How to interrupt Console.ReadLine

Question

Is it possible to stop the Console.ReadLine() programmatically?

I have a console application: the much of the logic runs on a different thread and in the main thread I accept input using Console.ReadLine(). I'd like to stop reading from console when the separated thread stop running.

How can I achieve this?

Answer 1

I know this question is old and predates .NET Core, but I thought it would be useful to add a more modern approach.

I've tested this approach in .NET 6. I've created a simple async ReadLine method that takes a cancellation token which can be used to interrupt it.

The key is to wrap Console.ReadLine() in a task. Obviously, the Console.ReadLine() call cannot be interrupted, so a Task.WhenAny is used in combination with a Task.Delay to get the cancellation token working.

In order to not lose any input, the read task is kept outside the method, so it can be awaited in the next call if the operation is cancelled.

Task<string?>? readTask = null;

async Task<string?> ReadLineAsync(CancellationToken cancellationToken = default)
{
    readTask ??= Task.Run(() => Console.ReadLine());

    await Task.WhenAny(readTask, Task.Delay(-1, cancellationToken));

    cancellationToken.ThrowIfCancellationRequested();

    string? result = await readTask;
    readTask = null;

    return result;
}

Answer 2

So here is a Solution working on Windows 10 and not using any fancy threading or dllimport magic. It worked fine for me I hope it helps.

I basically create a stream reader sitting on the standard input. Reading it kinda "async" and just dispose the stream reader if I want to cancel the readline.

Here is my Code:

    private System.IO.StreamReader stdinsr = new System.IO.StreamReader(Console.OpenStandardInput());
    [DebuggerHidden]
    private string ReadLine() {
        return stdinsr.ReadLineAsync().Result;
    }

    protected override void OnExit(ExitEventArgs e) {
        base.OnExit(e);

        commandLooper.Abort();
        stdinsr.Dispose();
    }

NOTE: Yes I read async but I wait for the task result so its basically still waiting for user input.

Answer 3

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.

---

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);
    }
}

Answer 4

Disclaimer: This is just a copy & paste answer.

Thanks to Gérald Barré for providing such a great solution:
https://www.meziantou.net/cancelling-console-read.htm

Documentation for CancelIoEX:
https://learn.microsoft.com/en-us/windows/win32/fileio/cancelioex-func

I tested it on Windows 10. It works great and is way less "hacky" than the other solutions (like reimplementing Console.ReadLine, sending return via PostMessage or closing the handle as in the accepted answer)

In case the site goes down I cite the code snippet here:

class Program
{
    const int STD_INPUT_HANDLE = -10;

    [DllImport("kernel32.dll", SetLastError = true)]
    internal static extern IntPtr GetStdHandle(int nStdHandle);

    [DllImport("kernel32.dll", SetLastError = true)]
    static extern bool CancelIoEx(IntPtr handle, IntPtr lpOverlapped);

    static void Main(string[] args)
    {
        // Start the timeout
        var read = false;
        Task.Delay(10000).ContinueWith(_ =>
        {
            if (!read)
            {
                // Timeout => cancel the console read
                var handle = GetStdHandle(STD_INPUT_HANDLE);
                CancelIoEx(handle, IntPtr.Zero);
            }
        });

        try
        {
            // Start reading from the console
            Console.WriteLine("Do you want to continue [Y/n] (10 seconds remaining):");
            var key = Console.ReadKey();
            read = true;
            Console.WriteLine("Key read");
        }
        // Handle the exception when the operation is canceled
        catch (InvalidOperationException)
        {
            Console.WriteLine("Operation canceled");
        }
        catch (OperationCanceledException)
        {
            Console.WriteLine("Operation canceled");
        }
    }
}

Answer 5

I just stumbled upon this little library on GitHub: https://github.com/tonerdo/readline

ReadLine is a GNU Readline like library built in pure C#. It can serve as a drop in replacement for the inbuilt Console.ReadLine() and brings along with it some of the terminal goodness you get from unix shells, like command history navigation and tab auto completion.

It is cross platform and runs anywhere .NET is supported, targeting netstandard1.3 means that it can be used with .NET Core as well as the full .NET Framework.

Although this library doesn't support interrupting input at the time of writing, it should be trivial to update it to do so. Alternatively, it can be an interesting example of writing a custom solution to counter the limitations of Console.ReadLine.

