How to Trap Ctrl+C (Sigint) in a C# Console App

How do I trap Ctrl+C (SIGINT) in a C# console app?

See MSDN:

Console.CancelKeyPress Event

Article with code samples:

Ctrl-C and the .NET console application

Catching ctrl+c event in console application (multi-threaded)

Based on your question there are two events you need to catch.

• First there is the console close event which is explained here: "On Exit" for a Console Application
• Second you want to catch control c which is explained here: How do I trap ctrl-c in a C# console app

If you put these two together with your example you get something like this:

static ConsoleEventDelegate handler;
private delegate bool ConsoleEventDelegate(int eventType);
[DllImport("kernel32.dll", SetLastError = true)]
private static extern bool SetConsoleCtrlHandler(ConsoleEventDelegate callback, bool add);

private static MyExternalProcess p1;

public static void Main()
{
    Console.CancelKeyPress += delegate
    {
        killEveryoneOnExit();
    };

    handler = new ConsoleEventDelegate(ConsoleEventCallback);
    SetConsoleCtrlHandler(handler, true);

    p1 = new MyExternalProcess();
    p1.startProcess();
}

public static void killEveryoneOnExit()
{
    p1.kill();
}

static bool ConsoleEventCallback(int eventType)
{
    if (eventType == 2)
    {
        killEveryoneOnExit();
    }
    return false;
}

For a working ctrl c (fun intended) paste example: http://pastebin.com/6VV4JKPY

How to avoid Console.Write after Ctrl-C during Console.ReadLine?

As AgentFire mentioned add input validation.

    class Program
    {
        static void Main()
        {
            Console.Write("login: ");
            string userId = Console.ReadLine();

            if (string.IsNullOrWhiteSpace(userId))
            {
                Environment.Exit(-1);
            }
            Console.Write("password: ");
            ....
        }

    }

Unable to trap Ctrl+C in a C# console app

Apparently there is a bug a work around from the Connect page is:

In the meantime, to work around this problem, you can enable mixed-mode debugging. Then, when you hit Ctrl-C and a dialog pops up notifying you of the first chance Ctrl-C exception, click "Continue". You should then hit the breakpoint in your handler.

Need to send CTRL+C (SIGINT) to Process object from main C# WPF app

I'm able to reproduce the second issue you report, i.e. the WPF process exiting when you try to send the signal to a console process. (The first issue, you've explained in your comment that it turned out to be a bug in your watch-dog code restarting the process even when it was signaled explicitly to exit.)

After investigation, it appears to me that this is caused by a race condition between the call to GenerateConsoleCtrlEvent() and the subsequent call to SetConsoleCtrlHandler(). It seems that if those calls occur too quickly, the Ctrl+C that is sent by GenerateConsoleCtrlEvent() remains visible to the default handling in the WPF app, causing the process to exit with the STATUS_CONTROL_C_EXIT code (i.e. the normal result from pressing Ctrl+C, but for the wrong process).

Interestingly, one of the things that stood out to me about the code you're using to send the signal, is that it restores the process state for the console and the signal handling in the same order those states were modified. This seems unusual to me, as one normally restores state in reverse order, to "back out" the state, as it were.

If one changes the code so that the signal handling is restored before freeing the attached console (i.e. the way one might normally write the code), then the problem with the host process receiving the signal and exiting reproduces the first time the method is called. I.e. it seems that the only reason it even works the first time is that there's some delay the first time that the host process calls the FreeConsole() function, that is sufficient to let the signal go unnoticed. The second time through, the delay no longer exists (possibly something got cached in the p/invoke layer…I didn't bother to investigate that part).

After that though, it works just like it would if you restored the state in the expected order.

Anyway…

I was able to reliably fix the issue by not restoring the current process's state until the target process had actually exited. In the proof-of-concept app I had to build in order to reproduce the issue, this was relatively simple, because I'd already implemented a TaskCompletionSource that is set when the Exited event is raised, and so I was able to pass the Task for that source to the StopProcess() method so it could await the Task before restoring the state.

I recommend you fix your code in a similar way. Note that you cannot call WaitForExit() on the Process itself, unless you do so from some thread other than the UI thread, because the Process class uses the UI thread to raise the Exited event, and so blocking the UI thread with a call to WaitForExit() will cause a deadlock. You could avoid that by putting the entire call to StopProcess() in a different thread, but that seems like overkill to me, especially when there's a more elegant way to implement the whole thing.

You can use any mechanism you like to wait for the process to terminate, as long as you take care to not deadlock the UI thread. But here's the code I wrote, in case you want to refer to it…

In the window class (note, completely broken for WPF as there's no MVVM here at all…this was just to get the basic minimal, complete example working):

private Process _process;
private TaskCompletionSource _processTask;

private async void startButton_Click(object sender, RoutedEventArgs e)
{
    startButton.IsEnabled = false;
    stopButton.IsEnabled = true;

    try
    {
        _process = new Process();
        _processTask = new TaskCompletionSource();

        _process.StartInfo.FileName = "tracert.exe";
        _process.StartInfo.Arguments = "google.com";
        _process.StartInfo.UseShellExecute = false;
        _process.StartInfo.CreateNoWindow = true;
        _process.StartInfo.RedirectStandardOutput = true;
        _process.StartInfo.RedirectStandardError = true;
        _process.StartInfo.RedirectStandardInput = true;
        _process.EnableRaisingEvents = true;
        _process.OutputDataReceived += (_, e) => _WriteLine($"stdout: \"{e.Data}\"");
        _process.ErrorDataReceived += (_, e) => _WriteLine($"stderr: \"{e.Data}\"");
        _process.Exited += (_, _) =>
        {
            _WriteLine($"Process exited. Exit code: {_process.ExitCode}");
            _processTask.SetResult();
        };

