Array of WaitHandles from process handles

[Mark Xa]

Hi,

I am trying to create an array of WaitHandles that contain process handles returned from Process.GetProcesses. The code compiles but at runtime I get a System.NullReferenceException - "Object reference not set to an instance of an object." at the statement whandles[i].SafeWaitHandle = swh;

The code is as follows:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;
using System.Threading;
using Microsoft.Win32.SafeHandles;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Process[] processlist = Process.GetProcesses();
            List<Process> closeProcesses = new List<Process>();

            foreach (Process p in processlist)
            {
                if (p.ProcessName == "FApp1" || p.ProcessName == "Fapp2" || p.ProcessName == "FApp3")
                {
                    closeProcesses.Add(p);
                }
            }

            int processesOpen = closeProcesses.Count;
            WaitHandle[] whandles = new WaitHandle[processesOpen];

            for (int i = 0; i < processlist.Length; ++i)
            {
                SafeWaitHandle swh = new SafeWaitHandle(closeProcesses[i].Handle, false);
                whandles[i].SafeWaitHandle = swh;
            }

            WaitHandle.WaitAll(whandles);
            System.Console.WriteLine("All closed!");
        }
    }
}

What's wrong?

[wildfrog]

            for (int i = 0; i < processlist.Length; ++i)

What you probably meant was:

            for (int i = 0; i < closeProcesses.Length; ++i)

Or better yet, use a for each statement.

- petter

[Mark Xa]

Thanks for your reply. Yes, that was definitely a mistake, but it doesn't solve the problem. Exactly the same runtime error occurs.

[wildfrog]

Ahh. Another snag. Your array of WaitHandles is initialized, but the individual elements are not. So when you try to access the whandles[i].SafeWaitHandle property you'll get an NullReferenceException.

To solve this you'll have to instantiate the WaitHandle. Only problem is that WaitHandle has protected access to its constructor. A workaround is to create an instance of a ManualResetEvent or similar (don't knnow of a better approach):

SafeWaitHandle swh = new SafeWaitHandle(closeProcesses[i].Handle, false);
whandles[i] = new AutoResetEvent(false); // first create a 'normal' wait handle
whandles[i].SafeWaitHandle = swh; // then assign our safe wait handle. This should release the original handle.

You should also make sure that the final array holds atleast 1 element, or your call to WaitAll(...) will fail.

- petter

[BillInPA]

Thanks for this - I was needing to detect exit of unmanaged base process from c++/cli shim
This is what i came up with, your last post was the hint i needed.

//Get a wait mechanism for the base process
m_eventArray[0] = new ManualResetEvent(false);
m_eventArray[0].SafeWaitHandle = new SafeWaitHandle(System.Diagnostics.Process.GetCurrentProcess().Handle, false);