Answer 6

This will process a Ctrl+C in a seperate thread while your app is waiting for a Console.Readline():

Console.CancelKeyPress += (_, e) =>
{
    e.Cancel = true;
    Environment.Exit(0);
};

Answer 7

I was also looking for a way to stop reading from console in certain conditions. the solution i came up with was to make a non blocking version of read line with these two methods.

static IEnumerator<Task<string>> AsyncConsoleInput()
{
    var e = loop(); e.MoveNext(); return e;
    IEnumerator<Task<string>> loop()
    {
        while (true) yield return Task.Run(() => Console.ReadLine());
    }
}

static Task<string> ReadLine(this IEnumerator<Task<string>> console)
{
    if (console.Current.IsCompleted) console.MoveNext();
    return console.Current;
}

this allows us to have ReadLine on separate thread and we can wait for it or use it in other places conditionally.

var console = AsyncConsoleInput();

var task = Task.Run(() =>
{
     // your task on separate thread
});

if (Task.WaitAny(console.ReadLine(), task) == 0) // if ReadLine finished first
{
    task.Wait();
    var x = console.Current.Result; // last user input (await instead of Result in async method)
}
else // task finished first 
{
    var x = console.ReadLine(); // this wont issue another read line because user did not input anything yet. 
}

Answer 8

The current accepted answer don't work any longer so I decided to create a new one. The only safe way to do this is to create your own ReadLine method I can think of many many scenarios requiring such functionality and the code here implements one of them:

