Question

I setup a test console application using the below code:

using System;

class Program
{
    static void Main(string[] args)
    {
        var myEventGenerator = new EventGenerator();
        for (int i = 0; i < 10; i++)
            new EventListener(myEventGenerator);

        while (Console.ReadLine().Length == 0)
            myEventGenerator.TriggerEvent();
    }
}

class EventListener
{
    private static int numberOfInstances = 0;
    private int instanceNumber;

    public EventListener(EventGenerator eg)
    {
        eg.EventHappened += EventHappened;
        instanceNumber = numberOfInstances++;
    }

    void EventHappened(object sender, EventArgs e)
    {
        Console.WriteLine("Event Caught in Instance " + instanceNumber);
    }
}

class EventGenerator
{
    public event EventHandler EventHappened;

    public void TriggerEvent()
    {
        Console.WriteLine("Event Begin");
        if (EventHappened != null)
            EventHappened.Invoke(this, new EventArgs());
        Console.WriteLine("Event End");
    }
}

And the output of the application is:

Event Begin
Event Caught in Instance 0
Event Caught in Instance 1
Event Caught in Instance 2
Event Caught in Instance 3
Event Caught in Instance 4
Event Caught in Instance 5
Event Caught in Instance 6
Event Caught in Instance 7
Event Caught in Instance 8
Event Caught in Instance 9
Event End

Seems that the event listeners are triggering based on the order that they subscribed to the event.

But what if I don’t want those event listeners to occur in that order? What if I want it to be random? Or, if I want to specify a different order? Is there a way to do that?

Answer 1

There isn’t any way to change the order in which the event handler invokes the events hooked up to it. Check out this page for more info.

Here is the important snippet:

Invocation of a delegate instance whose invocation list contains
multiple entries proceeds by invoking each of the methods in the
invocation list, synchronously, in order. Each method so called is
passed the same set of arguments as was given to the delegate
instance. If such a delegate invocation includes reference parameters
(Section 10.5.1.2), each method invocation will occur with a reference
to the same variable; changes to that variable by one method in the
invocation list will be visible to methods further down the invocation
list. If the delegate invocation includes output parameters or a
return value, their final value will come from the invocation of the
last delegate in the list.

If you want this kind of control, you’ll have to write some sort of class that does it for you.