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所属分类：.NET技术
前言
在Web 应用程序中，我们经常会遇到这样的场景，如用户信息，租户信息本次的请求过程中都是固定的，我们希望是这种信息在本次请求内，一次赋值，到处使用。本文就来探讨一下，如何在.NET Core 下去利用AsyncLocal 实现全局共享变量。
简介
我们如果需要整个程序共享一个变量，我们仅需将该变量放在某个静态类的静态变量上即可（不满足我们的需求，静态变量上，整个程序都是固定值）。我们在Web 应用程序中，每个Web 请求服务器都为其分配了一个独立线程，如何实现用户，租户等信息隔离在这些独立线程中。这就是今天要说的线程本地存储。针对线程本地存储 .NET 给我们提供了两个类 ThreadLocal 和 AsyncLocal。我们可以通过查看以下例子清晰的看到两者的区别：
[TestClass] public class TastLocal { private static ThreadLocal<string> threadLocal = new ThreadLocal<string>(); private static AsyncLocal<string> asyncLocal = new AsyncLocal<string>(); [TestMethod] public void Test() { threadLocal.Value = "threadLocal"; asyncLocal.Value = "asyncLocal"; var threadId = Thread.CurrentThread.ManagedThreadId; Task.Factory.StartNew(() => { var threadId = Thread.CurrentThread.ManagedThreadId; Debug.WriteLine($"StartNew:threadId:{ threadId}; threadLocal:{threadLocal.Value}"); Debug.WriteLine($"StartNew:threadId:{ threadId}; asyncLocal:{asyncLocal.Value}"); }); CurrThread(); } public void CurrThread() { var threadId = Thread.CurrentThread.ManagedThreadId; Debug.WriteLine($"CurrThread:threadId:{threadId};threadLocal:{threadLocal.Value}"); Debug.WriteLine($"CurrThread:threadId:{threadId};asyncLocal:{asyncLocal.Value}"); } } 输出结果：
CurrThread:threadId:4;threadLocal:threadLocal StartNew:threadId:11; threadLocal: CurrThread:threadId:4;asyncLocal:asyncLocal StartNew:threadId:11; asyncLocal:asyncLocal 从上面结果中可以看出 ThreadLocal 和 AsyncLocal 都能实现基于线程的本地存储。但是当线程切换后，只有 AsyncLocal 还能够保留原来的值。在Web 开发中，我们会有很多异步场景，在这些场景下，可能会出现线程的切换。所以我们使用AsyncLocal 去实现在Web 应用程序下的共享变量。
AsyncLocal 解读
源码查看：
public sealed class AsyncLocal<T> : IAsyncLocal { private readonly Action<AsyncLocalValueChangedArgs<T>>? m_valueChangedHandler; // // 无参构造函数 // public AsyncLocal() { } // // 构造一个带有委托的AsyncLocal<T>，该委托在当前值更改时被调用 // 在任何线程上 // public AsyncLocal(Action<AsyncLocalValueChangedArgs<T>>? valueChangedHandler) { m_valueChangedHandler = valueChangedHandler; } [MaybeNull] public T Value { get { object? obj = ExecutionContext.GetLocalValue(this); return (obj == null) ? default : (T)obj; } set => ExecutionContext.SetLocalValue(this, value, m_valueChangedHandler != null); } void IAsyncLocal.OnValueChanged(object? previousValueObj, object? currentValueObj, bool contextChanged) { Debug.Assert(m_valueChangedHandler != null); T previousValue = previousValueObj == null ? default! : (T)previousValueObj; T currentValue = currentValueObj == null ? default! : (T)currentValueObj; m_valueChangedHandler(new AsyncLocalValueChangedArgs<T>(previousValue, currentValue, contextChanged)); } } // // 接口，允许ExecutionContext中的非泛型代码调用泛型AsyncLocal<T>类型 // internal interface IAsyncLocal { void OnValueChanged(object? previousValue, object? currentValue, bool contextChanged); } public readonly struct AsyncLocalValueChangedArgs<T> { public T? PreviousValue { get; } public T? CurrentValue { get; } // // If the value changed because we changed to a different ExecutionContext, this is true. If it changed // because someone set the Value property, this is false. // public bool ThreadContextChanged { get; } internal AsyncLocalValueChangedArgs(T? previousValue, T? currentValue, bool contextChanged) { PreviousValue = previousValue!; CurrentValue = currentValue!; ThreadContextChanged = contextChanged; } } // // Interface used to store an IAsyncLocal => object mapping in ExecutionContext. // Implementations are specialized based on the number of elements in the immutable // map in order to minimize memory consumption and look-up times. // internal interface IAsyncLocalValueMap { bool TryGetValue(IAsyncLocal key, out object? value); IAsyncLocalValueMap Set(IAsyncLocal key, object? value, bool treatNullValueAsNonexistent); } 我们知道在.NET 里面，每个线程都关联着执行上下文。我们可以通 Thread.CurrentThread.ExecutionContext 属性进行访问 或者通过 ExecutionContext.Capture() 获取。
