Following code errors out at the line commented with "---errors is here---". What is wrong?

Question

using System;
using System.Threading;

public class MutexSample
{
    static Mutex gM1;
    static Mutex gM2;
    const int ITERS = 100;
    static AutoResetEvent Event1 = new AutoResetEvent(false);
    static AutoResetEvent Event2 = new AutoResetEvent(false);
    static AutoResetEvent Event3 = new AutoResetEvent(false);
    static AutoResetEvent Event4 = new AutoResetEvent(false);

    public static void Main(String[] args)
    {
        Console.WriteLine("Mutex Sample ...");
        // Create Mutex initialOwned, with name of "MyMutex".
        gM1 = new Mutex(true, "MyMutex");
        // Create Mutex initialOwned, with no name.
        gM2 = new Mutex(true);
        Console.WriteLine(" - Main Owns gM1 and gM2");

        AutoResetEvent[] evs = new AutoResetEvent[4];
        evs[0] = Event1;    // Event for t1
        evs[1] = Event2;    // Event for t2
        evs[2] = Event3;    // Event for t3
        evs[3] = Event4;    // Event for t4

        MutexSample tm = new MutexSample();
        Thread thread1 = new Thread(new ThreadStart(tm.t1Start));
        Thread thread2 = new Thread(new ThreadStart(tm.t2Start));
        Thread thread3 = new Thread(new ThreadStart(tm.t3Start));
        Thread thread4 = new Thread(new ThreadStart(tm.t4Start));
        thread1.Start();   // Does Mutex.WaitAll(Mutex[] of gM1 and gM2)
        thread2.Start();   // Does Mutex.WaitOne(Mutex gM1)
        thread3.Start();   // Does Mutex.WaitAny(Mutex[] of gM1 and gM2)
        thread4.Start();   // Does Mutex.WaitOne(Mutex gM2)

        Thread.Sleep(2000);
        Console.WriteLine(" - Main releases gM1");
        gM1.ReleaseMutex();  // t2 and t3 will end and signal

        Thread.Sleep(1000);
        Console.WriteLine(" - Main releases gM2");
        gM2.ReleaseMutex();  // t1 and t4 will end and signal

        // Waiting until all four threads signal that they are done.
        WaitHandle.WaitAll(evs);
        Console.WriteLine("... Mutex Sample");
    }

    public void t1Start()
    {
        Console.WriteLine("t1Start started,  Mutex.WaitAll(Mutex[])");
        Mutex[] gMs = new Mutex[2];
        gMs[0] = gM1;  // Create and load an array of Mutex for WaitAll call
        gMs[1] = gM2;
        Mutex.WaitAll(gMs);  // Waits until both gM1 and gM2 are released
        Thread.Sleep(2000);
        Console.WriteLine("t1Start finished, Mutex.WaitAll(Mutex[]) satisfied");
        Event1.Set();      // AutoResetEvent.Set() flagging method is done
    }

    public void t2Start()
    {
        Console.WriteLine("t2Start started,  gM1.WaitOne( )");
        gM1.WaitOne();    // Waits until Mutex gM1 is released ---errors is here---
        Console.WriteLine("t2Start finished, gM1.WaitOne( ) satisfied");
        Event2.Set();     // AutoResetEvent.Set() flagging method is done
    }

    public void t3Start()
    {
        Console.WriteLine("t3Start started,  Mutex.WaitAny(Mutex[])");
        Mutex[] gMs = new Mutex[2];
        gMs[0] = gM1;  // Create and load an array of Mutex for WaitAny call
        gMs[1] = gM2;
        Mutex.WaitAny(gMs);  // Waits until either Mutex is released
        Console.WriteLine("t3Start finished, Mutex.WaitAny(Mutex[])");
        Event3.Set();       // AutoResetEvent.Set() flagging method is done
    }

    public void t4Start()
    {
        Console.WriteLine("t4Start started,  gM2.WaitOne( )");
        gM2.WaitOne();   // Waits until Mutex gM2 is released
        Console.WriteLine("t4Start finished, gM2.WaitOne( )");
        Event4.Set();    // AutoResetEvent.Set() flagging method is done
    }
}

Answer 1

After waiting on a Mutex you have to release it, using

Mutex.ReleaseMutex()

before the threads exits.

fixed t1start - t4start

public void t1Start()
{
    Console.WriteLine("t1Start started,  Mutex.WaitAll(Mutex[])");
    Mutex[] gMs = new Mutex[2];
    gMs[0] = gM1;  // Create and load an array of Mutex for WaitAll call 
    gMs[1] = gM2;
    Mutex.WaitAll(gMs);  // Waits until both gM1 and gM2 are released  
    Thread.Sleep(2000);
    Console.WriteLine("t1Start finished, Mutex.WaitAll(Mutex[]) satisfied");
    Event1.Set();      // AutoResetEvent.Set() flagging method is done
    gM1.ReleaseMutex();
    gM2.ReleaseMutex();
}
public void t2Start()
{
    Console.WriteLine("t2Start started,  gM1.WaitOne( )");
    gM1.WaitOne();    // Waits until Mutex gM1 is released ---errors is here---    
    Console.WriteLine("t2Start finished, gM1.WaitOne( ) satisfied");

    gM1.ReleaseMutex();
    Event2.Set();     // AutoResetEvent.Set() flagging method is done

}
public void t3Start()
{
    Console.WriteLine("t3Start started,  Mutex.WaitAny(Mutex[])");
    Mutex[] gMs = new Mutex[2];
    gMs[0] = gM1;  // Create and load an array of Mutex for WaitAny call  
    gMs[1] = gM2;

    int result = Mutex.WaitAny(gMs);  // Waits until either Mutex is released  
    gMs[result].ReleaseMutex();
    Console.WriteLine("t3Start finished, Mutex.WaitAny(Mutex[])"); Event3.Set();       // AutoResetEvent.Set() flagging method is done
}
public void t4Start()
{
    Console.WriteLine("t4Start started,  gM2.WaitOne( )");
    gM2.WaitOne();   // Waits until Mutex gM2 is released   
    Console.WriteLine("t4Start finished, gM2.WaitOne( )");
    Event4.Set();    // AutoResetEvent.Set() flagging method is done
    gM2.ReleaseMutex();
}