how I retreive computernameof clients in activedirectory ?

i wrote these :
Dim management_object As ManagementObject
        Dim m_local_domain_name As String
        Dim m_local_computer_name As String
        For Each management_object In queryCollection
            m_local_domain_name = management_object("Domain")
            m_local_computer_name = management_object("Name")
        Next management_object


        Dim a As DirectoryEntry = GetComputersInfoCollection("ioptc.org")
        Dim mySearcher As System.DirectoryServices.DirectorySearcher = _
        New System.DirectoryServices.DirectorySearcher(a)
        mySearcher.Filter = ("(objectClass=computer)")
        Dim resEnt As System.DirectoryServices.SearchResult
       
        For Each resEnt In mySearcher.FindAll()
            Try
                TextBox20.Text = TextBox20.text + (":Processing:" & Mid(resEnt.GetDirectoryEntry).Name.ToString(), 4))

            Catch ex As Exception
                Console.WriteLine("Trying to Connect to: " & _
                  resEnt.GetDirectoryEntry().Name.ToString() & vbCrLf & ex.Message.ToString())
            End Try

        Next



and



Public Function GetComputersInfoCollection(ByVal domain As String) As DirectoryEntry
        Dim domainEntry As DirectoryEntry = New DirectoryEntry("WinNT://" + domain, "ioptc\rpi", "ali ranji 321", AuthenticationTypes.Secure)
        domainEntry.Children.SchemaFilter.Add("computer")
        Return domainEntry
    End Function


but when i run i get this error message:The provider does not support searching and cannot search WinNT://ioptc.org