官网

https://github.com/dotnet/runtime/blob/16b6369b7509e58c35431f05681a9f9e5d10afaa/src/libraries/System.Private.CoreLib/src/System/Threading/AsyncLocal.cs#L45

AsyncLocal是一个在.NET中用来在同步任务和异步任务中保持全局变量的工具类。它允许你在不同线程的同一个对象中保留一个特定值，这样你可以在不同的函数和任务中访问这个值。这是在实现异步任务中维持一致性和优雅性的一种重要手段。

中间件中使用

 public class BaseClass{public string Id { get; set; }}public static class AmbientContext{public static readonly ConcurrentDictionary<string, AsyncLocal<object?>>_contexts = new(StringComparer.Ordinal);public static void Set<T>(string key, [MaybeNull] T val){AsyncLocal<object?> keyctx = _contexts.AddOrUpdate(key,k => new AsyncLocal<object?>(),(k, al) => al);keyctx.Value = (object?)val;}[return: MaybeNull]public static T Get<T>(string key){return _contexts.TryGetValue(key, out AsyncLocal<object?>? keyctx)? (T)(keyctx!.Value ?? default(T)!): default(T);}}public class AmbientContextMiddleware{private readonly RequestDelegate _next;public AmbientContextMiddleware(RequestDelegate next) =>_next = next;public async Task Invoke(HttpContext context){string corrId =context.Request.Headers["x-foocorp-correlationId"].FirstOrDefault(Guid.NewGuid().ToString());context.Request.Headers.Add("x-foocorp-correlationId", corrId);AmbientContext.Set<BaseClass>(corrId, new BaseClass() { Id=DateTime.Now.ToString()});await _next.Invoke(context);// TODO: emit corrid response header, esp if _we_ created it}}[HttpGet]
public async Task<DatasetSmallMovie> GetByID(string id,string indexName)
{Console.WriteLine(DateTime.Now.ToString());Request.Headers.TryGetValue("x-foocorp-correlationId", out var headersvalue);Console.WriteLine(headersvalue);await Task.Delay(2000);Console.WriteLine(AmbientContext.Get<BaseClass>(headersvalue).Id);return await _meilisearchHelper.GetByID<DatasetSmallMovie>(id, indexName);
}

普通使用

    using System;using System.Threading;using System.Threading.Tasks;public class LogContext { public string StackTrace { get; set; } public string UserInfo { get; set; } }public class TenantContext { public string Name { get; set; } }public class Program{private static AsyncLocal<LogContext> _logContext = new AsyncLocal<LogContext>(); private static AsyncLocal<TenantContext> _tenantContext = new AsyncLocal<TenantContext>();public static async Task Main(string[] args){_logContext.Value = new LogContext { StackTrace = "Main Stack Trace", UserInfo = "User1" }; _tenantContext.Value = new TenantContext { Name = "Tenant A" };Console.WriteLine($"Initial Log Context: {_logContext.Value.StackTrace}, User: {_logContext.Value.UserInfo}, Tenant: {_tenantContext.Value.Name}");await Task.Run(() => LogAndProcess(new LogContext { StackTrace = "Child Stack Trace", UserInfo = "User2" }, new TenantContext { Name = "Tenant B" }));Console.WriteLine($"After Task Log Context: {_logContext.Value.StackTrace}, User: {_logContext.Value.UserInfo}, Tenant: {_tenantContext.Value.Name}");}private static void LogAndProcess(LogContext logContext, TenantContext tenant){_logContext.Value = logContext; _tenantContext.Value = tenant;Console.WriteLine($"In Task Log Context: {_logContext.Value.StackTrace}, User: {_logContext.Value.UserInfo}, Tenant: {_tenantContext.Value.Name}");// Simulate some processing        Task.Delay(1000).Wait();Console.WriteLine($"After Processing Log Context: {_logContext.Value.StackTrace}, User: {_logContext.Value.UserInfo}, Tenant: {_tenantContext.Value.Name}");}}