Check if a user is in an AD group.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.DirectoryServices;

using System.DirectoryServices.AccountManagement;

using System.Security.Principal;

namespace test_adgroup

{

   class Program

    {

       static void Main(string[] args)

        {

           string username = "cykill";

           //string username = WindowsIdentity.GetCurrent().Name;

           PrincipalContext pc = new PrincipalContext(ContextType.Domain,"mydomainname");

           UserPrincipal up = UserPrincipal.FindByIdentity(pc, username);

           if(up.IsMemberOf(pc,IdentityType.SamAccountName,"mygroup"))

            {

               Console.WriteLine("In Group");

            }

           else

            {

               Console.WriteLine("Not In Group");

            }

        }

    }

}

Caveat:  This does not work for the "Domain Users" group.  It always return false.  I don't know why.

Update:  Seems it doesn't work for large AD groups.