Conditions

Use

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

Language

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

• relational operators: ==, !=, <, <=, >= and >
• boolean operators: and, or, not
• string literals which are always evaluated as layouts - '${somerenderer}'
• boolean literals - true and false
• numeric literals - 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

Functions

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.

• length(s) Returns the length of a string.

• 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.

Examples

Here are several when_filter examples with conditions:

<rules>
    <logger name="*" writeTo="file">
        <filters>
            <when condition="length(message) > 100" action="Ignore" />
            <when condition="equals('${logger}','MyApps.SomeClass')" action="Ignore" />
            <when condition="(level >= LogLevel.Debug and contains(message,'PleaseDontLogThis')) or level==LogLevel.Warn" action="Ignore" />
            <when condition="not starts-with('${message}','PleaseLogThis')" action="Ignore" />
        </filters>
    </logger>
</rules>

Extensibility

New condition functions are easy to add; just create an ordinary function and mark it with the [ConditionMethod] attribute.