Get users that are 'memberof' a group

If you can use System.DirectoryServices.AccountManagement:

var context = new PrincipalContext(ContextType.Domain, "YOUR_DOMAIN_NAME");
using (var searcher = new PrincipalSearcher())
{
    var groupName = "YourGroup";
    var sp = new GroupPrincipal(context, groupName);
    searcher.QueryFilter = sp;
    var group = searcher.FindOne() as GroupPrincipal;

    if (group == null)
        Console.WriteLine("Invalid Group Name: {0}", groupName);

    foreach (var f in group.GetMembers())
    {
        var principal = f as UserPrincipal;

        if (principal == null || string.IsNullOrEmpty(principal.Name))
            continue;

        Console.WriteLine("{0}", principal.Name);
    }
}

I have some VB code that'll do it the old way also, but this is definitely simpler with AccountManagement.

---

Here's the VB code I was referring to (again it isn't pretty but it's functional):

Public Function GetUsersByGroup(de As DirectoryEntry, groupName As String) As IEnumerable(Of DirectoryEntry)
    Dim userList As New List(Of DirectoryEntry)
    Dim group As DirectoryEntry = GetGroup(de, groupName)

    If group Is Nothing Then Return Nothing

    For Each user In GetUsers(de)
        If IsUserInGroup(user, group) Then
            userList.Add(user)
        End If
    Next

    Return userList
End Function

Public Function GetGroup(de As DirectoryEntry, groupName As String) As DirectoryEntry
    Dim deSearch As New DirectorySearcher(de)

    deSearch.Filter = "(&(objectClass=group)(SAMAccountName=" & groupName & "))"

    Dim result As SearchResult = deSearch.FindOne()

    If result Is Nothing Then
        Return Nothing
    End If

    Return result.GetDirectoryEntry()
End Function

Public Function GetUsers(de As DirectoryEntry) As IEnumerable(Of DirectoryEntry)
    Dim deSearch As New DirectorySearcher(de)
    Dim userList As New List(Of DirectoryEntry)

    deSearch.Filter = "(&(objectClass=person))"

    For Each user In deSearch.FindAll()
        userList.Add(user.GetDirectoryEntry())
    Next

    Return userList
End Function

Public Function IsUserInGroup(user As DirectoryEntry, group As DirectoryEntry) As Boolean
    Dim memberValues = user.Properties("memberOf")

    If memberValues Is Nothing OrElse memberValues.Count = 0 Then Return False

    For Each g In memberValues.Value
        If g = group.Properties("distinguishedName").Value.ToString() Then
            Return True
        End If
    Next

    Return False
End Function

And usage:

Dim entries = New DirectoryEntry("LDAP://...")
Dim userList As IEnumerable(Of DirectoryEntry) = GetUsersByGroup(entries, "GroupName")

using System.DirectoryServices;

DirectoryEntry objEntry = DirectoryEntry(Ldapserver, userid, password);
DirectorySearcher personSearcher = new DirectorySearcher(objEntry);
personSearcher.Filter = string.Format("(SAMAccountName={0}", username);
SearchResult result = personSearcher.FindOne();

if(result != null)
{
    DirectoryEntry personEntry = result.GetDirectoryEntry();
    PropertyValueCollection groups = personEntry.Properties["memberOf"];
    foreach(string g in groups)
    {
        Console.WriteLine(g); // will write group name
    }
}

I originally used a method similar to what you have posted and it took about 12 minutes to run through my entire company's AD and get the results. After switching to this method, it takes about 2 minutes. You will need to use the ldapserver address where I wrote ldapserver and the userid and password as well and username is the SAMAccountName for the person you're looking up.

If you check HERE you can do the following:

DirectoryEntry group = new DirectoryEntry("LDAP://CN=foo-group,DC=Cmp,DC=COM");
foreach(object dn in group.Properties["member"] )
    //do whatever