Code:
using System;

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

Test Run:
martinf@monodev03:~/prj/cs/oratest> mono envshow.exe LD_LIBRARY_PATH
LD_LIBRARY_PATH=/usr/lib/oracle/xe/app/oracle/product/10.2.0/client/lib:/usr/lib/oracle/xe/app/oracle/product/10.2.0/client/lib:/usr/lib/oracle/xe/app/oracle/product/10.2.0/client/lib:/usr/lib/oracle/xe/app/oracle/product/10.2.0/client/lib: