*■ Delegate*
• a delegate is a type that represents references to methods with a specific parameter list and return type.
• It is similar to a function pointer in C or C++, but it is type-safe and object-oriented.
• It allows you to store the address of a method and call it later, even without knowing which method you're calling at compile time.
• Method Reference : A delegate can hold a reference to a method, and you can invoke that method through the delegate. The method can be static or an instance method.
• Type-Safe : Delegates ensure that the method signature (parameters and return type) matches the delegate's signature, providing type safety.
• Delegates are type-safe, meaning they ensure that the method being called matches the expected signature, reducing runtime errors.
• Multicast : A delegate can reference more than one method at a time, meaning it can invoke multiple methods in a chain. This is useful for event handling.
➤ Rules for creating delegate
• method must have the same return type and parameter types as the delegate.
• Method name and delegate name can be same but it's generally not recommended as it can lead to confusion.
• We generally define delegate under namespace.
1) Defining a Delegate:
[modifier] delegate [return type] [delegate-name] ([parameter-lists])
For ex :-
public delegate int MathOperation(int x, int y);
2) creating a method matching the delegate signature
class Calculator {
public static int Add(int a, int b)
{
return a + b;
}
}
3) Instantiating a delegate
• Create an instance of the delegate and assign it the method you want it to reference.
There are two ways to do this
1)
MathOperation mo = new MathOperation(Add);
If method is non static
Calculator obj=new Calculator();
MathOperation mo = new MathOperation(obj. Add);
2)
You can assign a method directly to a delegate without using the 'new' keyword
MathOperation mo = Add;
4) Invoke the Delegate
• Call the method through the delegate using the delegate instance.
int result = mo(5, 3); // Calls the Add method
Console.WriteLine(result); // Output: 8
using System;
// 1. Delegate for methods with a return type
public delegate int CalcDelegate(int x, int y);
// 2. Delegate for void methods
public delegate void CalcVoidDelegate(int x, int y);
*➤ Example*
___________________________________
// declaring delegate for method with return type
public delegate int CalcDelegate(int x, int y);
// declaring delegate for void method
public delegate void CalcVoidDelegate(int x, int y);
// class
public class Calculator
{
// 1. Static method with return type
public static int Add(int a, int b)
{
return a + b;
}
// 2. Void static method
public static void Subtract(int a, int b)
{
Console.WriteLine("Subtract: " + (a - b));
}
// 3. Non-static method with return type
public int Multiply(int a, int b)
{
return a * b;
}
// 4. Void non-static method
public void Divide(int a, int b)
{
Console.WriteLine("Divide: " + (a / b));
}
// Main method
public static void Main()
{
/* Static methods */
CalcDelegate addObj = Add; // Static method with return type
CalcVoidDelegate subObj = Subtract; // Void static method
/*Non-static methods */
Calculator obj = new Calculator();
CalcDelegate mulObj = obj.Multiply; // Non-static method with return type
CalcVoidDelegate divObj = obj.Divide; // Void non-static method
/* Invoke static methods */
Console.WriteLine("Add: " + addObj(10, 5)); // Output: Add: 15
subObj(10, 5); // Output: Subtract: 5
/* Invoke non-static methods */
Console.WriteLine("Multiply: " + mulObj(10, 5)); // Output: Multiply: 50
divObj(10, 5); // Output: Divide: 2
}
}
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Delegates Can Be Multicast:
➤ Multicast Delegate
• A multicast delegate in C# is a delegate that can hold references to more than one method.
• When you invoke a multicast delegate, all methods it references are invoked in the order they were added.
• All of the functions that the multicast delegate references will be called when the delegate is called.
• All method signatures should match if you want to use a delegate to call multiple methods.
➤ Key points about multicast delegate
➔ return type handling
• If the delegate has a return type (non-void), only the result of the last method in the invocation list is returned. Previous return values are discarded. (below, after few lines are the example see & understand )
• If the delegate returns void, all methods in the invocation list are executed, and each method can affect the program independently.
➔ combining methods
• Methods are added to delegates using the + or += operator.
➔ Removing methods
• You can remove a method from a multicast delegate using the -= operator.
➔ Use
• Multicast delegates are commonly used for event handling, logging, or notifications where multiple methods need to be invoked in response to a single event.
➤ Multicast delegate example
___________________________________
using System;
// Declare a delegate with a return type of int
public delegate int CalcDelegate(int a, int b);
// calculator class
public class Calculator
{
public int Add(int a, int b)
{
int result = a + b;
Console.WriteLine("Add: " + result);
return result;
}
public int Subtract(int a, int b)
{
int result = a - b;
Console.WriteLine("Subtract: " + result);
return result;
}
public int Multiply(int a, int b)
{
int result = a * b;
Console.WriteLine("Multiply: " + result);
return result;
}
public int Divide(int a, int b)
{
int result = a / b;
Console.WriteLine("Divide: " + result);
return result;
}
// Main Method
public static void Main()
{
// creating instance of Calculator class
Calculator obj = new Calculator();
/* Create a multicast delegate */
CalcDelegate calc = obj.Add;
calc += obj.Subtract;
calc += obj.Multiply;
calc += obj.Divide;
/* Invoke the multicast delegate */
// &
/* Since the delegate has a return type, only the result of the last method is returned.
Only the value of Divide method(which is in last) is returned */
int final_res = calc(20, 5);
Console.WriteLine("Final Result: " + final_res); // Output: 4
}
}
Output :-
Add: 25
Subtract: 15
Multiply: 100
Divide: 4
Final Result: 4
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*■ Event*
Read about event from pdf in classroom (pdf name - delegate and thread)
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