为什么在 Threadlocal 委托中初始化 threadstatic 属性不会为第一个线程初始化它？答案

【问题标题】：Why Initializing threadstatic attribute in Threadlocal delegate doesn't initilize it for the first thread?为什么在 Threadlocal 委托中初始化 threadstatic 属性不会为第一个线程初始化它？
【发布时间】：2017-05-07 20:00:21
【问题描述】：

我已经创建了 3 个线程，并且所有线程都对 thread1 之外的 threadlocal 属性进行了递增操作。我还在 threadlocal 委托中通过值 11 初始化 threadstatic 属性。在这里，我总是在第一个线程中得到 num = 0 的值。为什么会这样？

class Program
    {
        //static int numDuplicate = 0;
        [ThreadStatic]
        static int num = 5;

        public static ThreadLocal<int> _field = new ThreadLocal<int>(() =>
        {
            num = 11;
            //numDuplicate = num;
            Console.WriteLine("Threadstatic variable value in Threadlocal's delegate = " + num.ToString());
            return Thread.CurrentThread.ManagedThreadId;
        });



        public static void Main(string[] args)
        {
            Thread t1 = new Thread(new ThreadStart(() =>
            {
                Console.WriteLine("Threadlocal attribute value for thread 1: " + _field + ". Threadstatic variable value = " + num.ToString());
            }));

            Thread t2 = new Thread(new ThreadStart(() =>
            {
                _field.Value++;
            Console.WriteLine("Threadlocal attribute value for thread 2: " + _field + ". Threadstatic variable value = " + num.ToString());
            }));

            Thread t3 = new Thread(new ThreadStart(() =>
            {
                _field.Value++;
                Console.WriteLine("Threadlocal attribute value for thread 3: " + _field + ". Threadstatic variable value = " + num.ToString());
            }));

            t1.Start();
            t2.Start();
            t3.Start();
            Console.ReadLine();
        }
    }

//Output:
Threadstatic variable value in Threadlocal's delegate = 11
Threadstatic variable value in Threadlocal's delegate = 11
Threadstatic variable value in Threadlocal's delegate = 11
Threadlocal attribute value for thread 1: 10. Threadstatic variable value = 0
Threadlocal attribute value for thread 3: 13. Threadstatic variable value = 11
Threadlocal attribute value for thread 2: 12. Threadstatic variable value = 11

【问题讨论】：

标签： c# multithreading threadstatic

【解决方案1】：

因为“不能”为 ThreadStatics 分配初始值。

不要为标有 ThreadStaticAttribute 的字段指定初始值，因为这样的初始化只发生一次，当类构造函数执行时，因此只影响一个线程。如果不指定初始值，如果是值类型，则可以依赖被初始化的字段为其默认值，如果是引用类型，则可以依赖为 null。

来源：MSDN

你可以看看ThreadLocal<>

static ThreadLocal<int> num  = new ThreadLocal<int>(() => 5);

Func<> 将作为每个新线程的初始化执行。在这种情况下，它只返回 5。

【讨论】：