Delegates and Event

Event :-
An event is a message sent by an object to signal the occurrence of an action. The action could be caused by user interaction, such as a mouse click, or it could be triggered by some other program logic. The object that raises the event is called the event sender. The object that captures the event and responds to it is called the event receiver.

In event communication, the event sender class does not know which object or method will receive (handle) the events it raises. What is needed is an intermediary (or pointer-like mechanism) between the source and the receiver. The .NET Framework defines a special type (Delegate) that provides the functionality of a function pointer.

A delegate is a class that can hold a reference to a method. Unlike other classes, a delegate class has a signature, and it can hold references only to methods that match its signature. A delegate is thus equivalent to a type-safe function pointer or a callback.

How to declare delegates and events :-

private delegate void dlgAction();

private event dlgAction m_evnt = null;

Events are variables of type delegates

Example

public delegate void Notify();  // delegate
                    
public class ProcessBusinessLogic
{
    public event Notify ProcessCompleted; // event

    public void StartProcess()
    {
        Console.WriteLine("Process Started!");
        // some code here..
        OnProcessCompleted();
    }

    protected virtual void OnProcessCompleted() //protected virtual method
    {
        //if ProcessCompleted is not null then call delegate
        ProcessCompleted?.Invoke(); 
    }
}

class Program
{
    public static void Main()
    {
        ProcessBusinessLogic bl = new ProcessBusinessLogic();
        bl.ProcessCompleted += bl_ProcessCompleted; // register with an event
        bl.StartProcess();
    }

    /// event handler
    public static void bl_ProcessCompleted()
    {
        Console.WriteLine("Process Completed!");
    }
}