C# 如何判断子线程全部结束？？？急急急

 System.Diagnostics.Process[] allProcess = System.Diagnostics.Process.GetProcesses();
            foreach (System.Diagnostics.Process thisprocess in allProcess)
            {
                string processName = thisprocess.ProcessName;
                if (processName.ToLower() == "进程名字")
                {
                    try
                    {
                        //执行没结束的操作
                    }
                    catch (Exception e)
                    {
                        MessageBox.Show(e.Message);
                        return;
                    }
                }
            }

int threads;
private void button1_Click(object sender, EventArgs e)
{
    int threadCount = threads = 4;
    AutoResetEvent eventFinished = new AutoResetEvent(false);
    for (int i = 0; i < threadCount; i++)
    {
        ThreadPool.QueueUserWorkItem(state =>
            {
                Thread.Sleep(Math.Abs(Guid.NewGuid().GetHashCode()) % 1000);
                if (Interlocked.Decrement(ref this.threads) == 0)
                {
                    (state as AutoResetEvent).Set();
                }
            },
            eventFinished
            );
    }

    eventFinished.WaitOne();
    //执行完毕 
}