Timer event handler doesn't write on console

Question

I know this might be strange but I have a timer and I have an event handler for Elapsed event that writes on Console, but when I start the application, the timer start properly, the event fire properly, too. However, the result doesn't show in console except after I press a button, which made me put two Console.ReadKey() so the application won't terminate.

Here is the code in Program.cs:

    static void Main(string[] args)
    {
        Timer timer = new Timer(100);
        timer.Elapsed += new ElapsedEventHandler(WriteOnConsole);
        timer.Start();
        Console.ReadKey();
        Console.ReadKey();
    }

    static void WriteOnConsole(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("A B C D");
    }

Please inform me if I haven't mentioned enough information.

What is the type of your timer? Which timer class (System.Timers, System.Threading....etc.)? — ken2k, Mar 01 '13 at 12:53
@ken2k, I think its `System.Timers.Timer` from the event and constructor — Habib, Mar 01 '13 at 12:57
possible duplicate of [Strange behaviour of Console.ReadKey() with multithreading](http://stackoverflow.com/questions/15143931/strange-behaviour-of-console-readkey-with-multithreading) — Jon Skeet, Mar 01 '13 at 12:59

score 8 · Answer 1 · answered Mar 01 '13 at 12:55

8

The ReadKey() blocks the WriteLine(). After you pressed one key, the first ReadKey is finished and the pending writes to the console are flushed.

As this obviously only is a small sample to demonstrate the problem it is hard to suggest a better alternative.

One could be to use some sort of wait handle to exit the application only when a certain condition is met.

answered Mar 01 '13 at 12:55

Daniel Hilgarth

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But only the first one. And only if you haven't already written to it. I think there's something weird in the way the console is initialized, basically. – Jon Skeet Mar 01 '13 at 12:56
1

@JonSkeet: Which "first one" do you mean? The first `ReadKey` or the first `WriteLine`? – Daniel Hilgarth Mar 01 '13 at 12:57
Coincidentally, I think this has the same cause as the thing I asked about yesterday: http://stackoverflow.com/questions/15143931/strange-behaviour-of-console-readkey-with-multithreading – Matthew Watson Mar 01 '13 at 12:57
@DanielHilgarth: The first `ReadKey`. Note how the second `ReadKey` call *doesn't* block anything. – Jon Skeet Mar 01 '13 at 12:58
@JonSkeet The first `ReadKey`doesn't block anything either on my machine. – ken2k Mar 01 '13 at 12:59
Thank you Daniel, thank you Eivind. I'm still a beginner in C#, I used `Console.ReadLine()` which I never thought I'd use it for that purpose – Imad Nabil Alnatsheh Mar 01 '13 at 13:00
@ken2k: Indeed, on mine neither. It outputs right away. Strange. – Daniel Hilgarth Mar 01 '13 at 13:00
@Jon Skeet: What do you mean the second call to ReadKey _doesn't_ block anything ? Do you mean to say that it isn't a blocking call (which returns only after the console received a key press) ? Because it is... – Eduard Dumitru Mar 01 '13 at 13:01
@EduardDumitru: It's a blocking call, but it doesn't prevent another thread from writing output. – Jon Skeet Mar 01 '13 at 13:04
@JonSkeet: Oh. sorry.. I misunderstood. Of course it doesn't. You know what I thought you meant... Dangerously homonymic terms ended up in the same little package :) `Console.ReadKey` – Eduard Dumitru Mar 01 '13 at 13:08
@EduardDumitru: The point is that the "of course" isn't really an "of course" because when it's broken, the first `ReadKey` *does* block the `WriteLine` in the other thread. – Jon Skeet Mar 01 '13 at 13:09

score 3 · Answer 2 · answered Mar 01 '13 at 12:58

Try writing an empty string from the 1st thread, before anything else, thus initializing the Console:

static void Main(string[] args)
{
    Console.Write(string.Empty);
    Timer timer = new Timer(100);
    timer.Elapsed += new ElapsedEventHandler(WriteOnConsole);
    timer.Start();
    Console.ReadKey();
    Console.ReadKey();
}

static void WriteOnConsole(object source, ElapsedEventArgs e)
{
    Console.WriteLine("A B C D");
}

score 2 · Answer 3 · answered Mar 01 '13 at 12:58

2

I've seen this in another question - you don't even need a timer, just any two threads where one thread is reading from the console and another is writing.

It looks like the initialization for Console is somewhat broken - the first call to it will effectively take out a lock until it completes, preventing any other console access.

If you write:

Console.WriteLine("Starting");

at the start of your code, all will be fine, for example. It's not like ReadKey (or any other read operation) blocks all console output in general - it's just in the first call.

answered Mar 01 '13 at 12:58

Jon Skeet

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I can't reproduce: the OP's code works fine on my machine (edit: I mean it outputs directly to the console without the need to press a key) – ken2k Mar 01 '13 at 13:00
1

This is a race condition, and it seems to only occur on some systems. – Matthew Watson Mar 01 '13 at 13:04

Timer event handler doesn't write on console

3 Answers3