How to Interrupt Console.Readline

How to interrupt Console.ReadLine

UPDATE: this technique is no longer reliable on Windows 10. Don't use it please.
Fairly heavy implementation changes in Win10 to make a console act more like a terminal. No doubt to assist in the new Linux sub-system. One (unintended?) side-effect is that CloseHandle() deadlocks until a read is completed, killing this approach dead. I'll leave the original post in place, only because it might help somebody to find an alternative.

UPDATE2: Look at wischi's answer for a decent alternative.

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It's possible, you have to jerk the floor mat by closing the stdin stream. This program demonstrates the idea:

using System;
using System.Threading;
using System.Runtime.InteropServices;

namespace ConsoleApplication2 {
    class Program {
        static void Main(string[] args) {
            ThreadPool.QueueUserWorkItem((o) => {
                Thread.Sleep(1000);
                IntPtr stdin = GetStdHandle(StdHandle.Stdin);
                CloseHandle(stdin);
            });
            Console.ReadLine();
        }

        // P/Invoke:
        private enum StdHandle { Stdin = -10, Stdout = -11, Stderr = -12 };
        [DllImport("kernel32.dll")]
        private static extern IntPtr GetStdHandle(StdHandle std);
        [DllImport("kernel32.dll")]
        private static extern bool CloseHandle(IntPtr hdl);
    }
}

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

I'm surprised to learn that after 5 years, all of the answers still suffer from one or more of the following problems:

• A function other than ReadLine is used, causing loss of functionality. (Delete/backspace/up-key for previous input).
• Function behaves badly when invoked multiple times (spawning multiple threads, many hanging ReadLine's, or otherwise unexpected behavior).
• Function relies on a busy-wait. Which is a horrible waste since the wait is expected to run anywhere from a number of seconds up to the timeout, which might be multiple minutes. A busy-wait which runs for such an ammount of time is a horrible suck of resources, which is especially bad in a multithreading scenario. If the busy-wait is modified with a sleep this has a negative effect on responsiveness, although I admit that this is probably not a huge problem.

I believe my solution will solve the original problem without suffering from any of the above problems:

class Reader {
  private static Thread inputThread;
  private static AutoResetEvent getInput, gotInput;
  private static string input;

  static Reader() {
    getInput = new AutoResetEvent(false);
    gotInput = new AutoResetEvent(false);
    inputThread = new Thread(reader);
    inputThread.IsBackground = true;
    inputThread.Start();
  }

  private static void reader() {
    while (true) {
      getInput.WaitOne();
      input = Console.ReadLine();
      gotInput.Set();
    }
  }

  // omit the parameter to read a line without a timeout
  public static string ReadLine(int timeOutMillisecs = Timeout.Infinite) {
    getInput.Set();
    bool success = gotInput.WaitOne(timeOutMillisecs);
    if (success)
      return input;
    else
      throw new TimeoutException("User did not provide input within the timelimit.");
  }
}

Calling is, of course, very easy:

try {
  Console.WriteLine("Please enter your name within the next 5 seconds.");
  string name = Reader.ReadLine(5000);
  Console.WriteLine("Hello, {0}!", name);
} catch (TimeoutException) {
  Console.WriteLine("Sorry, you waited too long.");
}

Alternatively, you can use the TryXX(out) convention, as shmueli suggested:

  public static bool TryReadLine(out string line, int timeOutMillisecs = Timeout.Infinite) {
    getInput.Set();
    bool success = gotInput.WaitOne(timeOutMillisecs);
    if (success)
      line = input;
    else
      line = null;
    return success;
  }

Which is called as follows:

Console.WriteLine("Please enter your name within the next 5 seconds.");
string name;
bool success = Reader.TryReadLine(out name, 5000);
if (!success)
  Console.WriteLine("Sorry, you waited too long.");
else
  Console.WriteLine("Hello, {0}!", name);

In both cases, you cannot mix calls to Reader with normal Console.ReadLine calls: if the Reader times out, there will be a hanging ReadLine call. Instead, if you want to have a normal (non-timed) ReadLine call, just use the Reader and omit the timeout, so that it defaults to an infinite timeout.

So how about those problems of the other solutions I mentioned?

• As you can see, ReadLine is used, avoiding the first problem.
• The function behaves properly when invoked multiple times. Regardless of whether a timeout occurs or not, only one background thread will ever be running and only at most one call to ReadLine will ever be active. Calling the function will always result in the latest input, or in a timeout, and the user won't have to hit enter more than once to submit his input.
• And, obviously, the function does not rely on a busy-wait. Instead it uses proper multithreading techniques to prevent wasting resources.

The only problem that I foresee with this solution is that it is not thread-safe. However, multiple threads can't really ask the user for input at the same time, so synchronization should be happening before making a call to Reader.ReadLine anyway.

How to abort another thread in .NET, when said thread is executing Console.ReadLine?

Use SendKeys and simulate ENTER key. Get help here.

Good luck.

How do I stop my C# console program from stopping when executing console.readline(); twice

Console applications close when they get to the end of Main. It's exiting after the Console.ReadLine in onCommandLineReturn();.

Add a bool variable called keepLooping, set it to true, and wrap your code in a while(keepLooping) statement. Somewhere in your program flow, check for input like "quit" or "exit" and set the keepLooping variable to false.

Here's an example of it in a dotnetfiddle: https://dotnetfiddle.net/Jguj5k

Listen on ESC while reading Console line

You will probably have to forego the use of ReadLine and roll your own using ReadKey:

static void Main(string[] args)
{
    Console.Clear();
    Console.Write("Enter your name and press ENTER.  (ESC to cancel): ");
    string name = readLineWithCancel();

    Console.WriteLine("\r\n{0}", name == null ? "Cancelled" : name);

    Console.ReadLine();
}

//Returns null if ESC key pressed during input.
private static string readLineWithCancel()
{
    string result = null;

    StringBuilder buffer = new StringBuilder();

    //The key is read passing true for the intercept argument to prevent
    //any characters from displaying when the Escape key is pressed.
    ConsoleKeyInfo info = Console.ReadKey(true);
    while (info.Key != ConsoleKey.Enter && info.Key != ConsoleKey.Escape)
    {
        Console.Write(info.KeyChar);
        buffer.Append(info.KeyChar);
        info = Console.ReadKey(true);
    } 

    if (info.Key == ConsoleKey.Enter)
    {
        result = buffer.ToString();
    }

    return result;
}

This code is not complete and may require work to make it robust, but it should give you some ideas.

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