Query Active Directory for distinct values in a given field (e.g. Country) via DirectoryServices

I need to programmatically retrieve all of the distinct values in Active Directory for a given field (for example, the country field "co" - but I will have others) I don't care how many times a given value occurs or who/what it is associated with - I just need the list of distinct countries currently in there.

I have this which will produce the list - but for obvious reasons it is unacceptably slow as it is querying every record in AD and adding new countries to the list as it discovers them.

Public Function GetAllCountries() As List(Of String)

    Dim searcher As DirectorySearcher = ActiveDirectory.Forest.GetCurrentForest.FindGlobalCatalog.GetDirectorySearcher
    Dim searchResults As SearchResultCollection
    Dim properties As ResultPropertyCollection
    Dim countryList As New List(Of String)
    Dim country As String

    Try
        With searcher
            .Filter = "(&(objectClass=user)(co=*))"
            .PageSize = 1000
            .SizeLimit = 0
            .SearchScope = SearchScope.Subtree
            .CacheResults = False
            .PropertiesToLoad.Add("co")
            searchResults = .FindAll
        End With

        For Each result As SearchResult In searchResults
            properties = result.Properties
            country = GetPropertyValue(properties("co")).SingleItem
            If Not countryList.Contains(country) Then countryList.Add(country)
        Next

    Catch ex As Exception

    End Try

    Return countryList

End Function

FYI GetPropertyValue is just a custom function I put together to pull the string value from the Property object...

Is there any way of using DirectoryServices (or a suitable alternative?) to query distinct values for a specific AD field in a more efficient manner?

Unfortunately, there is no equivalent to SQL's DISTINCT in LDAP. And I see you're already doing what you can to make the search as fast as possible (asking for only accounts that have a value in the co attribute, and using PropertiesToLoad so only the one attribute is returned).

The only improvement I think you could make is to use a HashSet instead of a List. It saves using List.Contains() over and over, which is an O(n) operation, whereas it's O(1) with HashSet. The improvement may not be noticeable if the list never gets that big, but it's worth a try, especially if this code needs to be run multiple times.

For example:

Dim countryList As New HashSet(Of String)(1000)

Then:

For Each result As SearchResult In searchResults
    properties = result.Properties
    country = GetPropertyValue(properties("co")).SingleItem
    countryList.Add(country)
Next

Notice that I set the size of the HashSet when declaring it. This saves it from resizing as elements are added.

You also don't need to check if it's already in the HashSet, since Add() just won't add it if it's already there.