Console delegate FileSystemWatcher foreach object Array Reflection static

Event

An event can have many handlers. With the event handler syntax, we create a notification system. Events are used in many .NET programs (including Windows Forms).

We attach additional methods without changing other parts of the code. This makes programs easier to maintain. It makes code clearer.

First example

We use the event keyword. It creates an original event. The .NET Framework has many existing events: these are covered elsewhere.

Info The delegate keyword is used to specify the EventHandler type. The event keyword is used to create an instance of an event.

Detail We use the static modifier on the event. We can access the static event from a static method (Main).

Part 1 The _show event has 4 method instances added to its invocation list with the plus "+=" operator.

Part 2 The Invoke method is called. This in turn causes each of the 4 method instances to be run. The strings are printed to the console.

using System;

public delegate void EventHandler();

class Program
{
    public static event EventHandler _show;

    static void Main()
    {
        // Part 1: add event handlers to Show event.
        _show += new EventHandler(Dog);
        _show += new EventHandler(Cat);
        _show += new EventHandler(Mouse);
        _show += new EventHandler(Mouse);

        // Part 2: invoke the event.
        _show.Invoke();
    }

    static void Cat()
    {
        Console.WriteLine("Cat");
    }

    static void Dog()
    {
        Console.WriteLine("Dog");
    }

    static void Mouse()
    {
        Console.WriteLine("Mouse");
    }
}Dog
Cat
Mouse
Mouse

`GetInvocationList`

We can call methods on an event type. Here we use GetInvocationList, which returns a Delegate object array. With reflection, we can print the names of the methods.

using System;

public delegate void EventHandler();

class Program
{
    public static event EventHandler _click;

    static void Main()
    {
        _click += new EventHandler(ClickItem);
        _click += new EventHandler(ClickItem);

        // Display all delegates.
        foreach (var item in _click.GetInvocationList())
        {
            Console.WriteLine(item.Method.Name);
        }
    }

    static void ClickItem()
    {
    }
}ClickItem
ClickItem

Discussion

Events let you add methods to be triggered upon an external event. You might add new event handlers dynamically for a specific action. Then, call Invoke to run them all.

Tip Events can reduce the number of code changes needed. We can just add events, and not worry about calling locations.

Thus The event handler notification system provides a way to isolate different parts of your program from excessive changes.

Uses, Windows Forms

Events are used in many objects in the .NET Framework. The Windows Forms framework uses events for many controls.

Tip These events allow you to instantly act upon button clicks and key presses.

Note With threading, we use events on the BackgroundWorker type. We can monitor file system changes with FileSystemWatcher.

With events we separate something that happens from the behaviors we want to occur. Events are often used on the built-in types in the .NET Framework.