NLog, ASP.NET Core and SQL - The server was not found or was not accessible

Question

I've been trying to get an ASP.NET Core site working with NLog. It works fine until I try to write to a SQL Database. I've tried local databases and Azure databases - all with the same problem. I've even added the nlog table to a known database, one the site already connects to (with EF).

I'm using the nuget package: NLog.Web.AspNetCore

No matter what, I get the following:

Error Error when writing to database. Exception: System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections.

Here is how I am configuring things (I have tried changing the order of these statements):

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
            loggerFactory.AddNLog();
            app.AddNLogWeb();
            env.ConfigureNLog("nlog.config");
            LogManager.Configuration.Variables["connectionString"] = Configuration.GetConnectionString("DefaultConnection");
            loggerFactory.AddConsole(Configuration.GetSection("Logging"));
            loggerFactory.AddDebug();
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                app.UseBrowserLink();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
            }
//etc...

Here is my nlog config:

<?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="Warn"
      internalLogFile="c:\temp\internal-nlog.txt">

  <!-- define various log targets -->
  <targets>
    <target name="db"
            xsi:type="Database"
            commandType="StoredProcedure"
            commandText="[dbo].[NLog_AddEntry_p]">
      <parameter name="@machineName"    layout="${machinename}" />
      <parameter name="@siteName"       layout="${iis-site-name}" />
      <parameter name="@logged"         layout="${date}" />
      <parameter name="@level"          layout="${level}" />
      <parameter name="@username"       layout="${aspnet-user-identity}" />
      <parameter name="@message"        layout="${message}" />
      <parameter name="@logger"         layout="${logger}" />
      <parameter name="@properties"     layout="${all-event-properties:separator=|}" />
      <parameter name="@serverName"     layout="${aspnet-request:serverVariable=SERVER_NAME}" />
      <parameter name="@port"           layout="${aspnet-request:serverVariable=SERVER_PORT}" />
      <parameter name="@url"            layout="${aspnet-request:serverVariable=HTTP_URL}" />
      <parameter name="@https"          layout="${when:inner=1:when='${aspnet-request:serverVariable=HTTPS}' == 'on'}${when:inner=0:when='${aspnet-request:serverVariable=HTTPS}' != 'on'}" />
      <parameter name="@serverAddress"  layout="${aspnet-request:serverVariable=LOCAL_ADDR}" />
      <parameter name="@remoteAddress"  layout="${aspnet-request:serverVariable=REMOTE_ADDR}:${aspnet-request:serverVariable=REMOTE_PORT}" />
      <parameter name="@callSite"       layout="${callsite}" />
      <parameter name="@exception"      layout="${exception:tostring}" />
    </target>
  </targets>

  <rules>
    <logger name="*" minlevel="Trace" writeTo="db" />
  </rules>
</nlog>

Last, here is my appsettings.json:

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\projectsV13;Database=XXXX;Trusted_Connection=True;MultipleActiveResultSets=true",
  },
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

Answer 1

You aren't using the variable "connectionString" in your db target.

You also need

<target name="db"
   .. 
   connectionString="${var:connectionString}"
>

Variables in NLog are not automatically bound to a target. You need to set and use them in your config.

update

Since NLog.Web.AspNetCore 4.8 (NLog.Extensions.Logging 1.4 for .NET Core console programs) you could directly read from your appSettings.json

<target name="db"
   .. 
   connectionString="${configsetting:name=ConnectionStrings.DefaultConnection}"
>

see docs