任务并行库 - 我不明白我做错了什么

Question

这是一个两部分问题.
我有一个类异步获取所有进程并轮询它们的CPU使用情况.昨天我有一个错误,它在这里解决了.
问题的第一部分是解决方案为何有所帮助.我不明白这个解释.
问题的第二部分是,"Object reference not set to an instance of object"当我尝试在流程结束时打印结果时,我偶尔会遇到异常.这是因为item.Key确实是空的.我不明白为什么那是因为我检查断点(process == null)并且它从未被击中.我究竟做错了什么？
代码如下.

class ProcessCpuUsageGetter
    {
        private IDictionary<Process, int> _usage;

        public IDictionary<Process, int> Usage { get { return _usage; } }

        public ProcessCpuUsageGetter()
        {
            while (true)
            {
                Process[] processes = Process.GetProcesses();
                int processCount = processes.Count();
                Task[] tasks = new Task[processCount];

                _usage = new Dictionary<Process, int>();
                for (int i = 0; i < processCount; i++)
                {
                    var localI = i;
                    var localProcess = processes[localI];
                    tasks[localI] = Task.Factory.StartNew(() => DoWork(localProcess));
                }
                Task.WaitAll(tasks);

                foreach (var item in Usage)
                {
                    Console.WriteLine("{0} - {1}%", item.Key.ProcessName, item.Value);
                }                
            }
        }

        private void DoWork(object o)
        {
            Process process = (Process)o;
            PerformanceCounter pc = new PerformanceCounter("Process", "% Processor Time", process.ProcessName, true);
            pc.NextValue();
            Thread.Sleep(1000);
            int cpuPercent = (int)pc.NextValue() / Environment.ProcessorCount;
            if (process == null)
            {
                var x = 5;
            }
            if (_usage == null)
            {
                var t = 6;
            }
            _usage.Add(process, cpuPercent);
        }
    }

Answer 1

这条线

 _usage.Add(process, cpuPercent);

正在从线程访问not-threadsafe集合.

使用ConcurrentDictionary<K,V>而不是普通的字典.

'null reference'错误只是一个随机症状,你也可能得到其他错误.