using System;
using System.Data.SqlClient;
using log4net.Appender;
using log4net.Core;

public sealed class CustomDbAppender : IAppender, IOptionHandler
{
    private SqlConnection dbConnection;

    public string Name { get; set; }

    public string ConnectionString { get; set; }

    public void ActivateOptions()
    {
        dbConnection = new SqlConnection(ConnectionString);
        dbConnection.Open();
    }

    public void Close()
    {
        if (dbConnection != null)
        {
            dbConnection.Close();
        }
    }

    public void DoAppend(LoggingEvent loggingEvent)
    {
        SqlCommand command = dbConnection.CreateCommand();
        command.CommandText = "INSERT INTO Log ([Date],[Thread],[Level],[Logger],[Message],[User],[Guid]) VALUES (@Date, @Thread, @Level, @Logger, @Message, @User,@Guid)";
        command.Parameters.AddWithValue("@Date", loggingEvent.TimeStamp);
        command.Parameters.AddWithValue("@Thread", loggingEvent.ThreadName);
        command.Parameters.AddWithValue("@Level", loggingEvent.Level.Name);
        command.Parameters.AddWithValue("@Logger", loggingEvent.LoggerName);
        command.Parameters.AddWithValue("@Message", loggingEvent.RenderedMessage);
        command.Parameters.AddWithValue("@User", loggingEvent.UserName);
        command.Parameters.AddWithValue("@Guid", Guid.NewGuid());
        command.ExecuteNonQuery();
    }
}

CREATE TABLE [dbo].[Log](
	[Id] [int] IDENTITY( 1 , 1 ) NOT NULL,
	[Date] [datetime] NOT NULL,
	[Thread] [varchar]( 255 ) COLLATE Cyrillic_General_CI_AS NOT NULL,
	[Level] [varchar]( 50 ) COLLATE Cyrillic_General_CI_AS NOT NULL,
	[Logger] [varchar]( 255 ) COLLATE Cyrillic_General_CI_AS NOT NULL,
	[User] [varchar]( 50 ) COLLATE Cyrillic_General_CI_AS NULL,
	[Guid] [uniqueidentifier] NULL,
	[Message] [varchar]( 4000 ) COLLATE Cyrillic_General_CI_AS NOT NULL
) ON [PRIMARY]

void Application_Start(object sender, EventArgs e) 
{
    log4net.Config.XmlConfigurator.Configure();        
}

<?xml version="1.0"?>
<configuration>
  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,Log4net"/>
  </configSections>

  <log4net>
    <logger name="MyProject">
      <appender-ref ref="AdoNetAppender" />
    </logger>
    <appender name="AdoNetAppender" type="CustomDbAppender">
      <bufferSize value="1" />
      <connectionString value="Server=.;Database=Test;Trusted_Connection=Yes;" />
    </appender>
  </log4net>

  <connectionStrings />
  <appSettings/>
  
  <system.web>
    <customErrors mode="Off">
    </customErrors>
    <compilation debug="true" targetFramework="4.0"/>
    <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID"/>
  </system.web>
  
</configuration>

ILog log = LogManager.GetLogger("MyProject");
   
log.Debug("Отладка");
log.Info("Информация");
log.Warn("Предупреждение");
log.Error("Ошибка");
log.Fatal("Критическая ошибка");

<?xml version="1.0"?>
<configuration>
  <configSections>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,Log4net"/>
  </configSections>

  <log4net>
    <logger name="MyProject">
      <appender-ref ref="AdoNetAppender" />
    </logger>
    <appender name="AdoNetAppender" type="log4net.Appender.AdoNetAppender">
      <bufferSize value="1" />
      <connectionType value="System.Data.SqlClient.SqlConnection" />
      <connectionString value="Server=.;Database=test;Trusted_Connection=Yes;" />
      <commandText value="INSERT INTO Log ([Date],[Thread],[Level],[Logger],[Message],[User]) VALUES (@log_date, @thread, @log_level, @logger, @message, @user)" />
      <parameter>
        <parameterName value="@log_date" />
        <dbType value="DateTime" />
        <layout type="log4net.Layout.PatternLayout" value="%date{yyyy'-'MM'-'dd HH':'mm':'ss'.'fff}" />
      </parameter>
      <parameter>
        <parameterName value="@thread" />
        <dbType value="String" />
        <size value="255" />
        <layout type="log4net.Layout.PatternLayout" value="%thread" />
      </parameter>
      <parameter>
        <parameterName value="@log_level" />
        <dbType value="String" />
        <size value="50" />
        <layout type="log4net.Layout.PatternLayout" value="%level" />
      </parameter>
      <parameter>
        <parameterName value="@logger" />
        <dbType value="String" />
        <size value="255" />
        <layout type="log4net.Layout.PatternLayout" value="%logger" />
      </parameter>
      <parameter>
        <parameterName value="@message" />
        <dbType value="String" />
        <size value="4000" />
        <layout type="log4net.Layout.PatternLayout" value="%message" />
      </parameter>
      <parameter>
        <parameterName value="@user" />
        <dbType value="String" />
        <size value="50" />
        <layout type="log4net.Layout.PatternLayout">
          <conversionPattern value="%X{user}" />
        </layout>
      </parameter>
    </appender>
  </log4net>

  <connectionStrings />

  <appSettings/>
  
  <system.web>
    <customErrors mode="Off">
    </customErrors>
    <compilation debug="true" targetFramework="4.0"/>
    <pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID"/>
  </system.web>
  
</configuration>

CREATE TABLE [dbo].[Log] 
( 
	[Id] [int] IDENTITY ( 1 ,  1 ) NOT NULL, 
	[Date] [datetime] NOT NULL, 
	[Thread] [varchar] ( 255 ) NOT NULL, 
	[Level] [varchar] ( 50 ) NOT NULL, 
	[Logger] [varchar] ( 255 ) NOT NULL,
        [User] [varchar] ( 50 ) NULL, 
	[Message] [varchar] ( 4000 ) NOT NULL
)