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C# event ExamplesUse an event type with an EventHandler delegate. Call Invoke to run all attached methods on the event.
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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).

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

Delegates

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.

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

Static

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.

Console

C# program that uses event
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.

Foreach

Object Array

Reflection

C# program that uses GetInvocationList
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

A 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.

BackgroundWorker

FileSystemWatcher

A summary. 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.

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