如何进行一系列异步调用，然后使用第一个线程返回的值

我想进行异步调用，然后处理第一个线程返回的值。我不想使用并行库。我只能使用.Net 2.0。我已经编写了以下程序，应该足以满足我的要求。但是有人可以指出是否有更好的解决方案吗？谢谢。

这是我的代码：

class Program
{
    private static Worker _worker2;
    private static Worker _worker3;
    private static Worker _worker4;
    private static Worker _worker1;
    delegate void DoWorkDelegate(object param);
    private static Worker _firstReturnedWorker = null;
    private static object lockObject = new object();
    public static void Main()
    {
        _worker1 = new Worker();
        _worker2 = new Worker();
        _worker3 = new Worker();
        _worker4 = new Worker();
        int i = 10;
        DoWorkDelegate d1 = new DoWorkDelegate(_worker1.OnDoWork);
        d1.BeginInvoke(1, OnThreadReturned, ++i);
        DoWorkDelegate d2 = new DoWorkDelegate(_worker2.OnDoWork);
        d2.BeginInvoke(2, OnThreadReturned, ++i);
        DoWorkDelegate d3 = new DoWorkDelegate(_worker3.OnDoWork);
        d3.BeginInvoke(3, OnThreadReturned, ++i);
        DoWorkDelegate d4 = new DoWorkDelegate(_worker4.OnDoWork);
        d4.BeginInvoke(4, OnThreadReturned, ++i);
        Console.ReadKey();
    }
    private static void OnThreadReturned(IAsyncResult ar)
    {
        lock (lockObject)
        {
            if (_firstReturnedWorker == null)
            {                    
                var workerIdentifier = (int)ar.AsyncState;
                switch (workerIdentifier)
                {
                    case 11:
                        _firstReturnedWorker = _worker1;
                        break;
                    case 12:
                        _firstReturnedWorker = _worker2;
                        break;
                    case 13:
                        _firstReturnedWorker = _worker3;
                        break;
                    case 14:
                        _firstReturnedWorker = _worker4;
                        break;
                }
            }
            else
            {
                return;
            }
        }
        Console.WriteLine("First result received was {0}", _firstReturnedWorker.ReturnedValue);
    }
}
public class Worker
{
    public string ReturnedValue { get; private set; }
    public void OnDoWork(object value)
    {
        ReturnedValue = Process((int)value);
    }
    private string Process(int numberToProcess)
    {
        return string.Format("{0} was processed by {1}", numberToProcess, Thread.CurrentThread.ManagedThreadId);
    }
}