How to get only specific field from the list

You can select the value you want first, like this:

filteredLessons.Select(l => l.lessonId).ToList();

And you'll get a list of ID's

If you want to get the the specific row value from list using linq use the following code:

var name = from r in objClientList
           where r.ClientCode == Convert.ToInt32(drpClientsInternal.Items[i].Value)
           select r.IsInternalClient;

foreach (bool c in name)
{
    if (c)
    {
        ClientNameInternal = ClientNameInternal + drpClientsInternal.Items[i].Text +", ";
        drpClientsInternal.Items[i].Selected = true;
    }
}