1 public class WebApiApplication : System.Web.HttpApplication
 2     {
 3         protected void Application_Start()
 4         {
 5             log4net.Config.XmlConfigurator.Configure(new System.IO.FileInfo(Server.MapPath("~/Web.config")));
 6             AreaRegistration.RegisterAllAreas();
 7             GlobalConfiguration.Configure(WebApiConfig.Register);
 
 8             FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
 9             RouteConfig.RegisterRoutes(RouteTable.Routes);
10             BundleConfig.RegisterBundles(BundleTable.Bundles);
11         }
12     }

關鍵行在於

log4net.Config.XmlConfigurator.Configure(new System.IO.FileInfo(Server.MapPath("~/Web.config")));

之前試過

在AssemblyInfo.cs裡面新增[assembly: log4net.Config.XmlConfigurator(ConfigFileExtension = "config", Watch = true)]，失敗；

在WebApiConfig.cs裡面新增log4net.Config.XmlConfigurator.Configure();，失敗；

最後發現

log4net.Config.XmlConfigurator.Configure(new System.IO.FileInfo(Server.MapPath("~/Web.config")));，可行