Delegates and Events

Question

Hi

Why do most .NET Framework events take the following form:-

Page_Load(Object sender, EventArgs e)

Looking at the Event example in the MSDN documentation it is as follows:-

// Declare the Delegate
public delegate void SampleEventHandler(Object sender, EventArgs e)

// Declare the Event
// public event SampleEventHandler SampleEvent;

//Raise the Event
SampleEvent(this, e);

So given Page_Load is an Event it doesnt fit any of the above syntax. The only one that it is similar to is the delegate but of course this is a Delegate.

I just need a bit of clarification please. I know the Page_Load is the Event Handler Method so is this used differently to the above as the Method has its own Object sender, EventArgs e parameters?

Regards

Steven

Vulpes · Answer

This simple example may help to clarify the matter: using System; class Test { static void Main() { Test t = new Test(); Page p = new Page(); p.Load += new EventHandler(t.Page_Load); p.OnLoad(t EventArgs.Empty); Console.ReadKey(); } public void Page_Load(Object sender EventArgs e) { Console.WriteLine(Hello from Page_Load); } } class Page { public event EventHandler Load; public void OnLoad(Object sender EventArgs e) { if(Load != null) { Load(sender e); } } } If you change the highlighted line to: Load.Invoke(); then you'll find that the program won't compile. However it will compile and run fine if you use this: Load.Invoke(sender e); which is what the compiler (in effect) translates C#'s special syntax: Load(sender e) into.

Vulpes · Answer

That's right Steven. The Page_Load eventhandler must be compatible with the System.EventHandler delegate. So it must take two parameters of types Object and EventArgs and have a return type of void.

S · Answer

Upon further reading:- The signature of the event-handling method is identical to the signature of the event-handler delegate. The event-handler signature observes the following conventions: The return type is Void. The first parameter is named sender and is of type Object. This is the object that raised the event. The second parameter is named e and is of type EventArgs or a derived class of EventArgs. This is the event-specific data. The method takes exactly two parameters. So from this it implies that the EventHander delegate has the two parameters and not the EventHandling method however Page_Load still does? Is this because the Page_Load Method has to match the Delegate signature ie Return Type and Parameter Types?

S · Answer

Ok brilliant thanks Vulpes. So can the 2 parameters (Object sender EventArgs e) which are commonly seen be either declared on the Delegate or the Method being invoked by the Delegate ie:- // Delegate declaration delegate void EventHandler(Object sender EventArgs e) or //Method declaration protected void Page_Load(Object sender EventArgs e) or Both? Regards Steven

Vulpes · Answer

Events are actually a special kind of delegate where the only operations you can apply to them (outside the class in which they're defined) are the += and -= operators which respectively add an eventhandler to or remove an eventhandler from the event's invocation list. So the event Load in this example is of type EventHandler which is a delegate defined in the System namespace and is typically used for events which don't generate any data: http://msdn.microsoft.com/en-us/library/system.eventhandler.aspx Delegates of the same type as the event are also used to add eventhandler methods to the invocation list. So this line: p.Load += new EventHandler(t.Page_Load); adds an EventHandler delegate to Load's invocation list. When the event is fired this delegate invokes the t.Page_Load eventhandler. So you'll gather from all this that whenever you're using events you're implicitly using delegates.

S · Answer

So where does the delegate come into it or doesnt it is this a pure Event example only showing how the Page_Load Event Handler works? For example public delegate void EventHandler(Object sender EventArgs e); public event EventHandler NoDataEventHandler; I assume that either a delegate or method can be used as the EventHander ie your example or this one? Thanks

S · Answer

Looking into it further... Am I right in saying that because the Page_Load Event Handler Method already has the 2 parameters Object sender and EventArgs e is it satisfactory to do this:- // Declare the Delegate public delegate void EventHandler(); // Declare the Event public event EventHandler Load // Add the Page_Load Method to the Invocation list Load += new EventHandler(Page_Load); // Invoke Load.Invoke(); This being different from the other way of specifying a Delegate which has the Object sender EventArgs e as the 2 parameters like so... // Declare the Delegate public delegate void EventHandler(Object sender EventArgs e); // Declare the Event public event EventHandler Load // Invoke Load(this e);