Extending NLog

Create custom Target

using NLog;
using NLog.Config;
using NLog.Targets;

namespace MyNamespace 
{ 
    
    [Target("MyFirst")] 
    public sealed class MyFirstTarget: TargetWithLayout  //or inherit from Target
    { 
        public MyFirstTarget()
        {
            //set defaults
            this.Host = "localhost";
        }

        [RequiredParameter] 
        public string Host { get; set; }

        protected override void Write(LogEventInfo logEvent) 
        { 
            string logMessage = this.Layout.Render(logEvent); 

            //TODO write to target
        } 
    } 
}

ConfigurationItemFactory
          .Default
          .Targets
          .RegisterDefinition("MyFirst", typeof(MyNamespace.MyFirstTarget));

Usage:

<?xml version="1.0" ?>
<nlog xmlns="https://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance">

    <targets>
       <target name="target1" type="MyFirst" Host="somehost.com" />
    </targets>

    <rules>
        <logger name="*" minlevel="Debug" writeTo="target1" />
    </rules>
</nlog>

Create custom Layout Renderer

[LayoutRenderer("hello-world")]
public class HelloWorldLayoutRenderer : LayoutRenderer
{

    /// <summary>
    /// I'm option and not required or default
    /// </summary>
    public string Config1 { get; set; }

    /// <summary>
    /// I'm required option. And error will be thrown if not set.
    /// </summary>
    [RequiredParameter]
    public string Config2 { get; set; }

    /// <summary>
    /// I'm the default parameter.
    /// The first parameter value (without name) will be set to this one
    /// You can set me as required also. 
    /// </summary>
    [DefaultParameter]
    public bool Caps {get;set;}

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
        //TODO use options 
        builder.Append("hello world!");
    }
}

//register under "hello-world"
ConfigurationItemFactory.Default.LayoutRenderers
                        .RegisterDefinition("hello-world", typeof(MyNamespace.HelloWorldLayoutRenderer ));

Usage

${hello-world} - raises exception: required parameter Config2 isn't set
${hello-world:Config2=abc} - OK, Config2 property set
${hello-world:true:config2=abc} - default parameter (Caps) set to true
${hello-world:true:config2=abc:config1=yes} - all the three properties set.