简介

ThreadLocal 用于在多线程环境中创建线程局部变量，可以让每个线程独立地访问自己的变量副本，互不影响。

而 AsyncLocal 是 ThreadLocal 的异步版本，专门用于异步编程场景，在异步操作中它可以正确处理上下文切换。

ThreadLocal

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)" : "");
};

// 启动八个线程，在4核及一下的电脑上，一些线程名会重复
Parallel.Invoke(action, action, action, action, action, action, action, action);

//释放对象
ThreadName.Dispose();

private static ThreadLocal<WebContext> threadLocal = new ThreadLocal<WebContext>(() =>
{
    var ctx = new WebContext();
    Console.WriteLine($"创建WebContext");
    return ctx;
});

Console.WriteLine($"主线程: {Thread.CurrentThread.ManagedThreadId}");
//模拟5个HTTP请求
for (var i = 0; i < 5; i++)
{
    var index = i;
    Task.Factory.StartNew(() =>
    {
        var ctx = threadLocal.Value;
        ctx.Name = "请求" + index;
        ctx.Id = index;
        Console.WriteLine($"请求结束:{index} ctx.Name={ctx.Name} ctx.Id={ctx.Id}");
    });
}

可以看到，不同的线程值是不同的

AsyncLocal

//父上下文
public static async Task Async()
{
    asyncLocal.Value = new WebContext
    {
        Id = 0,
        Name = "父"
    };
    Console.WriteLine("父:设定ctx:" + asyncLocal.Value);
    await Async1();
    Console.WriteLine("父:结束时ctx:" + asyncLocal.Value);

}

//子上下文
public static async Task Async1()
{
    Console.WriteLine("    子读取到ctx:" + asyncLocal.Value);
    await Async1_1();
    Console.WriteLine("    经过孙处理后再读取ctx:" + asyncLocal.Value);
    asyncLocal.Value = new WebContext
    {
        Name = "子",
        Id = 1,
    };
    Console.WriteLine("    子改动ctx为:" + asyncLocal.Value);
    await Async1_1();
    Console.WriteLine("    经过孙处理后再读取ctx:" + asyncLocal.Value);
}

//孙上下文
public static async Task Async1_1()
{
    Console.WriteLine("        孙读取到ctx:" + asyncLocal.Value);
    asyncLocal.Value = new WebContext
    {
        Name = "孙",
        Id = 2,
    };
    Console.WriteLine("        孙改动ctx为:" + asyncLocal.Value);
}


class WebContext
{
    public string Name { get; set; }
    public int Id { get; set; }

    public override string ToString()
    {
        return $"Name={Name},Id={Id}";
    }
}

AsyncLocal 使你能够更深入地访问操作的异步流中的相同状态。 AsyncLocal 值仅向下流动（更深入地进入异步调用堆栈），并且不会将更改向上传播到调用方。

//父上下文
public static async Task Async()
{
    threadLocal.Value = new WebContext
    {
        Id = 0,
        Name = "父"
    };
    Console.WriteLine("父:设定ctx:" + threadLocal.Value);
    await Async1();
    Console.WriteLine("父:结束时ctx:" + threadLocal.Value);

}

//子上下文
public static async Task Async1()
{
    Console.WriteLine("    子读取到ctx:" + threadLocal.Value);
    await Async1_1();
    Console.WriteLine("    经过孙处理后再读取ctx:" + threadLocal.Value);
    threadLocal.Value = new WebContext
    {
        Name = "子",
        Id = 1,
    };
    Console.WriteLine("    子改动ctx为:" + threadLocal.Value);
    await Async1_1();
    Console.WriteLine("    经过孙处理后再读取ctx:" + threadLocal.Value);
}

//孙上下文
public static async Task Async1_1()
{
    Console.WriteLine("        孙读取到ctx:" + threadLocal.Value);
    threadLocal.Value = new WebContext
    {
        Name = "孙",
        Id = 2,
    };
    Console.WriteLine("        孙改动ctx为:" + threadLocal.Value);
}


class WebContext
{
    public string Name { get; set; }
    public int Id { get; set; }

    public override string ToString()
    {
        return $"Name={Name},Id={Id}";
    }
}