介绍

本文写作年代比较久远，最新日志文档请查看：

.NET Core 和 ASP.NET Core 中的日志记录 | Microsoft Learn了解如何使用由 Microsoft Extension.Logging NuGet 包提供的日志记录框架。https://learn.microsoft.com/zh-cn/aspnet/core/fundamentals/logging/?view=aspnetcore-7.0

官方文档对日志部分洋洋洒洒与了一大堆，实际上对于应用程序，只要记住几个关键点就行了：

1、打开配置日志系统

2、选择提供程序

3、控制级别与分类

AspNetCore中自带了日志组件，不需要单独安装。

具体项目中，使用LoggerFactory，通过指定的ILoggerProvider来创建Logger对象。AspNetCore自带了四个ILoggerProvider的实现，分别是：

Console
Debug
EventSource
EventLog

不过，这四个都是针对Windows平台的，如果需要其他的实现，比如日志写入文本文件，就需要第三方的实现了，第三框架需要在Nuget中单独安装，具体安装步骤参考各个框架的文档。以下第三方的日志框架，可以直接链接到相应的项目网站查看文档：

elmah.io (GitHub repo)
Gelf (GitHub repo)
JSNLog (GitHub repo)
KissLog.net (GitHub repo)
Log4Net (GitHub repo)
Loggr (GitHub repo)
NLog (GitHub repo)
PLogger (GitHub repo)
Sentry (GitHub repo)
Serilog (GitHub repo)
Stackdriver (Github repo)

配置

要在AspNetCore中启用日志，需要在Setup.cs文件中进行配置，通过扩展方法创建工厂实例，并注册到IOC容器中去。

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.AddLogging(loggin=> {
                loggin.AddConsole();
                loggin.AddLog4Net();
            });
        }

这里同时还使用了Log4Net，所以还要在项目根目录下增加一个log4net.config文件：

<?xml version="1.0" encoding="utf-8" ?>
<log4net>
  <appender name="DebugAppender" type="log4net.Appender.DebugAppender" >
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
  </appender>
  <appender name="RollingFile" type="log4net.Appender.RollingFileAppender">
    <file value="logs/app.log" />
    <appendToFile value="true" />
    <maximumFileSize value="100KB" />
    <maxSizeRollBackups value="2" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date %5level %logger: %message%newline %exception" />
    </layout>
  </appender>
  <root>
    <level value="ALL"/>
    <appender-ref ref="DebugAppender" />
    <appender-ref ref="RollingFile" />
  </root>
</log4net>

使用

因为IOC中注册了日志工厂实例，接下来就可以直接通过构造函数注入到相应的代码中，这里注入的方式有两种，两种注入方式的差别在于日志分类的方式不一样。

1、直接注入泛型ILogger<T>的实例，此时使用泛型参数作为日志分类：

 public class HomeController : Controller

    {
        ILogger<HomeController> logger;

        public HomeController(ILogger<HomeController> logger)
        {
            this.logger = logger;
        }
        public IActionResult Index()
        {
            logger.LogInformation("Hello World!");
            return View();
        }
    }

2、注入工厂实例，再通过工厂得到非泛型ILogger的实例，此时使用给定的名称进行日志分类：

    public class HomeController : Controller

    {
        ILogger logger;

        public HomeController(ILoggerFactory factory)
        {
            this.logger = factory.CreateLogger("Home");
        }
        public IActionResult Index()
        {
            logger.LogInformation("Hello World!");
            return View();
        }
    }

日志级别

日志组件提供了7个级别。

LogLevel	Value	Method	Description
Trace	0	LogTrace	Contain the most detailed messages. These messages may contain sensitive app data. These messages are disabled by default and should not be enabled in production.
Debug	1	LogDebug	For debugging and development. Use with caution in production due to the high volume.
Information	2	LogInformation	Tracks the general flow of the app. May have long-term value.
Warning	3	LogWarning	For abnormal or unexpected events. Typically includes errors or conditions that don't cause the app to fail.
Error	4	LogError	For errors and exceptions that cannot be handled. These messages indicate a failure in the current operation or request, not an app-wide failure.
Critical	5	LogCritical	For failures that require immediate attention. Examples: data loss scenarios, out of disk space.
None	6		Specifies that a logging category should not write any messages.

在运行环境中使用哪个级别，需要在appsettings.json中配置：

{
  "Logging": {      // Default, all providers.
    "LogLevel": {
      "Microsoft": "Warning"
    },
    "Console": { // Console provider.
      "LogLevel": {
        "Microsoft": "Information"
      }
    }
  }
}

过滤

日志组件提供了条件输出的功能，同样需要在Setup.cs的ConfigureServices方法中进行配置，比如：

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
            services.AddLogging(loggin=> {
                loggin.AddConsole();
                loggin.AddLog4Net();

                loggin.AddFilter((provider, category, logLevel) =>
                {
                    if (provider.Contains("ConsoleLoggerProvider")
                        && category.Contains("Controller")
                        && logLevel >= LogLevel.Information)
                    {
                        return true;
                    }
                    else if (provider.Contains("ConsoleLoggerProvider")
                        && category.Contains("Microsoft")
                        && logLevel >= LogLevel.Information)
                    {
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                });
            });
            
        }

附录：ASP.NET Core 和EF Core 中的日志分类

Category	Notes
Microsoft.AspNetCore	General ASP.NET Core diagnostics.
Microsoft.AspNetCore.DataProtection	Which keys were considered, found, and used.
Microsoft.AspNetCore.HostFiltering	Hosts allowed.
Microsoft.AspNetCore.Hosting	How long HTTP requests took to complete and what time they started. Which hosting startup assemblies were loaded.
Microsoft.AspNetCore.Mvc	MVC and Razor diagnostics. Model binding, filter execution, view compilation, action selection.
Microsoft.AspNetCore.Routing	Route matching information.
Microsoft.AspNetCore.Server	Connection start, stop, and keep alive responses. HTTPS certificate information.
Microsoft.AspNetCore.StaticFiles	Files served.
Microsoft.EntityFrameworkCore	General Entity Framework Core diagnostics. Database activity and configuration, change detection, migrations.