c# - 如何在 DataFlow Block 中为每个线程创建对象，但不是每个请求的对象？

Question

我有一个代码示例

var options = new ExecutionDataflowBlockOptions();
var actionBlock = new ActionBlock<int>(async request=>{
         var rand = new Ranodm();
         //do some stuff with request by using rand
},options);

这段代码的问题在于，在每个请求中我都必须创建新rand对象。有一种方法可以为rand每个线程定义一个对象并在处理请求时重用同一个对象？

标签： c#task-parallel-librarydataflowtpl-dataflow

Answer 1

感谢Reed Copsey 的精彩文章和 Theodor Zoulias 的提醒ThreadLocal<T>。

新的 ThreadLocal 类为我们提供了一个强类型、本地范围的对象，我们可以使用它来设置为每个线程单独保存的数据。这允许我们使用每个线程存储的数据，而不必将静态变量引入我们的类型。在内部，ThreadLocal 实例将自动设置静态数据，管理其生命周期，执行与我们特定类型之间的所有转换。这使得开发变得更加简单。

一个 msdn 示例：

 // Demonstrates:
    //      ThreadLocal(T) constructor
    //      ThreadLocal(T).Value
    //      One usage of ThreadLocal(T)
    static void Main()
    {
        // Thread-Local variable that yields a name for a thread
        ThreadLocal<string> ThreadName = new ThreadLocal<string>(() =>
        {
            return "Thread" + Thread.CurrentThread.ManagedThreadId;
        });

        // Action that prints out ThreadName for the current thread
        Action action = () =>
        {
            // If ThreadName.IsValueCreated is true, it means that we are not the
            // first action to run on this thread.
            bool repeat = ThreadName.IsValueCreated;

            Console.WriteLine("ThreadName = {0} {1}", ThreadName.Value, repeat ? "(repeat)" : "");
        };

        // Launch eight of them.  On 4 cores or less, you should see some repeat ThreadNames
        Parallel.Invoke(action, action, action, action, action, action, action, action);

        // Dispose when you are done
        ThreadName.Dispose();
    }

所以你的代码会是这样的：

ThreadLocal<Random> ThreadName = new ThreadLocal<Random>(() =>
{
    return new Random();
});


var options = new ExecutionDataflowBlockOptions
{
    MaxDegreeOfParallelism = 4,
    EnsureOrdered = false,
    BoundedCapacity = 4 * 8
};
var actionBlock = new ActionBlock<int>(async request =>
{
    bool repeat = ThreadName.IsValueCreated;
    var random = ThreadName.Value; // your local random class
    Console.WriteLine($"Thread: {Thread.CurrentThread.ManagedThreadId},  
        repeat: ", repeat ? "(repeat)" : "");

 }, options);