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Showing content from https://github.com/NLog/NLog/wiki/When-filter below:

When Filter · NLog/NLog Wiki · GitHub

Filter events in the config.

Platforms Supported: All

<rules>
  <logger ... >
    <filters defaultAction="Neutral | Ignore | Log | LogFinal | IgnoreFinal">
      <when condition="Condition" action="Enum"/>
    </filters>
  </logger>
</rules>

defaultAction - default filter action when no match. Default Ignore.

Introduced with NLog 4.6. Before NLog 5.0 the default was Neutral.
- Ignore - The message should not be logged.
- IgnoreFinal - The message should not be logged and ignore any following logging-rules.
- Log - The message should be logged.
- LogFinal - The message should be logged and ignore any following logging-rules.
- Neutral - No decision
filterdefaultaction - Alternative name for defaultAction to match actual API.

Introduced with NLog 5.0

condition - Condition expression. Condition Required. See section Conditions below.
action - Action to be taken when filter matches. Required.
Possible values:
- Ignore - The message should not be logged.
- IgnoreFinal - The message should not be logged and ignore any following logging-rules.
- Log - The message should be logged.
- LogFinal - The message should be logged and ignore any following logging-rules.
- Neutral - The filter doesn't want to decide whether to log or discard the message.

Conditions are expressions used with the <when> filter. They consist of one or more tests. They are used in the filter to determine if an action will be taken.

message - LogEvent formatted message
logger - Logger name
level - LogLevel object
exception - Exception object (Introduced with NLog 5.0)
${layout} - All available layout-options (except stacktrace).

The filter expressions are written in a special mini-language. The language consists of:

relational operators: ==, !=, <, <=, >= and >
Note: Some predefined XML characters may need to be escaped. For example, if you try to use the '<' character, the XML parser will interpret it as an opening tag which results in an error in the configuration file. Instead, use the escaped version of '<' (<) in this context.
boolean operators: and, or, not
string literals which are always evaluated as layouts - ${somerenderer}
boolean literals - true and false
numeric literals - e.g. 12345 (integer literal) and 12345.678 (floating point literal)
log level literals - LogLevel.Trace, LogLevel.Debug, ... LogLevel.Fatal
predefined keywords to access the most common log event properties - level, message and logger
braces - to override default priorities and group expressions together
condition functions - to perform string and object tests
Single quotes should be escaped with another single quote.

The following condition functions are available:

contains(s1,s2) Determines whether the second string is a substring of the first one. Returns: true when the second string is a substring of the first string, false otherwise.
ends-with(s1,s2) Determines whether the second string is a suffix of the first one. Returns: true when the second string is a prefix of the first string, false otherwise.
equals(o1,o2) Compares two objects for equality. Returns: true when two objects are equal, false otherwise.

Notice that 'o1' == 'o2' or 'o1' != 'o2' will be faster and reduces memory allocations.
length(s) Returns the length of a string.

Notice that 's' != '' is faster than length('s') > 0, and reduces memory allocation.
starts-with(s1,s2) Determines whether the second string is a prefix of the first one. Returns: true when the second string is a prefix of the first string, false otherwise.
regex-matches(input, pattern, options) Introduced in NLog 4.5. Indicates whether the regular expression pattern finds a match in the specified input string. options is an optional comma separated list of values from the RegexOptions enumeration.
Returns : true when a match is found in the input string, false otherwise.
Example : regex-matches('${message}', '^foo$', 'ignorecase,singleline')

Single quotes should be escaped with another single quote. Example:

contains('${message}', 'Cannot insert the value NULL into column ''Col1')

New custom condition functions methods can also be added.

NLog 4.7 allows you to register your own condition methods using LogFactory.Setup(). Where a lambda can be registered like this:

LogManager.Setup().SetupExtensions(s => 
   s.RegisterConditionMethod("hasParameters", evt => evt.Parameters?.Length > 0)
);

And a static method like this:

LogManager.Setup().SetupExtensions(s =>
   s.RegisterConditionMethod("hasPriority", typeof(NLogConditionMethods).GetMethod("HasPriority", BindingFlags.Static))
);

Alternative way of adding custom condition methods is creating a public static class with a static function and mark the class and method with the attributes [ConditionMethods] and [ConditionMethod] respectively.

You can find a sample implementation of a custom filter here

Then you have to tell NLog where to find your assembly (Ex. NLog.ConditionMethodsAssembly.dll)

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.netfx35.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" >
    <extensions>
		<add assembly="NLog.ConditionMethodsAssembly" />
    </extensions>
   ...
<nlog>

Here are several examples with conditions:

<rules>
    <logger name="*" minLevel="Debug" writeTo="file">
        <filters defaultAction="Log">
            <when condition="exception != null" action="Log" />
            <when condition="length(message) > 100" action="Ignore" />
            <when condition="'${OnHasProperties:1}' != ''" action="Ignore" />
            <when condition="logger == 'MyApps.SomeClass'" action="Ignore" />
            <when condition="(level >= LogLevel.Debug and contains(message, 'PleaseDontLogThis')) or level==LogLevel.Warn" action="Ignore" />
            <when condition="not starts-with(message, 'PleaseLogThis')" action="Ignore" />
            <when condition="contains(message, '&quot;Bob&quot;')" action="Ignore" />  <!-- "Bob" -->
        </filters>
    </logger>
</rules>

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