C# – What happens when an event fires and tries to execute an event-handler in an object that no longer exists

In your case, everything is fine. It's the object which publishes the events which keeps the targets of the event handlers live. So if I have:

publisher.SomeEvent += target.DoSomething;

then publisher has a reference to target but not the other way round.

In your case, the publisher is going to be eligible for garbage collection (assuming there are no other references to it) so the fact that it's got a reference to the event handler targets is irrelevant.

The tricky case is when the publisher is long-lived but the subscribers don't want to be - in that case you need to unsubscribe the handlers. For example, suppose you have some data transfer service which lets you subscribe to asynchronous notifications about bandwidth changes, and the transfer service object is long-lived. If we do this:

BandwidthUI ui = new BandwidthUI();
transferService.BandwidthChanged += ui.HandleBandwidthChange;
// Suppose this blocks until the transfer is complete
transferService.Transfer(source, destination);
// We now have to unsusbcribe from the event
transferService.BandwidthChanged -= ui.HandleBandwidthChange;

(You'd actually want to use a finally block to make sure you don't leak the event handler.) If we didn't unsubscribe, then the BandwidthUI would live at least as long as the transfer service.

Personally I rarely come across this - usually if I subscribe to an event, the target of that event lives at least as long as the publisher - a form will last as long as the button which is on it, for example. It's worth knowing about this potential issue, but I think some people worry about it when they needn't, because they don't know which way round the references go.

EDIT: This is to answer Jonathan Dickinson's comment. Firstly, look at the docs for Delegate.Equals(object) which clearly give the equality behaviour.

Secondly, here's a short but complete program to show unsubscription working:

using System;

public class Publisher
{
    public event EventHandler Foo;

    public void RaiseFoo()
    {
        Console.WriteLine("Raising Foo");
        EventHandler handler = Foo;
        if (handler != null)
        {
            handler(this, EventArgs.Empty);
        }
        else
        {
            Console.WriteLine("No handlers");
        }
    }
}

public class Subscriber
{
    public void FooHandler(object sender, EventArgs e)
    {
        Console.WriteLine("Subscriber.FooHandler()");
    }
}

public class Test
{
    static void Main()
    {
         Publisher publisher = new Publisher();
         Subscriber subscriber = new Subscriber();
         publisher.Foo += subscriber.FooHandler;
         publisher.RaiseFoo();
         publisher.Foo -= subscriber.FooHandler;
         publisher.RaiseFoo();
    }
}

Results:

Raising Foo
Subscriber.FooHandler()
Raising Foo
No handlers

(Tested on Mono and .NET 3.5SP1.)

Further edit:

This is to prove that an event publisher can be collected while there are still references to a subscriber.

using System;

public class Publisher
{
    ~Publisher()
    {
        Console.WriteLine("~Publisher");
        Console.WriteLine("Foo==null ? {0}", Foo == null);
    }

    public event EventHandler Foo;
}

public class Subscriber
{
    ~Subscriber()
    {
        Console.WriteLine("~Subscriber");
    }

    public void FooHandler(object sender, EventArgs e) {}
}

public class Test
{
    static void Main()
    {
         Publisher publisher = new Publisher();
         Subscriber subscriber = new Subscriber();
         publisher.Foo += subscriber.FooHandler;

         Console.WriteLine("No more refs to publisher, "
             + "but subscriber is alive");
         GC.Collect();
         GC.WaitForPendingFinalizers();         

         Console.WriteLine("End of Main method. Subscriber is about to "
             + "become eligible for collection");
         GC.KeepAlive(subscriber);
    }
}

Results (in .NET 3.5SP1; Mono appears to behave slightly oddly here. Will look into that some time):

No more refs to publisher, but subscriber is alive
~Publisher
Foo==null ? False
End of Main method. Subscriber is about to become eligible for collection
~Subscriber

It depends. The System.Timers.Timer has two modes of operation.

If SynchronizingObject is set to an ISynchronizeInvoke instance then the Elapsed event will execute on the thread hosting the synchronizing object. Usually these ISynchronizeInvoke instances are none other than plain old Control and Form instances that we are all familiar with. So in that case the Elapsed event is invoked on the UI thread and it behaves similar to the System.Windows.Forms.Timer. Otherwise, it really depends on the specific ISynchronizeInvoke instance that was used.

If SynchronizingObject is null then the Elapsed event is invoked on a ThreadPool thread and it behaves similar to the System.Threading.Timer. In fact, it actually uses a System.Threading.Timer behind the scenes and does the marshaling operation after it receives the timer callback if needed.