从上面我们可以看出 AsyncLocal 的 Value 存取是通过 ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 进行操作的，我们可以继续从 ExecutionContext 里面取出部分代码查看（源码地址），为了更深入地理解 AsyncLocal 我们可以查看一下源码，看看内部实现原理。
internal static readonly ExecutionContext Default = new ExecutionContext(); private static volatile ExecutionContext? s_defaultFlowSuppressed; private readonly IAsyncLocalValueMap? m_localValues; private readonly IAsyncLocal[]? m_localChangeNotifications; private readonly bool m_isFlowSuppressed; private readonly bool m_isDefault; private ExecutionContext() { m_isDefault = true; } private ExecutionContext( IAsyncLocalValueMap localValues, IAsyncLocal[]? localChangeNotifications, bool isFlowSuppressed) { m_localValues = localValues; m_localChangeNotifications = localChangeNotifications; m_isFlowSuppressed = isFlowSuppressed; } public void GetObjectData(SerializationInfo info, StreamingContext context) { throw new PlatformNotSupportedException(); } public static ExecutionContext? Capture() { ExecutionContext? executionContext = Thread.CurrentThread._executionContext; if (executionContext == null) { executionContext = Default; } else if (executionContext.m_isFlowSuppressed) { executionContext = null; } return executionContext; } internal static object? GetLocalValue(IAsyncLocal local) { ExecutionContext? current = Thread.CurrentThread._executionContext; if (current == null) { return null; } Debug.Assert(!current.IsDefault); Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context"); current.m_localValues.TryGetValue(local, out object? value); return value; } internal static void SetLocalValue(IAsyncLocal local, object? newValue, bool needChangeNotifications) { ExecutionContext? current = Thread.CurrentThread._executionContext; object? previousValue = null; bool hadPreviousValue = false; if (current != null) { Debug.Assert(!current.IsDefault); Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context"); hadPreviousValue = current.m_localValues.TryGetValue(local, out previousValue); } if (previousValue == newValue) { return; } // Regarding 'treatNullValueAsNonexistent: !needChangeNotifications' below: // - When change notifications are not necessary for this IAsyncLocal, there is no observable difference between // storing a null value and removing the IAsyncLocal from 'm_localValues' // - When change notifications are necessary for this IAsyncLocal, the IAsyncLocal's absence in 'm_localValues' // indicates that this is the first value change for the IAsyncLocal and it needs to be registered for change // notifications. So in this case, a null value must be stored in 'm_localValues' to indicate that the IAsyncLocal // is already registered for change notifications. IAsyncLocal[]? newChangeNotifications = null; IAsyncLocalValueMap newValues; bool isFlowSuppressed = false; if (current != null) { Debug.Assert(!current.IsDefault); Debug.Assert(current.m_localValues != null, "Only the default context should have null, and we shouldn't be here on the default context"); isFlowSuppressed = current.m_isFlowSuppressed; newValues = current.m_localValues.Set(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications); newChangeNotifications = current.m_localChangeNotifications; } else { // First AsyncLocal newValues = AsyncLocalValueMap.Create(local, newValue, treatNullValueAsNonexistent: !needChangeNotifications); } // // Either copy the change notification array, or create a new one, depending on whether we need to add a new item. // if (needChangeNotifications) { if (hadPreviousValue) { Debug.Assert(newChangeNotifications != null); Debug.Assert(Array.IndexOf(newChangeNotifications, local) >= 0); } else if (newChangeNotifications == null) { newChangeNotifications = new IAsyncLocal[1] { local }; } else { int newNotificationIndex = newChangeNotifications.Length; Array.Resize(ref newChangeNotifications, newNotificationIndex + 1); newChangeNotifications[newNotificationIndex] = local; } } Thread.CurrentThread._executionContext = (!isFlowSuppressed && AsyncLocalValueMap.IsEmpty(newValues)) ? null : // No values, return to Default context new ExecutionContext(newValues, newChangeNotifications, isFlowSuppressed); if (needChangeNotifications) { local.OnValueChanged(previousValue, newValue, contextChanged: false); } } 从上面可以看出，ExecutionContext.GetLocalValue 和GetLocalValue.SetLocalValue 都是通过对 m_localValues 字段进行操作的。
