C# update a control from an external thread rising an event

Here is the context : in Winform, I use an object from an imported library, then I launch its main method. This object rises events to give process statut while it's proceed (on which I subscribe with one of my winform method to get this statuts). The main method returns a bool, only when it has ended its process (so the events rised allow to know the evolution of the process)

Here is the problem : my method subscribed to the event is well rised, it writes into the console the statut AND it add in a listbox the statut (using BeginInvoke) The informations writen into the console are put each time the event is rised (just what I need), BUT the listbox appears blanks until the main method as returned its value, and then is finally shows its contents.

I would like the listbox (or whatever control) shows the status returned from event at the exact moment it receives it, otherwise I can't inform my user during the process.

Here is my code, could you help me ?

Local method rised by object from library :

        private void ClientOnUpdate(object sender, EventParameters UpdateEventArgs)
    {
        //Instant write
        Console.WriteLine("Update Event Parameters: {0}", UpdateEventArgs);

        //Wait for the return of the Start method to show results inf lisbox
        lbErrors.BeginInvoke((Action)delegate()
        {
            lbErrors.Items.Add(String.Format("Update Event Parameters: {0}", UpdateEventArgs));
        });

    }

Launching the main method of the object from the library:

        private void Launch()
    {
        lbErrors.Items.Clear();
        Client cl1 = new Client();
        bool res = cl1.Start();

        if (res == true)
        {
            //do stuff ...
        }
        else
        {
            //do stuff ...
        }
    }

The results (status get from the fired event):

1. Start


2. Step 1 OK


3. Step 2 OK


4. Step 3 OK


5. End

Many thanks!