Order of firing delegates

[John A. Bailo]

I'm working through some code that uses events and delegates.

Pardon me, but I understand delegates to be a little like function
callbacks in c.

I coded up a code sample and started playing with it -- but what I don't
understand is this.

Shouldn't an event behave asynchronously?

Like if I fire an event, and it's handler takes 5 seconds to occur, and
then I fire a second event and it's handler takes 2 seconds to occur,
shouldn't the 2nd handler's results show up first?

When I run my code in Studio, it seems like it waits for the first
handler to finish, and then goes to the second handler. That seems
more like something I would expect from VB5 -- not c# .NET!

using System;
using System.Diagnostics;
using System.Threading;

namespace DelegateTest
{
/// <summary>
/// Summary description for Class1.
/// </summary>
///
//create delegate object
public delegate void MyHandler1(object sender, MyEventArgs e);
public delegate void MyHandler2(object sender, MyEventArgs e);
//create event handler methods
class A
{
public const string m_id="Class A";

public void OnHandler1(object sender, MyEventArgs e)
{

//this should delay 1 for a while
while(true) Thread.Sleep(1000);

Debug.WriteLine("I am in OnHandler1 "
+ "and MyEventArgs is {0}", e.m_id);
}

public void OnHandler2(object sender, MyEventArgs e)
{

//this should fire first.
Debug.WriteLine("I am in OnHandler2 " +
"and MyEventArgs is {0}", e.m_id);
}
public A(B b)
{
MyHandler1 d1= new MyHandler1(OnHandler1);
MyHandler2 d2= new MyHandler2(OnHandler2);
b.Event1 += d1;
b.Event2 += d2;
}

}

class B
{
public event MyHandler1 Event1;
public event MyHandler2 Event2;

public void FireEvent1(MyEventArgs e)
{
if(Event1 != null)
{

Event1(this, e);
}
}

public void FireEvent2(MyEventArgs e)
{
if(Event2 != null)
{

Event2(this, e);
}
}
}

public class MyEventArgs {
public string m_id;
}

class Delegator
{
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main(string[] args)
{
//
// TODO: Add code to start application here
//

B b = new B();
A a = new A(b);

MyEventArgs e1 = new MyEventArgs();
MyEventArgs e2 = new MyEventArgs();

e1.m_id="Event args for event 1";
e2.m_id="Event args for event 2";

b.FireEvent1(e1);
b.FireEvent2(e2);
}
}
}

[Truong Hong Thi]

>Shouldn't an event behave asynchronously?
No, event hanlers get invoked synchronously. Every delegate in C# is
multicast, it consists of a chain of elements which you can obtain by
using MulticastDelegete.GetInvocationList method. If you want to invoke
them asynchronously, call GetInvocationList first, then invoke each
delegate in the returned array in a separate thread.

-----
Thi - http://thith.blogspot.com

[Bruce Wood]

> Shouldn't an event behave asynchronously?

No. Multicast delegates are nothing more than a language shorthand for
calling a list of methods in sequence. It is the .NET version of the
Observable / Observer pattern, which, if you look to Java, works
synchronously as well.

You can introduce asynchrony as Truong pounts out, but you don't get it
if you don't ask for it.

In fact, this makes things much easier to understand, particularly when
you start driving the UI using events. Asynchrony adds complexity (just
look at the number of posts in this newsgroup about threading
problems), and in many cases it's unnecessary for the smooth running of
an application. Making every event delegate run in its own thread would
introduce a lot of additional complexity and overhead that would bring
much benefit.

It's easy to introduce multithreading in C# where you decide that it
makes a difference. Otherwise, what you get is synchronous behaviour.