NLog config file to get configuration setting values from a web.config

Updated Answer

NLog ver. 4.6 includes ${appsetting:SmtpHostServer} in the core NLog-nuget-package. No longer requires NLog.Extended. See also https://github.com/nlog/NLog/wiki/AppSetting-Layout-Renderer

NLog.Extensions.Logging ver. 1.4 includes ${configsetting} that allows one to read settings from appsettings.json. See also https://github.com/NLog/NLog/wiki/ConfigSetting-Layout-Renderer

Original Answer

Nowadays this is possible without custom code:

Use NLog.Extended and use ${appsetting:SmtpHostServer}.

See docs for ${appsetting}

Please note: this isn't supported in .NET Core / .NET standard yet.

I couldn't see any obvious way to do this other than creating my own LayoutRenderer (see below). If you're putting into your own assembly don't forget to add the following into your NLog.Config:

<extensions>
   <add assembly="YOURASSEMBLYNAMEHERE" />
</extensions>

Hope this helps someone else:

[LayoutRenderer("aspnet-config")]
public class AspNetConfigValueLayoutRenderer : LayoutRenderer
{
    [DefaultParameter]
    public string Variable
    {
        get;
        set;
    }

    protected override void Append(StringBuilder builder, LogEventInfo logEvent)
    {
        if (this.Variable == null)
        {
            return;
        }
        HttpContext context = HttpContext.Current;
        if (context == null)
        {
            return;
        }
        builder.Append(Convert.ToString(System.Configuration.ConfigurationManager.AppSettings[this.Variable], CultureInfo.InvariantCulture));
    }


}