1、安装引用log4net

使用log4net需要安装引用log4net，若是 ASP.NET Core 项目，则需要安装 Microsoft.Extensions.Logging.Log4Net.AspNetCore。

1）log4net

在Nuget管理程序中，搜索 "log4net"，然后点击安装。

2）Microsoft.Extensions.Logging.Log4Net.AspNetCore

在Nuget管理程序中，搜索 "Microsoft.Extensions.Logging.Log4Net.AspNetCore"，然后点击安装。

相关文档：VS(Visual Studio)中Nuget的使用

2、log4net 配置文件

log4net 中配置文件可以使用XML格式的， 具体如下,

1）配置说明

2）Appender

3）log4net.Appender.RollingFileAppender

<?xml version="1.0" encoding="utf-8"?>
<log4net>

	<appender name="rollingAppender" type="log4net.Appender.RollingFileAppender">
		<file value="log4\app.log" />
	
		<appendToFile value="true" />
		<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />

		<!-- 可以为:Once|Size|Date|Composite -->
		<!-- Composite为Size和Date的组合 -->
		<rollingStyle value="Composite" />

		<!-- 当备份文件时,为文件名加的后缀 -->
		<datePattern value="yyyyMMdd.TXT" />

		<!-- 日志最大个数,都是最新的 -->
		<!-- rollingStyle节点为Size时,只能有value个日志 -->
		<!-- rollingStyle节点为Composite时,每天有value个日志 -->
		<maxSizeRollBackups value="20" />

		<!-- 可用的单位:KB|MB|GB -->
		<maximumFileSize value="3MB" />

		<!-- true时当前最新日志文件名永远为file配置项中的名字 -->
		<staticLogFileName value="true" />

		<!-- 输出级别的日志 -->
		<filter type="log4net.Filter.LevelRangeFilter">
			<param name="LevelMin" value="ALL" />
			<param name="LevelMax" value="FATAL" />
		</filter>
		<layout type="log4net.Layout.PatternLayout">
			<conversionPattern value="%date [%thread] %-5level %logger - %message%newline"/>
		</layout>

		<layout type="log4net.Layout.PatternLayout">
			<conversionPattern value="%date [%thread] %-5level %logger - %message%newline"/>
		</layout>

	</appender>
	<root>
		<!-- 控制级别，由低到高: ALL|DEBUG|INFO|WARN|ERROR|FATAL|OFF -->
		<!-- OFF:0 -->
		<!-- FATAL:FATAL -->
		<!-- ERROR: ERROR,FATAL -->
		<!-- WARN: WARN,ERROR,FATAL -->
		<!-- INFO: INFO,WARN,ERROR,FATAL -->
		<!-- DEBUG: INFO,WARN,ERROR,FATAL -->
		<!-- ALL: DEBUG,INFO,WARN,ERROR,FATAL -->
		<priority value="ALL"/>

		<level value="INFO"/>
		<appender-ref ref="rollingAppender" />
	</root>
</log4net>

2）log4net.Appender.ConsoleAppender

<?xml version="1.0" encoding="utf-8"?>
<log4net>

  <appender name="ConsoleAppender" type="log4net.Appender.ConsoleAppender">
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
  </appender>

	<root>
		<!-- 控制级别，由低到高: ALL|DEBUG|INFO|WARN|ERROR|FATAL|OFF -->
		<!-- OFF:0 -->
		<!-- FATAL:FATAL -->
		<!-- ERROR: ERROR,FATAL -->
		<!-- WARN: WARN,ERROR,FATAL -->
		<!-- INFO: INFO,WARN,ERROR,FATAL -->
		<!-- DEBUG: INFO,WARN,ERROR,FATAL -->
		<!-- ALL: DEBUG,INFO,WARN,ERROR,FATAL -->
		<priority value="ALL"/>

		<level value="INFO"/>
		<appender-ref ref="ConsoleAppender" />
	</root>
</log4net>

3）log4net.Appender.SmtpAppender

<?xml version="1.0" encoding="utf-8"?>
<log4net>

  <appender name="SmtpAppender" type="log4net.Appender.SmtpAppender">
    <to value="to@example.com" />
    <from value="from@example.com" />
    <subject value="Log4Net Error" />
    <smtpHost value="smtp.example.com" />
    <bufferSize value="512" />
    <lossy value="false" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
  </appender>

	<root>

		<!-- 控制级别，由低到高: ALL|DEBUG|INFO|WARN|ERROR|FATAL|OFF -->
		<!-- OFF:0 -->
		<!-- FATAL:FATAL -->
		<!-- ERROR: ERROR,FATAL -->
		<!-- WARN: WARN,ERROR,FATAL -->
		<!-- INFO: INFO,WARN,ERROR,FATAL -->
		<!-- DEBUG: INFO,WARN,ERROR,FATAL -->
		<!-- ALL: DEBUG,INFO,WARN,ERROR,FATAL -->
		<priority value="ALL"/>

		<level value="INFO"/>
		<appender-ref ref="log4net.Appender.SmtpAppender" />
	</root>
</log4net>

3、log4net配置及使用示例

下面是log4net的配置文件配置及使用，如下，

using Microsoft.OpenApi.Models;

using NLog.Web;

var builder = WebApplication.CreateBuilder(args);

// 配置log4net

builder.Logging.AddLog4Net("log4net.Config");
builder.Services.AddControllers();


app.UseDefaultFiles();
app.UseStaticFiles();
app.MapControllers();

app.Run();

使用示例：

using Microsoft.AspNetCore.Mvc;
using UtilityWeb.API.Services;
using UtilityWeb.Core;

using UtilityWeb.Tools;

namespace UtilityWeb.API.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class WeatherForecastController : ControllerBase
    {
        private static readonly string[] Summaries = new[]
        {
        "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
    };

        private readonly ILogger<WeatherForecastController> _logger;


        public WeatherForecastController(ILogger<WeatherForecastController> logger)
        {
            _logger = logger;
        }

        [HttpGet(Name = "GetWeatherForecast")]
        public IEnumerable<string> Get([FromServices] UtilityWebContext context,
            [FromServices] IUser userService,
            [FromServices] SwaggerService swaggerService)
        {
            _logger.LogError("error message");
            return new List<string>() { _logger.ToString() };
            
        }
    }
}

4、log4net 与NLog 对比

相关文档：

.NET 6 配置使用NLog日志框架的方法及示例代码

.NET 6 (C#) Serilog 日志框架的使用

.NET 6 (C#) Microsoft.Extensions.Logging 日志框架的使用