be saved                       as 'MyProgram.java'

public static void main(String args[]) : This method is usually found in any java program, it is the main method that every program start processing from it .

Java Variables:

• Local Variables
• Class Variables (static variables)
• Instance Variables (Non-static variables)

Java Arrays:

Arrays are objects that store multiple variables of the same type. We will look into how to declare, construct and initialize in the upcoming chapters.

Comments in Java

Comments in java are similar to cpp and c all characters available inside any comment are ignored by Java compiler. 

public class MyFirstProgram{

   /* This is my first java program.
    * This will print 'Hello World' as the output
    * This is an example of multi-line comments.
    */

    public static void main(String []args){
       // This is an example of single line comment
       /* This is also an example of single line comment. */
       System.out.println("Hello World"); 
    }
}