Custom Logging with Personal Fields in Nlog

Setting the database string via

var databaseTarget = (DatabaseTarget)LogManager.Configuration.FindTargetByName("database");
databaseTarget.ConnectionString = connectionString;
LogManager.ReconfigExistingLoggers();

The above didn't work but I used the example below where I set it via the program's start.

var defaultConnection = Configuration.GetConnectionString("DefaultConnection");
NLog.GlobalDiagnosticsContext.Set("defaultConnection", defaultConnection);
var logger = LogManager.LoadConfiguration("nlog.config").GetCurrentClassLogger();

Then you can use ${gdc:item=defaultConnection} in your nlog.config:

<target xsi:type="Database" connectionString="${gdc:item=defaultConnection}" />

Rather than using GDC, which is for global static data and fails on concurrent logging, it is better to use EventProperties-Layout-Renderer which allows to pass of custom  properties specific to the event

The easiest way of overriding connection string is through the use of GDC:

<target name="db" type="Database" connectionString="${gdc:myConnectionstring}" ... />

Now in your code you can simply do:

GDC.Set("myConnectionString", "Server=.;database=.....");

For Custom Logging with the following...

LogEventInfo theEvent = new LogEventInfo(logLevel, "", message);
theEvent.Properties["OrderId"] =orderId;`

log.Log(theEvent);

... and in your NLog.config file: 
${event-context:item=OrderId}  -- obsolete
${event-properties:item=OrderId} -- renders OrderId

https://github.com/nlog/NLog/wiki/Database-target

<nlog throwExceptions="true" xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <targets>
      <target name="default" xsi:type="File" fileName="logs/app-log.txt" archiveFileName="logs/archives/app-log.{#}.txt" archiveEvery="Day" archiveNumbering="Rolling" maxArchiveFiles="7" />
      <target name="database" xsi:type="Database" dbProvider="System.Data.SqlClient"
        commandText="INSERT INTO [dbo].[AppLog] 
        ([database_name],[app_name], [version], [message_type], [message], [routine], [src_file], [src_line], [username], [computer_name])                   VALUES                                       ('300', @app_name, @version, @message_type,  @message, @routine , @src_file, @src_line, @username, @computer_name)">
        <parameter name="@app_name" layout="${processname}" />
        <parameter name="@version" layout="${assembly-version}" />
        <parameter name="@message_type" layout="${level}" />
        <parameter name="@message" layout="${message}" />
        <parameter name="@routine" layout="${callsite:className=true:includeSourcePath=true:methodName=true}" />
        <parameter name="@src_file" layout="${currentdir}" />
        <parameter name="@src_line" layout="${callsite-linenumber}" />
        <parameter name="@username" layout="${environment-user}" />
        <parameter name="@computer_name" layout="${machinename}" />
      </target>
    </targets>
    <rules>
      <logger name="*" writeTo="database" />
    </rules>
  </nlog>

Doing the rest...

1.         <target name="eventlog" xsi:type="EventLog" layout="${message}" log="Application" source=" My Custom Api Services" />  
2.         <target name="database" type="Database" connectionString="Data Source=your sql source;initial catalog=YourDbNameDb;user id=u1;password=p1;MultipleActiveResultSets=True;">  
3. <rules>  
4.         <!-- I am adding my 3 logging rules here -->  
5.         <logger name="*" minlevel="Debug" writeTo="database" />  
6.         <logger name="*" minlevel="Trace" writeTo="logfile" />  
7.         <logger name="*" minlevel="Trace" writeTo="eventlog" />  
8.     </rules>