NLog入门

将定义log输出到file.txt文件。

2. 在`<rules>`标签，添加：
```xml
<logger name="*" minlevel="Info" writeTo="logfile" />

这个代码将会把名字为”*” leve高于Info(Info, Warn, Error and Fatal)的所有log写入logfile。

完整的配置文件是这个样子的:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 
    <targets>
        <target name="logfile" xsi:type="File" fileName="file.txt" />
    </targets>
 
    <rules>
        <logger name="*" minlevel="Info" writeTo="logfile" />
    </rules>
</nlog>

当你运行程序的时候，log信息将会被写入file.txt文件内。

多个目标(输出到多个文件)

让我们试一下更复杂的东西。假如你想把很详细的信息写入一个文件，并且还想在控制台窗口看比较简略的信息。下述配置可以满足要求:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 
    <targets>
        <target name="logfile" xsi:type="File" fileName="file.txt" />
        <target name="console" xsi:type="Console" />
    </targets>
 
    <rules>
        <logger name="*" minlevel="Trace" writeTo="logfile" />
        <logger name="*" minlevel="Info" writeTo="console" />
    </rules>
</nlog>

特定Logger设置

另一种常见的情景是，需要某些组件，如当前开发的组件产生更详细的log信息，但是减少其他组件的输出信息，可以用如下配置:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 
    <targets>
        <target name="logfile" xsi:type="File" fileName="file.txt" />
    </targets>
 
    <rules>
        <logger name="SomeNamespace.Component.*" minlevel="Trace" writeTo="logfile" final="true" />
        <logger name="*" minlevel="Info" writeTo="logfile" />
    </rules>
</nlog>

第一个规则：名字以SomeNamespace.Component开头level是Trace或更高的loggers将写log信息到logfile。属性final="true"表示与制定logger相关的log在写入文件后，将不在执行后续配置的操作。

第二个规则：剩余的logs，如果level高于Info将写入logfile。

包装器Wrappers

NLog支持特殊类型的目标，这类目标不作任何记录，但是修改其他logger的动作。这些目标被称为包装器/wrappers。最常用的有:

• ImpersonatingWrapper(ImpersonatingWrapper-target) - Impersonates another user for the duration of the write.
• AsyncWrapper(AsyncWrapper-target) - Provides asynchronous, buffered execution of target writes.
• FallbackGroup(FallbackGroup-target) - Provides fallback-on-error.
• FilteringWrapper(FilteringWrapper-target) - Filters log entries based on a condition.

更多的包装器请参考官网帮助。

包装器使用target标签并包含一个<target />元素，具体如下:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 
    <targets>
        <target name="asyncFile" xsi:type="AsyncWrapper">
            <target name="logfile" xsi:type="File" fileName="file.txt" />
        </target>
    </targets>
 
    <rules>
        <logger name="*" minlevel="Info" writeTo="asyncFile" />
    </rules>
</nlog>

上述配置使用了异步包装器，将log异步写入文件，可以改善调用logger线程的相应速度。

###Layouts Layouts提供了log内容写入文件时格式化内容的方法。有两种layouts:

• 简单layout - which is composed of Layout Renderers
• 结构化layouts - 可以输出XML, CSV, 以及其他复杂的格式。

简单layout用${ 和 }之间的内容表示。例如，下述配置表示每个log信息前添加yyyyMMddHHmmss格式的日期和时间:

<target name="logfile" xsi:type="File" fileName="file.txt" layout="${date:format=yyyyMMddHHmmss} ${message}" />

NLog高级

###Expose logger to sub-classes When we wish to expose the logger into sub classes a the following pattern could be used.

class BaseClass
{      
    protected BaseClass()
    {
        Log = LogManager.GetLogger(GetType().FullName);
    }

    protected Logger Log { get; private set; }
}

class ExactClass : BaseClass
{
    public ExactClass() : base() { }
    ...
}

The LogManager.GetLogger() method should NOT be invoked in the constructor of the BaseClass with Type parameter argument i.e.

protected BaseClass()
{
    Log = LogManager.GetLogger(GetType().FullName, GetType());
}

as an exception will be thrown.

In case of the ExactClass has a default constructor which invokes the BaseClass constructor a System.StackOverflowException is thrown. This is because the ExactClass calls the BaseClass the GetLogger(String, Type) attempts to construct a ExactClass until the exception is thrown.

ExactClass => BaseClass => ExactClass => BaseClass => ...

When there is no default constructor at the ExactClass the GetLogger(String, Type) method call does not know how to construct the ExactClass and it fails with a NLog.NLogConfigurationException.

参考文章

nlog tutorial