m_localValues 的类型是 IAsyncLocalValueMap ,IAsyncLocalValueMap 的实现 和 AsyncLocal.cs 在一起，感兴趣的可以进一步查看 IAsyncLocalValueMap 是如何创建，如何查找的。
可以看到，里面最重要的就是ExecutionContext 的流动，线程发生变化时ExecutionContext 会在前一个线程中被默认捕获，流向下一个线程，它所保存的数据也就随之流动。在所有会发生线程切换的地方，基础类库(BCL) 都为我们封装好了对执行上下文的捕获 (如开始的例子，可以看到 AsyncLocal 的数据不会随着线程的切换而丢失)，这也是为什么 AsyncLocal 能实现 线程切换后，还能正常获取数据，不丢失。
总结
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AsyncLocal 本身不保存数据，数据保存在 ExecutionContext 实例。
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ExecutionContext 的实例会随着线程切换流向下一线程（也可以禁止流动和恢复流动），保证了线程切换时，数据能正常访问。
在.NET Core 中的使用示例
- 先创建一个上下文对象
点击查看代码
using System; using System.Collections.Generic; using System.Linq; using System.Threading; using System.Threading.Tasks; namespace NetAsyncLocalExamples.Context { /// <summary> /// 请求上下文 租户ID /// </summary> public class RequestContext { /// <summary> /// 获取请求上下文 /// </summary> public static RequestContext Current => _asyncLocal.Value; private readonly static AsyncLocal<RequestContext> _asyncLocal = new AsyncLocal<RequestContext>(); /// <summary> /// 将请求上下文设置到线程全局区域 /// </summary> /// <param name="userContext"></param> public static IDisposable SetContext(RequestContext userContext) { _asyncLocal.Value = userContext; return new RequestContextDisposable(); } /// <summary> /// 清除上下文 /// </summary> public static void ClearContext() { _asyncLocal.Value = null; } /// <summary> /// 租户ID /// </summary> public string TenantId { get; set; } } } namespace NetAsyncLocalExamples.Context { /// <summary> /// 用于释放对象 /// </summary> internal class RequestContextDisposable : IDisposable { internal RequestContextDisposable() { } public void Dispose() { RequestContext.ClearContext(); } } } - 创建请求上下文中间件
点击查看代码
using Microsoft.AspNetCore.Http; using Microsoft.Extensions.Logging; using NetAsyncLocalExamples.Context; using System; using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; namespace NetAsyncLocalExamples.Middlewares { /// <summary> /// 请求上下文 /// </summary> public class RequestContextMiddleware : IMiddleware { protected readonly IServiceProvider ServiceProvider; private readonly ILogger<RequestContextMiddleware> Logger; public RequestContextMiddleware(IServiceProvider serviceProvider, ILogger<RequestContextMiddleware> logger) { ServiceProvider = serviceProvider; Logger = logger; } public virtual async Task InvokeAsync(HttpContext context, RequestDelegate next) { var requestContext = new RequestContext(); using (RequestContext.SetContext(requestContext)) { requestContext.TenantId = $"租户ID:{DateTime.Now.ToString("yyyyMMddHHmmsss")}"; await next(context); } } } } - 注册中间件
点击查看代码
public void ConfigureServices(IServiceCollection services) { services.AddTransient<RequestContextMiddleware>(); services.AddRazorPages(); } public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } else { app.UseExceptionHandler("/Error"); // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts. app.UseHsts(); } app.UseHttpsRedirection(); app.UseStaticFiles(); app.UseRouting(); app.UseAuthorization(); //增加上下文 app.UseMiddleware<RequestContextMiddleware>(); app.UseEndpoints(endpoints => { endpoints.MapRazorPages(); }); } - 一次赋值，到处使用
点击查看代码
namespace NetAsyncLocalExamples.Pages { public class IndexModel : PageModel { private readonly ILogger<IndexModel> _logger; public IndexModel(ILogger<IndexModel> logger) { _logger = logger; _logger.LogInformation($"测试获取全局变量1：{RequestContext.Current.TenantId}"); } public void OnGet() { _logger.LogInformation($"测试获取全局变量2：{RequestContext.Current.TenantId}"); } } } 
