How to refer from NLog to my connection string in app.config

Question

I have a .net core 3.1 Console project. I'm trying to set it up to use NLog. I use 2 targets, db and file. I can write to the file but can't to the db. I'm sure the connection string is correct because I use it in other projects but they were written in 4.7 framework.

This is my app.config setting:

<connectionStrings>
  <add name="NLog" connectionString="Data Source=mySource;Initial Catalog=MyLogging;Integrated Security=SSPI;" />
</connectionStrings>

and this is my attempt to set it up in the target:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"
      internalLogLevel="Info"
      internalLogFile="c:\temp\nlog-internal.log">

  <!-- enable asp.net core layout renderers -->
  <extensions>
    <add assembly="NLog.Web.AspNetCore"/>
  </extensions>

  <!-- the targets to write to -->
  <targets>  
    <target name="database" xsi:type="Database" connectionString="${configsetting:name=NLog}" commandType="StoredProcedure" commandText="[dbo].[MyLogEntry]">
    </target>   
  </targets>
  <rules>
    <logger name="*" minlevel="Trace" writeTo="Database" />
  </rules>
</nlog>

and the error message from the c:\temp\nlog-internal.log is:

2021-02-10 16:11:48.3381 Error DatabaseTarget(Name=database): Error when writing to database. Exception: System.InvalidOperationException: The ConnectionString property has not been initialized.
at System.Data.SqlClient.SqlConnection.PermissionDemand()
at System.Data.SqlClient.SqlConnectionFactory.PermissionDemand(DbConnection outerConnection)
at System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
at System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)
at System.Data.SqlClient.SqlConnection.Open()
at NLog.Targets.DatabaseTarget.OpenConnection(String connectionString, LogEventInfo logEventInfo)
at NLog.Targets.DatabaseTarget.EnsureConnectionOpen(String connectionString, LogEventInfo logEventInfo)
at NLog.Targets.DatabaseTarget.WriteLogEventToDatabase(LogEventInfo logEvent, String connectionString)

is the problem at the: connectionString="${configsetting:name=NLog}"

Answer 1

The app.config isn't really suitable in ASP.NET Core. It's possible, but it's dirty.

It's recommend to move to JSON application settings, like appsettings.json (see Configuration in ASP.NET Core)

For example:

appsettings.json:

{
    "ConnectionStrings":{
        "NLog":"Data Source=mySource;Initial Catalog=MyLogging;Integrated Security=SSPI;",
    }
}

and usages: ${configsetting:item=ConnectionStrings.NLog}

Please note, for ${configsetting} you need one of these packages:

• NLog.Extensions.Logging,
• NLog.Extensions.Hosting and/or
• NLog.Web.AspNetCore

See https://nlog-project.org/config/?tab=layout-renderers&search=configsetting