.NET Core 项目使用的是 SkyWalking.Agent.AspNetCore -Version 0.6.0 包来加载 SkyWalking 探针，但是在多线程中跟踪时会发生如下异常：

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object.
    at SkyWalking.Diagnostics.SqlClient.SqlClientTracingDiagnosticProcessor.BeforeExecuteCommand(SqlCommand sqlCommand)

之前配置 SkyWalking 时没有注意，导致请求的详细信息在记录不久之后（2 个小时不到）就查不到了。

查到在官方文档 TTL 中的如下描述：

1. 安装 SkyWalking.Agent.AspNetCore 包

   当前最新版本是 0.6.0。

   Install-Package SkyWalking.Agent.AspNetCore
   

2. 通过 ContextManager 中的 GlobalTraceId 获取当前的 TraceId

   /// <summary>
   /// Trace Id
   /// </summary>
   public string TraceId { get; set; } = SkyWalking.Context.ContextManager.GlobalTraceId;
   

3. GlobalTraceId 属性的源码

   namespace SkyWalking.Context
   {
       /// <summary>
       /// Context manager controls the whole context of tracing. Since .NET server application runs as same as Java,
       /// We also provide the CONTEXT propagation based on ThreadLocal mechanism.
       /// Meaning, each segment also related to singe thread.
       /// </summary>
       public class ContextManager : ITracingContextListener, IIgnoreTracerContextListener
       {
           static ContextManager()
           {
               var manager = new ContextManager();
               TracingContext.ListenerManager.Add(manager);
               IgnoredTracerContext.ListenerManager.Add(manager);
           }
   
           private static readonly AsyncLocal<ITracerContext> _context = new AsyncLocal<ITracerContext>();
   
           public static string GlobalTraceId
           {
               get
               {
                   if (_context.Value != null)
                   {
                       return _context.Value.GetReadableGlobalTraceId();
                   }
   
                   return "N/A";
               }
           }
       }
   }