.NET: Clarification Request -Processing Threads

I am a beginner.Based on the Albahari's Producer/Consumer Solution , I have developed a code.Coding is working fine.But I have some doubts on my implementation.

class FloodDisaterManagement :IDisposable
    {

        Queue<string>MedicinePack  = new Queue<string>();
        Thread[] volunteers;
        readonly object locker = new object();

    public FloodDisaterManagement(int volunteerCount)
    {
        volunteers = new Thread[volunteerCount];

        for (int i = 0; i < volunteerCount; i++)
        {
            volunteers[i] = new Thread(SupplyMedicine);
            volunteers[i].Start();
        }
    }

    public void DumpMedicine(string pack)
    {
        lock (locker)
        {
            MedicinePack.Enqueue(pack);
            Monitor.PulseAll(locker);
        }
    }

    public void Dispose()
    {

        foreach (Thread volunteer in volunteers) DumpMedicine(null);
        foreach (Thread volunteer in volunteers) volunteer.Join();
    }

    public void SupplyMedicine()
    {
        while (true)
        {
            string pack;
            lock (locker)
            {
                while (MedicinePack .Count == 0) Monitor.Wait(locker);
                pack= MedicinePack.Dequeue();
            }
            if (pack == null) return; 
            Console.WriteLine(pack+"is supplied");
            Thread.Sleep(1000); 
        }

    }


    static void Main()
    {
       string[] medicinePacks = new string[]  
        { "Pack1", "Pack2", "Pack3", "Pack4", "Pack5", "Pack6", 
          "Pack7", "Pack8", "Pack9", "Pack10"
        };

        using (FloodDisaterMana开发者_开发技巧gement q = new FloodDisaterManagement(2))
        {
            foreach (string pack in medicinePacks)
                q.DumpMedicine(pack);
            Console.WriteLine("Enqueued 10 Medicine Packs");

        }

        Console.WriteLine("..Medicines Distribution Completed");
        Console.ReadKey(true);
    }


}

Questions:

1. As We have Using(...) in Main() ,Why still there is a need for Dispose() in FloodDisasterManagement class ?
2. What is the exact purpose of having two foreach inside Dispose() ?
3. Where does the control return when we use "return statement" inside the SupplyMedicine()? (i.e)

public void SupplyMedicine()
{
    while (true)
    {
        ....  
        if (pack == null) return; 
        ... 
       //Whether the control return to Main() or SomeWhere?
    }

}

1: because the using calls the Dispose(); that is the purpose of the using.

2: to close each of the encapsulated resources, in this case threads (asking them to exit, then waiting for them to exit).

3: it returns to the caller; in this case the caller is a Thread, so this cleanly exits the thread

Or in English:

• the using in this case acts to wait for all the threads to exit; without the using we would close too early
• the foreach signals each thread to exit when it has finished processing its load, and waits for them all
• the null is detected as a "now exit" signal, causing each thread to exit

About 3:

Since the SupplyMedicine method runs on a separate thread, and this is the first and only method run by the thread- the return statement simply means that this thread finished its work and should be terminated (or returned to the thread pool in case you are using a thread pool thread).