public static string CancellableReadLine(CancellationToken cancellationToken)
{
    StringBuilder stringBuilder = new StringBuilder();
    Task.Run(() =>
    {
        try
        {
            ConsoleKeyInfo keyInfo;
            var startingLeft = Con.CursorLeft;
            var startingTop = Con.CursorTop;
            var currentIndex = 0;
            do
            {
                var previousLeft = Con.CursorLeft;
                var previousTop = Con.CursorTop;
                while (!Con.KeyAvailable)
                {
                    cancellationToken.ThrowIfCancellationRequested();
                    Thread.Sleep(50);
                }
                keyInfo = Con.ReadKey();
                switch (keyInfo.Key)
                {
                    case ConsoleKey.A:
                    case ConsoleKey.B:
                    case ConsoleKey.C:
                    case ConsoleKey.D:
                    case ConsoleKey.E:
                    case ConsoleKey.F:
                    case ConsoleKey.G:
                    case ConsoleKey.H:
                    case ConsoleKey.I:
                    case ConsoleKey.J:
                    case ConsoleKey.K:
                    case ConsoleKey.L:
                    case ConsoleKey.M:
                    case ConsoleKey.N:
                    case ConsoleKey.O:
                    case ConsoleKey.P:
                    case ConsoleKey.Q:
                    case ConsoleKey.R:
                    case ConsoleKey.S:
                    case ConsoleKey.T:
                    case ConsoleKey.U:
                    case ConsoleKey.V:
                    case ConsoleKey.W:
                    case ConsoleKey.X:
                    case ConsoleKey.Y:
                    case ConsoleKey.Z:
                    case ConsoleKey.Spacebar:
                    case ConsoleKey.Decimal:
                    case ConsoleKey.Add:
                    case ConsoleKey.Subtract:
                    case ConsoleKey.Multiply:
                    case ConsoleKey.Divide:
                    case ConsoleKey.D0:
                    case ConsoleKey.D1:
                    case ConsoleKey.D2:
                    case ConsoleKey.D3:
                    case ConsoleKey.D4:
                    case ConsoleKey.D5:
                    case ConsoleKey.D6:
                    case ConsoleKey.D7:
                    case ConsoleKey.D8:
                    case ConsoleKey.D9:
                    case ConsoleKey.NumPad0:
                    case ConsoleKey.NumPad1:
                    case ConsoleKey.NumPad2:
                    case ConsoleKey.NumPad3:
                    case ConsoleKey.NumPad4:
                    case ConsoleKey.NumPad5:
                    case ConsoleKey.NumPad6:
                    case ConsoleKey.NumPad7:
                    case ConsoleKey.NumPad8:
                    case ConsoleKey.NumPad9:
                    case ConsoleKey.Oem1:
                    case ConsoleKey.Oem102:
                    case ConsoleKey.Oem2:
                    case ConsoleKey.Oem3:
                    case ConsoleKey.Oem4:
                    case ConsoleKey.Oem5:
                    case ConsoleKey.Oem6:
                    case ConsoleKey.Oem7:
                    case ConsoleKey.Oem8:
                    case ConsoleKey.OemComma:
                    case ConsoleKey.OemMinus:
                    case ConsoleKey.OemPeriod:
                    case ConsoleKey.OemPlus:
                        stringBuilder.Insert(currentIndex, keyInfo.KeyChar);
                        currentIndex++;
                        if (currentIndex < stringBuilder.Length)
                        {
                            var left = Con.CursorLeft;
                            var top = Con.CursorTop;
                            Con.Write(stringBuilder.ToString().Substring(currentIndex));
                            Con.SetCursorPosition(left, top);
                        }
                        break;
                    case ConsoleKey.Backspace:
                        if (currentIndex > 0)
                        {
                            currentIndex--;
                            stringBuilder.Remove(currentIndex, 1);
                            var left = Con.CursorLeft;
                            var top = Con.CursorTop;
                            if (left == previousLeft)
                            {
                                left = Con.BufferWidth - 1;
                                top--;
                                Con.SetCursorPosition(left, top);
                            }
                            Con.Write(stringBuilder.ToString().Substring(currentIndex) + " ");
                            Con.SetCursorPosition(left, top);
                        }
                        else
                        {
                            Con.SetCursorPosition(startingLeft, startingTop);
                        }
                        break;
                    case ConsoleKey.Delete:
                        if (stringBuilder.Length > currentIndex)
                        {
                            stringBuilder.Remove(currentIndex, 1);
                            Con.SetCursorPosition(previousLeft, previousTop);
                            Con.Write(stringBuilder.ToString().Substring(currentIndex) + " ");
                            Con.SetCursorPosition(previousLeft, previousTop);
                        }
                        else
                            Con.SetCursorPosition(previousLeft, previousTop);
                        break;
                    case ConsoleKey.LeftArrow:
                        if (currentIndex > 0)
                        {
                            currentIndex--;
                            var left = Con.CursorLeft - 2;
                            var top = Con.CursorTop;
                            if (left < 0)
                            {
                                left = Con.BufferWidth + left;
                                top--;
                            }
                            Con.SetCursorPosition(left, top);
                            if (currentIndex < stringBuilder.Length - 1)
                            {
                                Con.Write(stringBuilder[currentIndex].ToString() + stringBuilder[currentIndex + 1]);
                                Con.SetCursorPosition(left, top);
                            }
                        }
                        else
                        {
                            Con.SetCursorPosition(startingLeft, startingTop);
                            if (stringBuilder.Length > 0)
                                Con.Write(stringBuilder[0]);
                            Con.SetCursorPosition(startingLeft, startingTop);
                        }
                        break;
                    case ConsoleKey.RightArrow:
                        if (currentIndex < stringBuilder.Length)
                        {
                            Con.SetCursorPosition(previousLeft, previousTop);
                            Con.Write(stringBuilder[currentIndex]);
                            currentIndex++;
                        }
                        else
                        {
                            Con.SetCursorPosition(previousLeft, previousTop);
                        }
                        break;
                    case ConsoleKey.Home:
                        if (stringBuilder.Length > 0 && currentIndex != stringBuilder.Length)
                        {
                            Con.SetCursorPosition(previousLeft, previousTop);
                            Con.Write(stringBuilder[currentIndex]);
                        }
                        Con.SetCursorPosition(startingLeft, startingTop);
                        currentIndex = 0;
                        break;
                    case ConsoleKey.End:
                        if (currentIndex < stringBuilder.Length)
                        {
                            Con.SetCursorPosition(previousLeft, previousTop);
                            Con.Write(stringBuilder[currentIndex]);
                            var left = previousLeft + stringBuilder.Length - currentIndex;
                            var top = previousTop;
                            while (left > Con.BufferWidth)
                            {
                                left -= Con.BufferWidth;
                                top++;
                            }
                            currentIndex = stringBuilder.Length;
                            Con.SetCursorPosition(left, top);
                        }
                        else
                            Con.SetCursorPosition(previousLeft, previousTop);
                        break;
                    default:
                        Con.SetCursorPosition(previousLeft, previousTop);
                        break;
                }
            } while (keyInfo.Key != ConsoleKey.Enter);
            Con.WriteLine();
        }
        catch
        {
            //MARK: Change this based on your need. See description below.
            stringBuilder.Clear();
        }
    }).Wait();
    return stringBuilder.ToString();
}

Place this function somewhere in your code and this gives you a function that can be cancelled via a CancellationToken also for better code I have used

using Con = System.Console;

This function returns an empty string upon cancellation (which was good for my case) you can throw an exception inside the marked catch expression above if you wish.

Also in the same catch expression you can remove the stringBuilder.Clear(); line and that will cause the code to return what user entered so far. Combine this with a successful or canceled flag and you can keep what used entered so far and use it in further requests.

Other thing you can change is that you can set a timeout besides the cancellation token in the loop if you want to get a timeout functionality.

I tried to be as clean as I need but this code can be cleaner. The method can become async itself and timeout and cancellation token passed in.