        _process.Start();
        _process.BeginOutputReadLine();
        _process.BeginErrorReadLine();

        await _processTask.Task;
    }
    finally
    {
        _process?.Dispose();
        _process = null;
        _processTask = null;
        startButton.IsEnabled = true;
        stopButton.IsEnabled = false;
    }
}

private async void stopButton_Click(object sender, RoutedEventArgs e)
{
    try
    {
        await Win32Process.StopProcess(_process, _processTask.Task);
    }
    catch (InvalidOperationException exception)
    {
        _WriteLine(exception.Message);
    }
}

private void _WriteLine(string text)
{
    Dispatcher.Invoke(() => consoleOutput.Text += $"{text}{Environment.NewLine}");
}

Here's the updated version of the StopProcess() method (which I put into its own helper class):

public static async Task StopProcess(Process process, Task processTask)
{
    if (AttachConsole((uint)process.Id))
    {
        // NOTE: each of these functions could fail. Error-handling omitted
        // for clarity. A real-world program should check the result of each
        // call and handle errors appropriately.
        SetConsoleCtrlHandler(null, true);
        GenerateConsoleCtrlEvent(ConsoleCtrlEvent.CTRL_C, 0);
        await processTask;
        SetConsoleCtrlHandler(null, false);
        FreeConsole();
    }
    else
    {
        int hresult = Marshal.GetLastWin32Error();
        Exception e = Marshal.GetExceptionForHR(hresult);

        throw new InvalidOperationException(
            $"ERROR: failed to attach console to process {process.Id}: {e?.Message ?? hresult.ToString()}");
    }
}

You can probably infer what the XAML is — just a couple of buttons and a TextBlock to display messages — but for completeness, here it is anyway:

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition Height="Auto"/>
      <RowDefinition/>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition Width="Auto"/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>

    <Button x:Name="startButton" Grid.Row="0" Grid.Column="0" Content="Start" Click="startButton_Click"/>
    <Button x:Name="stopButton" Grid.Row="0" Grid.Column="1" Content="Ctrl-C" Click="stopButton_Click" IsEnabled="False"/>
    <ScrollViewer Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3">
      <TextBlock x:Name="consoleOutput"/>
    </ScrollViewer>
  </Grid>
</Window>

Console app won't exit when given ctrl + c in debug mode

One way is to achieve this is to use Console.CancelKeyPress

Occurs when the Control modifier key (Ctrl) and either the
ConsoleKey.C console key (C) or the Break key are pressed
simultaneously (Ctrl+C or Ctrl+Break).

When the user presses either Ctrl+C or Ctrl+Break, the CancelKeyPress
event is fired and the application's ConsoleCancelEventHandler event
handler is executed. The event handler is passed a
ConsoleCancelEventArgs object

---

Example

private static bool keepRunning = true;

public static void Main(string[] args)
{
   Console.CancelKeyPress += delegate(object sender, ConsoleCancelEventArgs e) {
         e.Cancel = true;
         keepRunning = false;
      };

   while (keepRunning) 
   {
      // Do stuff
   }
   Console.WriteLine("exited gracefully");
}

How implement exit from console application using ctrl + x?

The CTRL+C and CTRL+BREAK key combinations receive special handling by console processes. By default, when a console window has the keyboard focus, CTRL+C or CTRL+BREAK is treated as a signal (SIGINT or SIGBREAK) and not as keyboard input. By default, these signals are passed to all console processes that are attached to the console. (Detached processes are not affected. See Creation of a Console.) The system creates a new thread in each client process to handle the event. The thread raises an exception if the process is being debugged. The debugger can handle the exception or continue with the exception unhandled.

CTRL+BREAK is always treated as a signal, but an application can change the default CTRL+C behavior in two ways that prevent the handler functions from being called:

• The SetConsoleMode function can disable the ENABLE_PROCESSED_INPUT input mode for a console's input buffer, so CTRL+C is reported as keyboard input rather than as a signal.
• When SetConsoleCtrlHandler is called with NULL and TRUE values for its parameters, the calling process ignores CTRL+C signals. Normal CTRL+C processing is restored by calling SetConsoleCtrlHandler with NULL and FALSE values. This attribute of ignoring or not ignoring CTRL+C signals is inherited by child processes, but it can be enabled or disabled by any process without affecting existing processes.

So we can do some basic things and disable CTRL+C and CTRL+Break first.

Console.TreatControlCAsInput = false;
Console.CancelKeyPress += delegate (object? sender, ConsoleCancelEventArgs e) {
        e.Cancel = true;
 };
Console.WriteLine("Hello, World!");
Console.ReadKey();

The next step is hook up and add ctrl+x

Console.TreatControlCAsInput = false;

    Console.CancelKeyPress += delegate (object? sender, ConsoleCancelEventArgs e) {
        e.Cancel = true;
    };
    
    
    ConsoleKeyInfo cki;
    
    do
    {
        Console.WriteLine("Hello, World!");
        //todo any work here
        cki = Console.ReadKey(true);
    } while (cki.Modifiers != ConsoleModifiers.Control && cki.Key != ConsoleKey.X);

C# prevent ctrl-c to child processes

Apparently you can set Console.TreatCtrlCAsInput = true which will allow you to handle that keystroke, stop the other processes, and then exit yourself. According to the MSDN docs, this property...

Gets or sets a value indicating whether the combination of the Control modifier key and C console key (Ctrl+C) is treated as ordinary input or as an interruption that is handled by the operating system.

---

Related Topics