Event in C#

Event in C# is based on the mechanism that delegate used to work.  Event is useful for broadcasting information (or message) to and receiving data (or message) from other parts of the program.  

Event allows the program to communicate asynchronously and help us build a more responsive application.  Besides, Event can be switched on and off as needed.  

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Basic Syntax – Event Handler

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace BasicEvents
{
    // define the delegate for the event handler
    public delegate void myEventHandler(string value);


    class EventPublisher
    {
        private string theVal;

        // declare the event
        public event myEventHandler valueChanged;

        public string Val
        {
            set
            {
                this.theVal = value;
                // when the value changes, fire the event
                this.valueChanged(theVal);
            }
        }
    }


    class Program
    {
        static void Main(string[] args)
        {

            EventPublisher obj = new EventPublisher();
            obj.valueChanged += obj_valueChanged;

            string str;
            do {
                Console.WriteLine("Enter a value: ");
                str = Console.ReadLine();
                if (!str.Equals("exit")) {
                    obj.Val = str;
                }
            } while (!str.Equals("exit"));

            Console.WriteLine("Goodbye!");

            static void obj_valueChanged(string value)
            {
                Console.WriteLine("The value changed to {0}", value);
            }
        }

    }
}

The code above is an example of Event Handler in C# programming. The primary purpose of the Event Handler in this code snippet is observing the change of theVal property in EventPublisher class.

We define the delegate for the event handler by the following line of code, and it looks exactly the same as the delegate declaration.

// define the delegate for the event handler
public delegate void myEventHandler(string value);

Once we declared the delegate for the event handler, we need to declare the event in the designated class as a property. In the code above, we declare the event in EventPublisher class, and we name it as valueChanged. 

// declare the event
public event myEventHandler valueChanged;

After we declare the event, we need to trigger the event as needed. In the example above, we trigger the event when the value of Val property changed. In the meantime, we pass in the new value of Val property as a parameter.

public string Val
        {
            set
            {
                this.theVal = value;
                // when the value changes, fire the event
                // trigger the event
                this.valueChanged(theVal);
            }
        }

Everything is set and we need to switch on the Event. In the main function of program class, we switch on the event by the following code:

obj.valueChanged += obj_valueChanged;

The obj_valueChanged is the implementation method of the delegate.

Every time, the obj.Val changed, we fire the event.

if (!str.Equals("exit")) {
    obj.Val = str;
}

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Anonymous Event Handler

As mentioned before, Event is based on Delegation in C#; so we can implement the Event by using Anonymous Event Handler.

The amend code snippet:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace BasicEvents
{
    // define the delegate for the event handler
    public delegate void myEventHandler(string value);


    class EventPublisher
    {
        private string theVal;

        // declare the event
        public event myEventHandler valueChanged;

        public string Val
        {
            set
            {
                this.theVal = value;
                // when the value changes, fire the event
                // trigger the event
                this.valueChanged(theVal);
            }
        }
    }


    class Program
    {
        static void Main(string[] args)
        {

            EventPublisher obj = new EventPublisher();
            //Anonymous Event Implementation
            obj.valueChanged += delegate (string val) {
                Console.WriteLine("The value changed to {0}", val);
            };

            string str;
            do {
                Console.WriteLine("Enter a value: ");
                str = Console.ReadLine();
                if (!str.Equals("exit")) {
                    obj.Val = str;
                }
            } while (!str.Equals("exit"));

            Console.WriteLine("Goodbye!");

        }

    }
}

As you can see, we delete the static void obj_valueChanged(string value) function. We switch on and implement the event at the same time via the following code.

//Anonymous Event Implementation
obj.valueChanged += delegate (string val) {
     Console.WriteLine("The value changed to {0}", val);
};

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.NET Framework Guideline For Defining Event Handlers

The code snippets above are customized Event Handlers, whereas the .NET framework provides us with a predefined event handler for our development as well.

The .NET Framework defines a predefined event handler as a template that you can use to send events using the same format that .NET does. .NET event handlers typically take the form of having two arguments. The first one is the object where the event originated. The second is a custom sub-class derived from the .NET event args base-class. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ChainedEvents
{
    

    class MyClass
    {
        private string theVal;
        // declare the event handle      
        public event EventHandler<ObjChangeEventArgs> objChanged;

        public string Val
        {
            set
            {
                this.theVal = value;
                // when the value changes, fire the event
                
                this.objChanged(this, new ObjChangeEventArgs() { propChanged = "Val" });
            }
        }
    }

    class ObjChangeEventArgs : EventArgs
    {
        public string propChanged;
    }

    class Program
    {
        static void Main(string[] args)
        {
            // create the test class
            MyClass obj = new MyClass();

            obj.objChanged += delegate(object sender, ObjChangeEventArgs e) {
                Console.WriteLine("{0} had the '{1}' property changed", sender.GetType(), e.propChanged);
            };

            string str;
            do {
                Console.WriteLine("Enter a value: ");
                str = Console.ReadLine();
                if (!str.Equals("exit")) {
                    obj.Val = str;
                }
            } while (!str.Equals("exit"));
            Console.WriteLine("Goodbye!");
        }

    }
}

First, We declare the event handler by the following code. We define the event by using the generic syntax of C#, instead of the delegate declaration. The name of the event is objChanged.

// declare the event handler      
public event EventHandler<ObjChangeEventArgs> objChanged;

We declare a custom sub-class derived from the .NET event args base-class.  In this case, the name of the class is ObjChangeEventArgs and it is a subclass of EventArgs, which is a sub-class derived from the .NET event args base-class.

class ObjChangeEventArgs : EventArgs
{
    public string propChanged;
}

We set up the event by the following code. There are two parameters in the implementation. this refers to the original class, in this case, it is MyClass. new ObjChangeEventArgs() refers to the subclass of EventArgs, which is a sub-class derived from the .NET event args base-class.

// when the value changes, fire the event
this.objChanged(this, new ObjChangeEventArgs() { 
propChanged = "Val" 
});

After that, we switch on and implement the event by the following code:

obj.objChanged += delegate(object sender, ObjChangeEventArgs e) 
{
Console.WriteLine("{0} had the '{1}' property changed", sender.GetType(), e.propChanged);
};

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Event in C# programming is widely used in our development, especially when we are doing the web, UI development. It takes time to sink in, but it is very rewarding in the end. Good Luck.

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