Why does this short program never complete?

Question

Through debugging a problem of my own, I have managed to recreate a tiny program which behaves very unusually:

using System;
using System.Threading;

namespace CancelBug
{
    class Program
    {
        static void Main(string[] args)
        {
            var unused = new ManualResetEvent(false);
            var cancelled = new ManualResetEvent(false);
            Console.CancelKeyPress += (s, e) => cancelled.Set();
            Console.WriteLine("Running. The only thing to do now is ctrl+c or close the window...");
            WaitHandle.WaitAny(new[] { unused, cancelled });
            Console.WriteLine("Press enter to continue...");
            Console.Read();
        }
    }
}

I would expect this program to:

• display the first line
• wait until the user tries to exit the program
• display the second line
• wait until the user presses enter
• exit

However, once this makes it past the call to WaitHandle.WaitAny , it seems to hang on random lines. Sometimes the last line will never be printed, sometimes it will be printed but the enter key is never read. With a larger code base, it can execute more lines of code and still hang at a seemingly random position.

Can anyone explain this strange behaviour?

Answer 1

You need to cancel the CTRL+C command or else your process will be terminated:

Console.CancelKeyPress += (s, e) =>
{
    e.Cancel = true;
    cancelled.Set();
};

From https://msdn.microsoft.com/en-us/library/system.consolecanceleventargs(v=vs.110).aspx :

If the Cancel property is set to true in the event handler, the process is resumed; otherwise, the process is terminated. By default, the value of the ConsoleCancelEventArgs property is false, and the process terminates.

Answer 2

Ctrl + C is global command to close the command window. So, This key combination will close the window before the actual program ends. Try using other key.

Answer 3

Please run the application without the debugger (directly from the command line).

Here's my test app that behaves as you expect, according to my test.

        var cancelled = new ManualResetEvent(false);
        Console.CancelKeyPress += (s, e) =>
        {
            e.Cancel = true;
            Console.WriteLine("Ctrl+C detected...");
            cancelled.Set();
        };
        Console.WriteLine("Running. The only thing to do now is ctrl+c or close the window...");
        WaitHandle.WaitAny(new[] { cancelled });
        Console.WriteLine("Press enter to exit...");
        Console.ReadLine();