Properties constraints for custom extensions

Custom Targets, layout etc. could have properties. When configuring from the XML config, there are some limitations on the types.

Supported types

Supported types for Targets, Layouts and Layout renderers:

• .NET primitive types: e.g. bool, char, decimal, double, float, int, uint, etc.
• Enums (use short name in the XML)
• Encoding
• CultureInfo
• Type
• LineEndingMode
• Uri
• NLog types: Layout, SimpleLayout & ConditionExpression
• Types which has an implicit conversion from string
• Types which are using TypeDescriptor from string
• Collections, introduced in NLog 4.4. See section Collection types

Collection types

Introduced in NLog 4.4, collection types could be used.

Usage in XML: comma separated string. If the value contains a comma, single quote the whole value.

Examples:

• value="one arg"
• value="1,2"
• value="value1,'value2, with comma'"

Supported types:

• IList<T> / IList
• IEnumerable<T> / IEnumerable
• ISet<T> / HashSet<T>

with the following types:

• .NET built in types (string, int, double, object)
• enums
• culture, encoding, Type
• not supported: Layout

Not supported:

• Arrays
• Non-generic List
• Non-gereric IList
• Custom class implementing/inheriting types listed above (because of performance)

PS: .NET 3.5 hasn't ISet<T>, so use HashSet<T>

XML Elements

For targets and layout renderers there is also support for XML elements.

E.g. the JSON layout:

<target name="jsonFile" xsi:type="File" fileName="${logFileNamePrefix}.json">
      <layout xsi:type="JsonLayout">
              <attribute name="time" layout="${longdate}" />
              <attribute name="level" layout="${level:upperCase=true}"/>
              <attribute name="message" layout="${message}" />
       </layout>
</target>

Usage in C#

[ArrayParameter(typeof(JsonAttribute), "attribute")]
public IList<JsonAttribute> Attributes { get; private set; }

[NLogConfigurationItem]
public class JsonAttribute {} //needs default ctor

Another example is the Database target