如何在.NET Core托管服务中访问Serilog上下文外的CorrelationId？

后台托管服务没有HTTP上下文，默认的CorrelationId自然拿不到，要让日志带上CorrelationId，按下面几步来：

1. 实现Correlation上下文访问器 #

异步线程要传递CorrelationId，得用AsyncLocal存储，先定义接口和实现：

public interface ICorrelationContextAccessor
{
    string CorrelationId { get; set; }
}

public class CorrelationContextAccessor : ICorrelationContextAccessor
{
    private readonly AsyncLocal<string> _correlationIdHolder = new AsyncLocal<string>();

    public string CorrelationId
    {
        get => _correlationIdHolder.Value;
        set => _correlationIdHolder.Value = value;
    }
}

然后在Program.cs里注册为单例：

builder.Services.AddSingleton<ICorrelationContextAccessor, CorrelationContextAccessor>();

2. 在托管服务中设置/传递CorrelationId #

如果是定时任务，直接生成CorrelationId；如果是处理外部消息（比如消息队列），就从消息头部获取。示例：

public class MyBackgroundService : BackgroundService
{
    private readonly ILogger<MyBackgroundService> _logger;
    private readonly ICorrelationContextAccessor _correlationAccessor;

    public MyBackgroundService(ILogger<MyBackgroundService> logger, ICorrelationContextAccessor correlationAccessor)
    {
        _logger = logger;
        _correlationAccessor = correlationAccessor;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            // 生成CorrelationId，或从外部消息/配置中读取
            var correlationId = Guid.NewGuid().ToString();
            _correlationAccessor.CorrelationId = correlationId;

            // 用日志范围包裹业务逻辑，让日志自动带上CorrelationId
            using (_logger.BeginScope(new { CorrelationId = correlationId }))
            {
                _logger.LogInformation("后台任务执行中");
            }

            await Task.Delay(60000, stoppingToken);
        }
    }
}

3. 配置日志显示CorrelationId #

打开appsettings.json，开启控制台日志的Scope显示，让CorrelationId出现在日志里：

{
  "Logging": {
    "LogLevel": {
      "Default": "Information"
    },
    "Console": {
      "FormatterOptions": {
        "IncludeScopes": true,
        "TimestampFormat": "HH:mm:ss "
      }
    }
  }
}

4. 用Serilog的简化方案 #

如果项目用Serilog，直接通过日志上下文注入CorrelationId更省心：

1. 安装NuGet包：Serilog.AspNetCore、Serilog.Enrichers.Context
2. 在Program.cs配置Serilog：

builder.Host.UseSerilog((ctx, config) => config
    .ReadFrom.Configuration(ctx.Configuration)
    .Enrich.FromLogContext()
    .WriteTo.Console(outputTemplate: "[{Timestamp:HH:mm:ss} {Level}] {Message} | CorrelationId:{CorrelationId}{NewLine}"));

3. 在托管服务中推送CorrelationId到日志上下文：

using (LogContext.PushProperty("CorrelationId", correlationId))
{
    _logger.LogInformation("处理消息中");
}

内容的提问来源于stack exchange，提问作者hashbytes