Parent process and child process relation

Question

I have parent process that opens child process . I need to perform some functionality only when the parent process is no more running.

What is the best way to know that the parent process is not running ? Because it can be terminated violently then I don't want to make some functionality that will send signal to my child process on the closing event.

Or just looking for my parent process like that:

In the parent make this and pass it to the child Process.GetCurrentProcess().Id And in the child every several milliseconds check this one

Process localById = Process.GetProcessById(1234);

Any ideas ? Recommendations ..

Answer 1

Here is a simple example how to use Process.WaitForExit to check for a parent process whose id has been passed on the command line:

using System;
using System.Diagnostics;
using System.Threading;

class Program
{
    static AutoResetEvent _autoResetEvent;

    static void Main(string[] args)
    {
        int parentProcessId = int.Parse(args[0]);

        _autoResetEvent = new AutoResetEvent(false);

        WaitCallback callback = delegate(object processId) { CheckProcess((int)processId); };
        ThreadPool.QueueUserWorkItem(callback, parentProcessId);

        _autoResetEvent.WaitOne();
    }

    static void CheckProcess(int processId)
    {
        try
        {
            Process process = Process.GetProcessById(processId);
            process.WaitForExit();
            Console.WriteLine("Process [{0}] exited.", processId);
        }
        catch (ArgumentException)
        {
            Console.WriteLine("Process [{0}] not running.", processId);
        }

        _autoResetEvent.Set();
    }
}

Using the Process.Exited event could be done like this:

using System;
using System.Diagnostics;
using System.Threading;

class Program
{
    static AutoResetEvent _autoResetEvent;

    static void Main(string[] args)
    {
        int parentProcessId = int.Parse(args[0]);
        Process process = Process.GetProcessById(parentProcessId);
        process.EnableRaisingEvents = true;
        process.Exited += new EventHandler(process_Exited);

        _autoResetEvent = new AutoResetEvent(false);
        _autoResetEvent.WaitOne();
    }

    static void process_Exited(object sender, EventArgs e)
    {
        Console.WriteLine("Process exit event triggered.");
        _autoResetEvent.Set();
    }
}

Note that in both samples the purpose of the AutoResetEvent is solely to prevent your main thread from exiting. In a Windows Forms application you would not need to use it as your program will be in a message loop and only exit if you close it.

Answer 2

The underlying Win32 process handle is waitable, and will be signalled when the process exits.

In native code:

DWORD res = WaitForSIngleObject(hProcess, INFINITE);
if (res == WAIT_OBJECT_0) {
  // process has exited.
}

In native code, you'll need to either create a custom subtype of WaitHandle for process handles, or use P/Invoke. The disadvantage of P/Invoke is that it is harder to combine (using WaitForMultipleObjects multiple waits, so you are not dedicating a thread just to wait on one thing).

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Thanks to 0xA3: Just use Process.WaitForExit (there is an overload with a timeout to avoid indefinite waits, and don't do this on your UI thread).