Array of WaitHandles from process handles

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:

Code:

---

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?

Code:

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

---

What you probably meant was:

Code:

---

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

---

Or better yet, use a for each statement.

- petter

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

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):

Code:

---

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

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);