Using GlobalDiagnosticsContext in NLog internalLogFile - asp.net-core

Question

Is it possible to add a custom variable to the internalLogFile attribute in the nlog.config file, like ProjectName in internalLogFile="c:\logs\${gdc:ProjectName}\internal-nlog.txt"?
I tried this:
<nlog internalLogFile="c:\logs\${gdc:ProjectName}\internal-nlog.txt">
...
</nlog>
and
private static void ConfigureLogging(IServiceCollection services)
{
GlobalDiagnosticsContext.Set("ProjectName", "some-project-name");
LogManager.LoadConfiguration("nlog.config");
services.AddLogging(loggingBuilder =>
{
loggingBuilder.AddConfiguration(Config.GetSection("Logging"));
loggingBuilder.SetMinimumLevel(Microsoft.Extensions.Logging.LogLevel.Debug);
loggingBuilder.AddNLog();
});
}
But LoadConfiguration throws an error (caught as first-chance exception in VS, so it's actually silently ignored):
System.IO.IOException: The directory name is invalid. : 'c:\logs\${gdc:ServiceRunnerFolder}'
I stepped through NLog code and it seems it only calls ExpandFilePathVariables to expand some known environment variables before setting the filename.
Is there a way to use a custom variable for this filename, without creating environment variables?

Answer 1

Never mind, I found the docs, it seems it's not possible as of NLog 5:
When you configure NLog using Configuration File, you can enable
internal logging by setting the following attribute on the
element:
internalLogFile="file.txt" - adding
internalLogFile cause NLog to write its internal debugging messages to
the specified file. This includes any exceptions that may be thrown
during logging.
Note: only a few layouts are supported, as the
internal log needs to be as stable as possible.
NLog 4.6+: Environment
Variables are also supported: e.g. %appdata% or %HOME%
NLog 4.6+:
Supports renderers (without options): ${currentdir}, ${basedir},
${tempdir}
NLog 4.7.1+: Supports renderer (without options):
${processdir}
NLog 5.0+: Supports renderers (without options):
${commonApplicationDataDir}, ${userApplicationDataDir},
${userLocalApplicationDataDir}

Answer 2

Instead of using ${gdc} and GlobalDiagnosticsContext.Set("ProjectName", "some-project-name"); then just update the InternalLogger directly:
var projectName = "some-project-name";
NLog.Common.InternalLogger.LogFile = $"c:\logs\{some-project-name}\internal-nlog.txt";
See also: https://github.com/NLog/NLog/wiki/Internal-Logging#example-with-internal-logging-from-code