使用Log4Net动态发送UDP日志

在Log4Net的配置文件中可以添加多个UdpAppender节点，用来向不同的UDP服务器发送日志。

但是，如果需要动态接收UDP日志呢，每个未知的客户端都可以接收Log4Net的UDP日志，那么就需要在软件中开启一个监听UDP消息的线程，接收到UDP主机的IP和端口后，动态添加UDP Server信息到Log4Net的配置中，这样Log4Net就能同时向多个UDP Server发送日志了。

由于是动态添加UDP Server，所以Log4Net的配置文件中就可以没有UdpAppender节点了（也可以配置几个固定的IP、端口），下面上主要代码。

1、软件中监听UDP Server消息的线程

/// <summary>
/// UDP Server发现线程回调函数
/// </summary>
private void RecvUdpServerMessage()
{
	IPEndPoint remoteIp = new IPEndPoint(IPAddress.Any, 0);
	while (true)
	{
		try
		{
			byte[] bytRecv = _udpRecv.Receive(ref remoteIp);
			string message = Encoding.UTF8.GetString(bytRecv, 0, bytRecv.Length);
			message = message.Trim();   //类似"UDPServer:2018"
			if (!message.StartsWith("UDPServer"))
			{
				continue;
			}

			var index = message.IndexOf(":", StringComparison.CurrentCulture);
			if (index < 0)
			{
				continue;
			}

			//解析UDP端口
			message = message.Substring(index + 1);
			if (!int.TryParse(message, out var port))
			{
				continue;
			}

			remoteIp.Port = port;
			UdpServerDiscovered?.Invoke(remoteIp);
		}
		catch (Exception e)
		{
			Console.WriteLine(e.Message);
			break;
		}
	}
}

2、UDP发现线程接收到消息后动态添加UdpAppender

/// <summary>
/// UDPServer发现回调
/// </summary>
/// <param name="remoteIp">远程UDP服务器IP信息对象</param>
private void OnUdpServerDiscovered(IPEndPoint remoteIp)
{
	if (!_listUdpServer.Contains(remoteIp))
	{
		try
		{
			//创建UdpAppender
			var appender = new log4net.Appender.UdpAppender()
			{
				Name = remoteIp.ToString(),
				LocalPort = _localUdpPort++,
				Encoding = Encoding.UTF8,
				Layout = new log4net.Layout.PatternLayout(log4net.Layout.PatternLayout.DetailConversionPattern),
				RemoteAddress = remoteIp.Address,
				RemotePort = remoteIp.Port
			};
			appender.AddFilter(new log4net.Filter.LevelRangeFilter()
			{
				LevelMin = log4net.Core.Level.Debug,
				LevelMax = log4net.Core.Level.Fatal
			});
			appender.ActivateOptions();

			//添加UdpAppender
			var root = ((log4net.Repository.Hierarchy.Hierarchy)log4net.LogManager.GetRepository()).Root;
			root.CloseNestedAppenders();
			root.AddAppender(appender);

			_listUdpServer.Add(remoteIp);

			UdpServerDiscovered?.Invoke(_listUdpServer);
		}
		catch (Exception e)
		{
			Console.WriteLine(e.Message);
		}
	}
}