.NET Framework Standard Library Annotated Reference, Volume 1: Base Class Library and Extended Numerics Library

Example

using System;
using System.Security;
using System.Security.Permissions;
namespace Samples
{
  public class EnvironmentPermissionSample
  {
    public static void Main()
    {
      string ev = "USERNAME";
      EnvironmentPermission p =
             new EnvironmentPermission(
                     EnvironmentPermissionAccess.Read,
                     ev);
      p.Assert();
      TestAccess(ev);
      CodeAccessPermission.RevertAssert();
      p.Deny();
      TestAccess(ev);
      CodeAccessPermission.RevertDeny();
      p.PermitOnly();
      TestAccess(ev);
      TestAccess("TEMP");
      CodeAccessPermission.RevertPermitOnly();
    }
    public static void TestAccess(string s)
    {
      try
      {
         Console.WriteLine("Variable {0} Value: {1}",
                 s,
                 Environment.GetEnvironmentVariable(s));
      }
      catch(Exception)
      {
        Console.WriteLine("Variable {0} cannot be read",
                          s);
      }
    }
  }
}

The output is

 Variable USERNAME ...