Answer 9

This is a modified version of Contango's answer. Instead of using the current process's MainWindowhandle, this code uses GetForegroundWindow() to get the Console's MainWindowHandle if launched from cmd.

using System;
using System.Runtime.InteropServices;

public class Temp
{
    //Just need this
    //==============================
    static IntPtr ConsoleWindowHnd = GetForegroundWindow();
    [DllImport("user32.dll")]
    static extern IntPtr GetForegroundWindow();
    [DllImport("User32.Dll")]
    private static extern bool PostMessage(IntPtr hWnd, uint msg, int wParam, int lParam);
    const int VK_RETURN = 0x0D;
    const int WM_KEYDOWN = 0x100;
    //==============================

    public static void Main(string[] args)
    {
        System.Threading.Tasks.Task.Run(() =>
        {
            System.Threading.Thread.Sleep(2000);

            //And use like this
            //===================================================
            PostMessage(ConsoleWindowHnd, WM_KEYDOWN, VK_RETURN, 0);
            //===================================================

        });
        Console.WriteLine("Waiting");
        Console.ReadLine();
        Console.WriteLine("Waiting Done");
        Console.Write("Press any key to continue . . .");
        Console.ReadKey();
    }
}

---

Optional

Check to see if the foreground window was cmd. If it wasn't, then the current process should launch the console window so go ahead and use that. This shouldn't matter because the foreground window should be the current process window anyway, but this helps you feel good about it by double checking.

    int id;
    GetWindowThreadProcessId(ConsoleWindowHnd, out id);
    if (System.Diagnostics.Process.GetProcessById(id).ProcessName != "cmd")
    {
        ConsoleWindowHnd = System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle;
    }

Answer 10

Send [enter] to the currently running console app:

    class Program
    {
        [DllImport("User32.Dll", EntryPoint = "PostMessageA")]
        private static extern bool PostMessage(IntPtr hWnd, uint msg, int wParam, int lParam);

        const int VK_RETURN = 0x0D;
        const int WM_KEYDOWN = 0x100;

        static void Main(string[] args)
        {
            Console.Write("Switch focus to another window now.\n");

            ThreadPool.QueueUserWorkItem((o) =>
            {
                Thread.Sleep(4000);

                var hWnd = System.Diagnostics.Process.GetCurrentProcess().MainWindowHandle;
                PostMessage(hWnd, WM_KEYDOWN, VK_RETURN, 0);
            });

            Console.ReadLine();

            Console.Write("ReadLine() successfully aborted by background thread.\n");
            Console.Write("[any key to exit]");
            Console.ReadKey();
        }
    }

This code sends [enter] into the current console process, aborting any ReadLine() calls blocking in unmanaged code deep within the windows kernel, which allows the C# thread to exit naturally.

I used this code instead of the answer that involves closing the console, because closing the console means that ReadLine() and ReadKey() are permanently disabled from that point on in the code (it will throw an exception if its used).

This answer is superior to all solutions that involve SendKeys and Windows Input Simulator, as it works even if the current app does not have the focus.

Answer 11

I needed a solution that would work with Mono, so no API calls. I'm posting this just encase anyone else is in the same situation, or wants a pure C# way of doing this. The CreateKeyInfoFromInt() function is the tricky part (some keys are more than one byte in length). In the code below, ReadKey() throws an exception if ReadKeyReset() is called from another thread. The code below is not entirely complete, but it does demonstrate the concept of using existing Console C# functions to create an interuptable GetKey() function.

static ManualResetEvent resetEvent = new ManualResetEvent(true);

/// <summary>
/// Resets the ReadKey function from another thread.
/// </summary>
public static void ReadKeyReset()
{
    resetEvent.Set();
}

/// <summary>
/// Reads a key from stdin
/// </summary>
/// <returns>The ConsoleKeyInfo for the pressed key.</returns>
/// <param name='intercept'>Intercept the key</param>
public static ConsoleKeyInfo ReadKey(bool intercept = false)
{
    resetEvent.Reset();
    while (!Console.KeyAvailable)
    {
        if (resetEvent.WaitOne(50))
            throw new GetKeyInteruptedException();
    }
    int x = CursorX, y = CursorY;
    ConsoleKeyInfo result = CreateKeyInfoFromInt(Console.In.Read(), false);
    if (intercept)
    {
        // Not really an intercept, but it works with mono at least
        if (result.Key != ConsoleKey.Backspace)
        {
            Write(x, y, " ");
            SetCursorPosition(x, y);
        }
        else
        {
            if ((x == 0) && (y > 0))
            {
                y--;
                x = WindowWidth - 1;
            }
            SetCursorPosition(x, y);
        }
    }
    return result;
}