C# – Validando usuário no domínio Windows

using (PrincipalContext pc = new PrincipalContext(ContextType.Domain, "MY-DOMAIN")){
   UserPrincipal up = UserPrincipal.FindByIdentity(
       pc, IdentityType.SamAccountName,
       "USERNAME");
      if(up != null){
        //Usuario existe
      }
 }