Question

How do you make a event-driven program where the execution of the main flow is suspended whenever an event is raised and handled, until the event handler finishes?

I created the following program

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Timers;

namespace EventTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Timer t = new Timer(2000);
            t.Enabled = true;
            t.Elapsed += new ElapsedEventHandler(TimerHandler);
            for (int i = 0; i < 10000; i++)
            {
                for (int j = 0; j < 100000000; j++) ;
                Console.WriteLine(i);
            }
        }
        public static void TimerHandler(object source, EventArgs e)
        {
            Console.WriteLine("Event started");
            for (int i = 0; i < 5; i++)
            {
                for (int j = 0; j < 100000000; j++) ;
                Console.WriteLine("Event "+i);
            }
            Console.WriteLine("Event finished");
        }

    }
}

The main method is supposed to print sequential numbers, and an event is fired every 2 seconds. During the event handling, the program is supposed to print “Event started”, “Event 1” to “Event 5”, then “Event Finished”, then returns.

I expected Main() flow to stop while TimerHandler() is called, however my output suggests that Main() and TimerHandler() both are run simultaneously

0
1
2
3
4
5
6
7
8
9
10
11
Event started
12
Event 0
13
Event 1
14
Event 2
15
Event 3
16
Event 4
Event finished
17
18
19
20
21
22
23
Event started
24
Event 0
25
Event 1
26
Event 2
27
Event 3
28
Event 4
Event finished
29
30
31
32
33

Is it possible to make the Main() method to stop while the event is handled?

Answer 1

You’re using System.Timers.Timer, which processes ticks on a background thread, typically using the thread pool.

As such, yes, it will run simultaneously with the rest of your Main code.

As for your second question, possible to get Main to stop, I need to know what the expected behavior is.

If you want an entire tick in TimerHandler to run together, pausing Main while it does so, a simple lock would suffice:

namespace EventTest
{
    class Program
    {
        static object _Lock = new object();

        static void Main(string[] args)
        {
            Timer t = new Timer(2000);
            t.Enabled = true;
            t.Elapsed += new ElapsedEventHandler(TimerHandler);
            for (int i = 0; i < 10000; i++)
            {
                for (int j = 0; j < 100000000; j++) ;
                lock (_Lock)
                    Console.WriteLine(i);
            }
        }
        public static void TimerHandler(object source, EventArgs e)
        {
            lock (_Lock)
            {
                Console.WriteLine("Event started");
                for (int i = 0; i < 5; i++)
                {
                    for (int j = 0; j < 100000000; j++) ;
                    Console.WriteLine("Event "+i);
                }
                Console.WriteLine("Event finished");
            }
        }
    }
}