C# multithreading - updating the GUI with background events

Question

I'm a newbie to C# and multithreading so I apologise if this is a duplicate question but being a novice it appears my question is slightly different to others I have read.

My GUI runs in one (main) thread. It calls a background task (in a dll -- that I am writing too) that runs in a separate thread. The dll has NO knowledge of the GUI (i.e. it cannot reference the GUI class).

Now, say I want to update a progress bar on the GUI based on the status of the dll thread -> What I'm doing is creating an event in the dll that every X per cent will fire and the GUI will subscribe to this event. When the event is fired, the GUI will update the progress bar.

My questions:

1. Is the method of creating events the best way (bearing in mind the dll cannot reference the GUI)?
2. How do I ensure my above method is 'event safe'? Should I pass the progress percentage in the event to be thread safe or is there more to it?
3. Do I need to use Invoke when updating the GUI? I saw a post that suggested I did but I don't understand why since the updating of the bar is being done in the GUI thread??!

Hope you can clarify this for me!

Thanks

Answer 1

1.-I use that method all the time and yes it will work

2.-Just pass a int to the event handler and the variable will be safe to read. however when you are fireing the event from code do it like this

private void UpdatePercentage(int a)
{
    var myEvent = PercentageUpdatedEvent
    if(myEvent != null)
         myEvent(this, new ProgressBarEventArgs(a));
}

The reason for this is so if the event is unsubcribed between the null check and the calling you won't get a exception.

3.-As everyone else has mentioned you will need to call Invoke as the event will be running on the dll's thread. However with controls it is legal to call a BeginInvoke without a EndEnvoike so the call will be non blocking on the dll's thread.

Here is the pattern I always use

private myClass_OnPercentageUpdatedEvent(object a, ProgressBarEventArgs e)
{
    if(progressBar.InvokeRequired)
        progressBar.BeginInvoke((Action<object,ProgressBarEventArgs>)myCless_OnPercentageUpdatedEvent, a, e);
    else
    {
        progressBar.Value = e.Value;
    }
}