Question

Example program: Listen FileSystem events on some folder and print FileSystem events info to console when Timer event fires.

class Program
{
    public static string location = @"D:\TestEvents";
    public static double interval = 15000; 

    public static System.Timers.Timer timer;
    public static List<string> listOfChanges = new List<string>();

    static void Main(string[] args)
    {
        StartWatch();
        StartTimer();

        Console.ReadLine();
    }

    private static void StartWatch()
    {
        FileSystemWatcher Watcher = new FileSystemWatcher();
        Watcher.Path = location;
        Watcher.Created += new FileSystemEventHandler(OnFileCreatedOrDeleted);
        Watcher.Deleted += new FileSystemEventHandler(OnFileCreatedOrDeleted);
        Watcher.EnableRaisingEvents = true;
    }

    static void OnFileCreatedOrDeleted(object sender, FileSystemEventArgs e)
    {
        listOfChanges.Add(String.Format("Change Type: {0}, Name: {1}, Time: {2}", e.ChangeType, e.Name, DateTime.Now));
    }

    private static void StartTimer()
    {
        timer = new System.Timers.Timer();
        timer.AutoReset = false;
        timer.Elapsed += new System.Timers.ElapsedEventHandler(OnTimerEpleased);
        timer.Interval = interval;
        timer.Start();
    }

    private static void OnTimerEpleased(object sender, System.Timers.ElapsedEventArgs e)
    {
        Console.WriteLine("Timer event fired: " + DateTime.Now);
        foreach (var item in listOfChanges)
        {
            Console.WriteLine(item);
        }
        Console.WriteLine();
        listOfChanges.Clear();

        timer.Interval = interval;
        timer.Start();
    }
}

Is it safe to access the same storage static List<string> listOfChanges from both event handlers?
I don’t really understand how events works underneath. Is it creates some global event handler queue and runs all event handlers one-by-one despite of event type? Or it creates different threads for each event handler type?

Edit:
I guess the best solution is to use BlockingCollection with ConcurrentQueue, so it should be like this:

public static BlockingCollection<string> listOfChanges = new BlockingCollection<string>();

static void OnFileCreatedOrDeleted(object sender, FileSystemEventArgs e)
{
    listOfChanges.Add(String.Format("Change Type: {0}, Name: {1}, Time: {2}", e.ChangeType, e.Name, DateTime.Now));
}

private static void OnTimerEpleased(object sender, System.Timers.ElapsedEventArgs e)
{
    Console.WriteLine("Timer event fired: " + DateTime.Now);
    while (listOfChanges.Count > 0)
    {
        string item;
        bool b = listOfChanges.TryTake(out item);
        if (b)
        {
            Console.WriteLine(item);
        }
    }
    Console.WriteLine();

    timer.Interval = interval;
    timer.Start();
}

Answer 1

A generic List is not thread safe. It will be possible that you will get an error when listOfChanges is traversed (in OnTimerEpleased), and a new entry would have added an entry to the list (OnFileCreatedOrDeleted). See http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx. Either you synchronize access to the list or use a built-in thread safe collection: http://msdn.microsoft.com/en-us/library/system.collections.concurrent.aspx