Helper for accessing the always unique Active Directory user ID (ObjectGUID).

AdHelper.cs

public class AdHelper
{
    public WindowsAccount GetWindowsAccount(string username = "")
    {
        // NOTE: If the username is not specified, use the current users account
        //       The username should also include the domain if available
        
        var identity = String.IsNullOrWhiteSpace(username)
              	? WindowsIdentity.GetCurrent().Name
            	: username;
            	
      try
      {
          var objGuidBytes = (Byte[]) GetUserFromAd(identity).Properties["objectGUID"][0];
    
          return new WindowsAccount(identity, new Guid(objGuidBytes));
      }
      catch (Exception exception)
      {
          return new WindowsAccount();
      }
    }
    
    static SearchResult GetUserFromAd(string samAccountName)
    {
      var directory = new DirectorySearcher();
      var cleansedAccountName = StripDomainName(samAccountName);
      var filter = string.Format("(&(objectCategory=user)(objectClass=user)(|(samaccountname={0})))", cleansedAccountName);
    
      directory.Filter = filter;
      directory.SizeLimit = int.MaxValue;
      directory.CacheResults = true;
      directory.PropertiesToLoad.AddRange(new[] { "objectGUID", "sAMAccountName" });
    
      var searchResult = directory.FindOne();
    
      return searchResult;
    }
    
    static string StripDomainName(string username)
    {
      var lastInstanceOfForwardSlash = username.LastIndexOf('\\');
    
      if (lastInstanceOfForwardSlash > -1 && username.Length > username.LastIndexOf('\\') + 1)
      {
          return username.Substring(lastInstanceOfForwardSlash + 1);
      }
    
      return username;
    }
}

public class WindowsAccount
{
   public WindowsAccount()
       : this(String.Empty, Guid.Empty) { }

   public WindowsAccount(
       string name,
       Guid objectId)
   {
       Name = name;
       ObjectId = objectId;
   }

   public string Name { get; private set; }

   public Guid ObjectId { get; private set; }

   public bool IsValid
   {
       get { return ObjectId != Guid.Empty; }
   }

   public override string ToString()
   {
       return String.Format("{0} ({1:D})", Name, ObjectId);
   }
}