C#

class Program 
{ 
static Service1[] clients = null; 
static Thread[] threads = null; 
static ManualResetEvent[] doneEvents = null; 

static void ThreadJob (object index) 
{ 
// query 100 times 
for (int i = 0; i < 100; i++) 
{ 
clients[(int)index].HelloWorld(); 
} 

doneEvents[(int)index].Set(); 
} 

static void Main(string[] args) 
{ 
Console.WriteLine("Specify number of threads: "); 
int number = Int32.Parse(Console.ReadLine()); 

clients = new Service1[number]; 
threads = new Thread[number]; 
doneEvents = new ManualResetEvent[number]; 

for (int i = 0; i < number; i++) 
{ 
doneEvents[i] = new ManualResetEvent(false); 
clients [i] = new Service1(); 
clients[i].EnableDecompression = true; 
ThreadPool.QueueUserWorkItem(ThreadJob, i); 
// ParameterizedThreadStart starter = new ParameterizedThreadStart(ThreadJob); 
// threads[i] = new Thread(starter); 
} 

DateTime begin = DateTime.Now; 

/* 
for (int i = 0; i < number; i++) 
{ 
threads[i].Start(i); 
} 

for (int i = 0; i < number; i++) 
{ 
threads[i].Join(); 
} 
*/ 

WaitHandle.WaitAll(doneEvents); 

Console.WriteLine("Total elapsed time (s): " + (DateTime.Now - begin).TotalSeconds); 

Console.ReadLine(); 

return; 
} 
} 

Stack<Service1> clients ...;

Stack<Items> stack ...;

public void MyThread(object whocares){
    while(go){
        Item item = null;
        lock(stack){
            item = stack.Pop();
        }
        if(item != null){
            //Process Item
        }
    }
}

System.Threading.ThreadPool.QueueUserWorkItem(new System.Threading.WaitCallback(MyThread)); 

class Program
{
    static Service1[] clients = null;
    static IAsyncResult[] results = null;
    static AsyncMethodCaller[] callers = null;

    public delegate void AsyncMethodCaller(object index);

    static void ThreadJob (object index)
    {
        // query 100 times
        for (int i = 0; i < 100; i++)
        {
            clients[(int)index].HelloWorld();
        }
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Specify number of threads: ");
        int number = Int32.Parse(Console.ReadLine());

        clients = new Service1[number];
        results = new IAsyncResult[number];
        callers = new AsyncMethodCaller[number];

        for (int i = 0; i < number; i++)
        {
            clients[i] = new Service1();
            clients[i].EnableDecompression = true;
            callers[i] = new AsyncMethodCaller(ThreadJob);
            results[i] = callers[i].BeginInvoke(i, null, null);
        }

        DateTime begin = DateTime.Now;

        int j = 0;
        foreach (IAsyncResult res in results)
        {
            callers[j].EndInvoke(res);
            j++;
        }

        Console.WriteLine("Total elapsed time (s): " + (DateTime.Now - begin).TotalSeconds);

        Console.ReadLine();

        return;
    }
}

class Program
{
    static Service1[] clients = null;

    public delegate void AsyncMethodCaller(int index);

    static void ThreadJob (int index)
    {
        // query 100 times
        for (int i = 0; i < 100; i++)
        {
            clients[index].HelloWorld();
        }
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Specify number of threads: ");
        int number = Int32.Parse(Console.ReadLine());
        //note: this doesn't actually specify the number of threads
        //the number of threads will be something like "at most 15 time the number of available cores"
        //which means that it will only specify the number of threads that are used if
        //the number if small. And any number of threads may exists - but they wouldn't be used.

        clients = new Service1[number];
        IAsyncResult[] results = new IAsyncResult[number];

        AsyncMethodCaller TJ = new AsyncMethodCaller(ThreadJob)
        for (int i = 0; i < number; i++)
        {
            clients[i] = new Service1();
            clients[i].EnableDecompression = true;
            results[i] = TJ.BeginInvoke(i, null, null);
        }

        DateTime begin = DateTime.Now;

        foreach (IAsyncResult res in results)
            TJ.EndInvoke(res);

        Console.WriteLine("Total elapsed time (s): " + (DateTime.Now - begin).TotalSeconds);

        Console.ReadLine();

        return;
    }
}

/// <summary>
/// Summary description for Service1
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
public class Service1 : System.Web.Services.WebService
{

    [WebMethod]
    public double HelloWorld()
    {
        return new Random().NextDouble();
    }
}