Question

pseudo code:

form1
{
    int i;
    label1;

    Add()
    {
     i++;
     label1 = i.ToString(); //#ErrorLine
    }

    backgroundworker worker;

    worker_DoWork()
    { 
     FileGuard guard = new FileGuard();
     guard.FileKilled += guard.KillH(Add);
     guard.StarGuarding(); //there is system watcher inside 
                           //this guard and some processing code
                           //that will fire event FileKilled();

    } 
} 

Afer calling StartGuarding() worker will be compleated
But when there is event FileKilled fired I goth this error on line #ErrorLine

Cross-thread operation not valid: Control ‘form1’ accessed from a thread other than the thread it was created on.

Answer 1

This has nothing to do with the events themselves, but rather the fact that you are accessing UI controls from another thread. In Windows Forms, you are not allowed to interact with the UI from any other thread than the main UI thread.

You can use InvokeRequired to check whether you are on a thread that has no access to the UI, and then use Invoke to run code on the UI thread if required. It might look something like this:

private void DoStuffWithGUI() 
{
    if (InvokeRequired) 
    {
        Action work = DoStuffWithGUI;
        Invoke(work);
    }
    else
    {
        // Your normal logic
    }
}

You can use the UI directly from ProgressChanged and RunWorkerCompletedEvents (because they are automatically marshalled to the UI thread). But all work you do within DoWork, (and therefore all events you might raise as part of the work) runs in a separate thread, and must be marshalled to the UI thread using Invoke. From MSDN for BackgroundWorker:

You must be careful not to manipulate
any user-interface objects in your
DoWork event handler. Instead,
communicate to the user interface
through the ProgressChanged and
RunWorkerCompleted events.