How to add a Timeout to Console.ReadLine()?

Question

I have a console app in which I want to give the user x seconds to respond to the prompt. If no input is made after a certain period of time, program logic should

Answer 1

EDIT: fixed the problem by having the actual work be done in a separate process and killing that process if it times out. See below for details. Whew!

Just gave this a run and it seemed to work nicely. My coworker had a version which used a Thread object, but I find the BeginInvoke() method of delegate types to be a bit more elegant.

namespace TimedReadLine
{
   public static class Console
   {
      private delegate string ReadLineInvoker();

      public static string ReadLine(int timeout)
      {
         return ReadLine(timeout, null);
      }

      public static string ReadLine(int timeout, string @default)
      {
         using (var process = new System.Diagnostics.Process
         {
            StartInfo =
            {
               FileName = "ReadLine.exe",
               RedirectStandardOutput = true,
               UseShellExecute = false
            }
         })
         {
            process.Start();

            var rli = new ReadLineInvoker(process.StandardOutput.ReadLine);
            var iar = rli.BeginInvoke(null, null);

            if (!iar.AsyncWaitHandle.WaitOne(new System.TimeSpan(0, 0, timeout)))
            {
               process.Kill();
               return @default;
            }

            return rli.EndInvoke(iar);
         }
      }
   }
}

The ReadLine.exe project is a very simple one which has one class which looks like so:

namespace ReadLine
{
   internal static class Program
   {
      private static void Main()
      {
         System.Console.WriteLine(System.Console.ReadLine());
      }
   }
}

Answer 2

Example implementation of Eric's post above. This particular example was used to read information that was passed to a console app via pipe:

 using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;

namespace PipedInfo
{
    class Program
    {
        static void Main(string[] args)
        {
            StreamReader buffer = ReadPipedInfo();

            Console.WriteLine(buffer.ReadToEnd());
        }

        #region ReadPipedInfo
        public static StreamReader ReadPipedInfo()
        {
            //call with a default value of 5 milliseconds
            return ReadPipedInfo(5);
        }

        public static StreamReader ReadPipedInfo(int waitTimeInMilliseconds)
        {
            //allocate the class we're going to callback to
            ReadPipedInfoCallback callbackClass = new ReadPipedInfoCallback();

            //to indicate read complete or timeout
            AutoResetEvent readCompleteEvent = new AutoResetEvent(false);

            //open the StdIn so that we can read against it asynchronously
            Stream stdIn = Console.OpenStandardInput();

            //allocate a one-byte buffer, we're going to read off the stream one byte at a time
            byte[] singleByteBuffer = new byte[1];

            //allocate a list of an arbitary size to store the read bytes
            List<byte> byteStorage = new List<byte>(4096);

            IAsyncResult asyncRead = null;
            int readLength = 0; //the bytes we have successfully read

            do
            {
                //perform the read and wait until it finishes, unless it's already finished
                asyncRead = stdIn.BeginRead(singleByteBuffer, 0, singleByteBuffer.Length, new AsyncCallback(callbackClass.ReadCallback), readCompleteEvent);
                if (!asyncRead.CompletedSynchronously)
                    readCompleteEvent.WaitOne(waitTimeInMilliseconds);

                //end the async call, one way or another

                //if our read succeeded we store the byte we read
                if (asyncRead.IsCompleted)
                {
                    readLength = stdIn.EndRead(asyncRead);
                    if (readLength > 0)
                        byteStorage.Add(singleByteBuffer[0]);
                }

            } while (asyncRead.IsCompleted && readLength > 0);
            //we keep reading until we fail or read nothing

            //return results, if we read zero bytes the buffer will return empty
            return new StreamReader(new MemoryStream(byteStorage.ToArray(), 0, byteStorage.Count));
        }

        private class ReadPipedInfoCallback
        {
            public void ReadCallback(IAsyncResult asyncResult)
            {
                //pull the user-defined variable and strobe the event, the read finished successfully
                AutoResetEvent readCompleteEvent = asyncResult.AsyncState as AutoResetEvent;
                readCompleteEvent.Set();
            }
        }
        #endregion ReadPipedInfo
    }
}

Answer 3

Another cheap way to get a 2nd thread is to wrap it in a delegate.

Answer 4

One way or another you do need a second thread. You could use asynchronous IO to avoid declaring your own:

• declare a ManualResetEvent, call it "evt"
• call System.Console.OpenStandardInput to get the input stream. Specify a callback method that will store its data and set evt.
• call that stream's BeginRead method to start an asynchronous read operation
• then enter a timed wait on a ManualResetEvent
• if the wait times out, then cancel the read

If the read returns data, set the event and your main thread will continue, otherwise you'll continue after the timeout.

Answer 5

Calling Console.ReadLine() in the delegate is bad because if the user doesn't hit 'enter' then that call will never return. The thread executing the delegate will be blocked until the user hits 'enter', with no way to cancel it.

Issuing a sequence of these calls will not behave as you would expect. Consider the following (using the example Console class from above):

System.Console.WriteLine("Enter your first name [John]:");

string firstName = Console.ReadLine(5, "John");

System.Console.WriteLine("Enter your last name [Doe]:");

string lastName = Console.ReadLine(5, "Doe");

The user lets the timeout expire for the first prompt, then enters a value for the second prompt. Both firstName and lastName will contain the default values. When the user hits 'enter', the first ReadLine call will complete, but the code has abandonded that call and essentially discarded the result. The second ReadLine call will continue to block, the timeout will eventually expire and the value returned will again be the default.

BTW- There is a bug in the code above. By calling waitHandle.Close() you close the event out from under the worker thread. If the user hits 'enter' after the timeout expires, the worker thread will attempt to signal the event which throws an ObjectDisposedException. The exception is thrown from the worker thread, and if you haven't setup an unhandled exception handler your process will terminate.

Answer 6

I think you will need to make a secondary thread and poll for a key on the console. I know of no built in way to accomplish this.