Why does Console.In.ReadLineAsync block?

Question

Start a new console app using the following code -

class Program
{
    static void Main(string[] args)
    {
        while (true)
        {
            Task<string> readLineTask = Console.In.ReadLineAsync();

            Debug.WriteLine("hi");
        }
    }
}

Console.In.ReadLineAsync is blocking and doesn't return until a line is entered in the console.. so "Hi" never gets written to the console.

Using await on Console.In.ReadLineAsync also blocks.

It was my understanding that the new Async CTP methods do not block.

What is the reason for this?

Here is another example

static void Main(string[] args)
{
    Task delayTask = Task.Delay(50000);

    Debug.WriteLine("hi");
}

This behaves as I expect, it goes straight to the next line and prints "hi" since Task.Delay does not block.

score 7 · Accepted Answer · answered Feb 12 '13 at 21:12

7

daryal provided the answer here http://smellegantcode.wordpress.com/2012/08/28/a-boring-discovery/

It seems ReadLineAsync is not actually doing what it's supposed to do. Bug in the framework.

My solution is to use ThreadPool.QueueUserWorkItem in a loop so each call to ReadLineAsync is done on a new thread.

answered Feb 12 '13 at 21:12

NoPyGod

4,905
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3

Bug is reported here https://connect.microsoft.com/VisualStudio/feedback/details/779079/console-readlineasync-is-actually-blocking-when-it-should-be-returning-a-task-immediately – NoPyGod Feb 12 '13 at 22:43
For other readers - connect.microsoft.com has been retired several years ago. – Eugene Mayevski 'Callback Sep 24 '21 at 09:54

score 4 · Answer 2 · edited Jan 12 '18 at 07:33

4

This can now be found in the documentation:

Read operations on the standard input stream execute synchronously. That is, they block until the specified read operation has completed. This is true even if an asynchronous method, such as ReadLineAsync, is called on the TextReader object returned by the In property.

edited Jan 12 '18 at 07:33

user247702

23,641
15
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answered Aug 30 '17 at 18:55

score 2 · Answer 3 · answered Jul 18 '18 at 14:07

Another solution:

static void Main()
{
    using (var s = Console.OpenStandardInput())
    using (var sr = new StreamReader(s))
    {
        Task readLineTask = sr.ReadLineAsync();
        Debug.WriteLine("hi");
        Console.WriteLine("hello");

        readLineTask.Wait();// When not in Main method, you can use await. 
                            // Waiting must happen in the curly brackets of the using directive.
    }
    Console.WriteLine("Bye Bye");
}

Why does Console.In.ReadLineAsync block?

3 Answers3

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