Question

My code will run in Windows (non-mono) and in Linux (mono). Currently, I am using System.DirectoryServices, which works great in Windows. But in Linux:

System.NullReferenceException: Object reference not set to an instance of an object
  at System.DirectoryServices.DirectorySearcher.InitBlock () [0x00000] 
  at System.DirectoryServices.DirectorySearcher.DoSearch () [0x00000] 
  at System.DirectoryServices.DirectorySearcher.get_SrchColl () [0x00000] 
  at System.DirectoryServices.DirectorySearcher.FindOne () [0x00000] 
  at (wrapper remoting-invoke-with-check) System.DirectoryServices.DirectorySearcher:FindOne ()

Answer 1

RemObjects also has a LDAP implementation in their Internet Pack for .NET,

http://blogs.remobjects.com/blogs/ck/2010/02/08/p1043