Code:
using System;

public class Foo
{
        static void Main(string []args)
        {
                Console.WriteLine("{0}={1}", args[0],
                System.Environment.ExpandEnvironmentVariables(args[0])
);
        }
}

Sanity check:
martinf@monodev03:~/prj/cs/oratest> echo $DISPLAY
:0.0

Test Run:
martinf@monodev03:~/prj/cs/oratest> mono envshow.exe DISPLAY
DISPLAY=DISPLAY