AsyncLocal特性解析

# 1、跨线程性质
AsyncLocal与方法调用上下文（ExecutionContext）绑定，简单理解就是：相当于局部变量。

## 实验代码
    using System;
    using System.Threading;
    using System.Threading.Tasks;
    namespace ConsoleApp2
    {
        class Program
        {
            static AsyncLocal<string> d = new AsyncLocal<string>();
            static Random rd = new Random();
    
            static void Main(string[] args)
            {
                Console.WriteLine("Hello World!");
                for (int i = 0; i < 10; i++)
                {
                    Task.Factory.StartNew(TMain);//10个并发
                }
                Console.ReadLine();
            }
    
            public static async Task TMain()
            {
                int scope = rd.Next(0, 10000); //产生一个随机数scope
                d.Value = scope.ToString();    //赋给AsyncLocal
                System.Diagnostics.Trace.WriteLine("[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "] set scope:" + scope); //记录scope所在线程
                new System.Threading.Thread(Main1).Start(scope);    //开新线程，传入scope值，
                Task.Run(() => { Main2(scope); });                  //Task方式跨线程
                await Main3(scope);                                 //异步方式跨线程
            }
            public static void Main1(object scope)
            {
                System.Threading.Thread.Sleep(rd.Next(1, 10)); //随机休眠
                System.Diagnostics.Trace.WriteLine("main1[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]scope:" + scope + "，d:" + d.Value);
            }
    
            public static async Task Main3(int scope)
            {
                await Task.Delay(rd.Next(1, 10));
                System.Diagnostics.Trace.WriteLine("main3[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]scope:" + scope + "，d:" + d.Value);
            }
    
            public static void Main2(int scope)
            {
                System.Threading.Thread.Sleep(rd.Next(1, 10));
                System.Diagnostics.Trace.WriteLine("main2[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]scope:" + scope + "，d:" + d.Value);
            }
        }
    }
## 输出结果
    [5] set scope:7810
    [4] set scope:3704
    [6] set scope:2435
    [7] set scope:1011
    main1[10]scope:2435，d:2435
    main3[7]scope:3704，d:3704
    main1[8]scope:3704，d:3704
    main3[7]scope:2435，d:2435
    main2[6]scope:2435，d:2435
    main2[4]scope:3704，d:3704
    main2[5]scope:7810，d:7810
    [4] set scope:3774
    [5] set scope:5339
    main3[6]scope:1011，d:1011
    main2[7]scope:1011，d:1011
    [6] set scope:3421
    [7] set scope:2080
    main1[9]scope:7810，d:7810
    main1[11]scope:1011，d:1011
    main1[12]scope:3774，d:3774
    main2[5]scope:5339，d:5339
    main1[13]scope:5339，d:5339
    [5] set scope:1308
    main2[7]scope:2080，d:2080
    main3[6]scope:5339，d:5339
    [7] set scope:8773
    main1[14]scope:3421，d:3421
    main3[4]scope:3421，d:3421
    main1[15]scope:2080，d:2080
    main2[5]scope:1308，d:1308
    main3[4]scope:1308，d:1308
    main3[5]scope:7810，d:7810
    main2[6]scope:3421，d:3421
    main3[4]scope:3774，d:3774
    main3[5]scope:2080，d:2080
    main3[7]scope:8773，d:8773
    main1[16]scope:1308，d:1308
    main1[17]scope:8773，d:8773
    main2[4]scope:3774，d:3774
    main2[6]scope:8773，d:8773
## 结论
你会发现，d是个静态实例，但无论怎么跨线程，从d.Value取到的值，都跟传进来的实参scope是一致的，就是说，可以替代scope实参的传递，不会产生跨线程问题。

那么，你可以这样理解，d其实只是个取值的方法，调d.Value的时候，实际上从当前调用上下文里存取数据，并不会保存在静态对象本身上。
## 用处
对于Aop用处比较大，比如想传一些Context的参数，但又不能放进接口定义中。
再具体点：
比如，在mvc里，要从Middleware或Filter中传值给Action。
# 2、作用域
## 试验代码
     static void Main(string[] args)
            {
                Console.WriteLine("Hello World!");
                TScope().Wait();
                Console.ReadLine();
            }
    
            public static async Task TScope()
            {
                int scope = rd.Next(0, 10000); //产生一个随机数scope
                d.Value = scope.ToString();    //赋给AsyncLocal
                System.Diagnostics.Trace.WriteLine("TScope[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]set scope:" + scope + "，d:" + d.Value);
                await Tscope2(scope);
                System.Diagnostics.Trace.WriteLine("Tscope[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]scope:" + scope  + "，d:" + d.Value);
            }
    
            public static async Task Tscope2(int pscope)
            {
                System.Diagnostics.Trace.WriteLine("Tscope2[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]pscope:" + pscope + "，d:" + d.Value);
                int scope = rd.Next(0, 10000); //产生一个随机数scope
                d.Value = scope.ToString();    //赋给AsyncLocal
                System.Diagnostics.Trace.WriteLine("Tscope2[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]pscope:" + pscope + ",set scope:" + scope + "，d:" + d.Value);
                await Tscope3(scope);
                System.Diagnostics.Trace.WriteLine("Tscope2[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]pscope:" + pscope + ",scope:" + scope + "，d:" + d.Value);
            }
    
            public static async Task Tscope3(int pscope)
            {
                System.Diagnostics.Trace.WriteLine("Tscope3[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]pscope:" + pscope + "，d:" + d.Value);
                int scope = rd.Next(0, 10000); //产生一个随机数scope
                d.Value = scope.ToString();    //赋给AsyncLocal
                System.Diagnostics.Trace.WriteLine("Tscope3[" + System.Threading.Thread.CurrentThread.ManagedThreadId + "]pscope:" + pscope + ",set scope:" + scope + "，d:" + d.Value);
            }
## 输出结果
    TScope[1]set scope:2251，d:2251
    Tscope2[1]pscope:2251，d:2251
    Tscope2[1]pscope:2251,set scope:4812，d:4812
    Tscope3[1]pscope:4812，d:4812
    Tscope3[1]pscope:4812,set scope:29，d:29
    Tscope2[1]pscope:2251,scope:4812，d:4812
    Tscope[1]scope:2251，d:2251
## 结论
对d.Value的赋值，作用域为当前方法，以及子方法，也调是调用链下方，不会返回到上一级方法。
    这也跟局部变量(非ref和out类型)相似，局部变量 可以 作为实参往下传，但不能往上传。
    （用过python的同学，有没有觉得跟python里变量的作用域一样一样的？）

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