How to make an input parameter/flag default to a named Environment Variable?

[x10an14]

As the title says;

• I want a flag/input parameter named --foo-bar. (No probs there).
• I want --help to show that --foo-bar gets its default value from environment variable FOO_BAR.
  (Maybe also show the value of the current envvar FOO_BAR if it exists in current shell/machine)?

[moh-hassan]

All properties of OptionAttribute are constant and can't be modified at run-time.
But you can add dynamic contents to help text before /or after options which can be computed using a function at runtime.

           var helpText= HelpText.AutoBuild(result, h =>
		 {
		     h.AddPostOptionsLine(dynamicData());	//add post options dynamic content	
		    return h;
		 } , e=>e);	

       static Func<string> dynamicData = () =>{
	   var header ="-------Environment Variables----------";
	   var FOO_BAR=Environment.GetEnvironmentVariable("FOO_BAR");
	  return $"{header}\nFOO_BAR={FOO_BAR}";
	};

output:

myapp.exe 1.0.0.0
Copyright (C) 2019 global.com

  -r, --read         Input files to be processed.
  --verbose          (Default: false) Prints all messages to standard output.
  --stdin            (Default: false) Read from stdin
  --help             Display this help screen.
  --version          Display version information.
  offset (pos. 0)    File offset.

-------Environment Variables----------
FOO_BAR=server123

For more details of custom help see HelpText-Configuration