Calling Console.WriteLine from multiple threads

Question

Why does Console.WriteLine work from multiple threads?

Hoodwink

Why shouldn't it? – Discontinue

@Discontinue - The question is why does it work, what is the reason it works. There is nothing saying that it doesn't work. Your comment is snide and offensive. – Hoodwink

I'm sorry, it wasn't meant to sound this way. What it says is: The very fact that you ask this question suggests that you think it's a suprise that it does work. I really would have liked to know why, because I know pretty mouch nothing about multi-threading and its caveats. Apology if it sounded rude. – Discontinue

Answer 1

The console class handles the thread synchronization for you.

From the documentation of Console:

I/O operations using these streams are synchronized, which means multiple threads can read from, or write to, the streams.

Masteratarms

@Reed is it synchronized by locking or with some other synchronization mechanism? – Couturier

@Lirik It's handled on the native side, not using a managed lock. – Masteratarms

I would like to add that the Console window (in Visual Studio) allows writing text from multiple threads, but you should be aware that the order in which the text is output may not be as expected, e.g. when adding large strings (like stack traces) they can even end up "garbled" or mixed. There is apparently no locking that ensures one Write is completed before another call starts pumping text into the stream. – Benedicto

I'm wondering, how long the text has to be to experience the garbling? I tried two similar programs, one in C++ with cout and another in C# with Parallel.For and Console.Write. While the C++ program did indeed give mixed output from different threads, the C# program displayed correct output and I couldn't find any way to corrupt it. – Williford

@Benedicto Do you mean that the order of calling Console.WriteLine from multiple threads is not the same with the order in which text appears in the console? In other words, does Console.WriteLine writes asynchronously and not in the same order it was called? – Hayott

@WriteEatSleepRepeat: The calls are posted to Write in the correct order and the output starts in the correct order, but longer texts (e.g. stack traces) may be interrupted by shorter texts, sent later, but before the first text was fully output. At least that was my experience... – Benedicto

@WriteEatSleepRepeat: This does not apply to WriteLine. – Benedicto

Answer 2

There is a bug in .NET 4.5 CLR which makes Console.WriteLine not work from multiple threads if you use Console.ReadKey. It is fixed in some Windows versions, but in 8.1 Windows Update does not find it yet.

Infrequent hangs in a multi-threaded C# console application when using Console.Writeline() or Console.Write()

Using Console.WriteLine in a Timer why it would appear to exit?

Inquisitor

Answer 3

Multiple threads write to the same output when using Console.WriteLine, generally your screen by default.

Adipose

So Console.WriteLine is coded as thread safe then? One thread blocks while the other writes? – Hoodwink

It is - see my answer for the details from MSDN. – Masteratarms

Answer 4

The console and your code are 2 seperate applications

The console window is not your application. A console application is invisable, your application does not contain the console. The console is installed into windows itself.

Console.Writeline() is a Message

It writes to a TextStream. Microsoft's console process simply waits for new streamtext to display.

The 'Console Application' name does not make it a console

The difference with ui projects is that a console application comes with a simplified set of references, and relies on input / output stream to communicate with the user.

You dont even need the console, you could build your own console, or use a debugger to view and write to it.

Comparison with notepad

If a file was a message, and notepad opened the filestream saved by your code. It wouldnt have mattered from which thread you saved the file. You are not accesing any ui.

Chavira